Some climate change effects, clockwise from top left: Wildfire caused by heat and dryness, bleached coral caused by ocean acidification and heating, coastal flooding caused by storms and sea level rise, and environmental migration caused by desertification

The effects of climate change span the impacts on physical environment, ecosystems and human societies due to ongoing human-caused climate change. Many physical impacts of climate change are obvious: extreme weather, glacier retreat,^[1] sea level rise, declines in Arctic sea ice,^[2] and changes in the timing of seasonal events (such as earlier spring flowering).^[3] Since 1970 the ocean has absorbed more than 90% of the excess heat in the climate system, raising temperatures and damaging coral reefs. Even if global surface temperature is stabilized, sea levels will continue to rise and the ocean will continue to absorb excess heat from the atmosphere for many centuries.^[4] As well as the heat, 20% to 30% of human-induced atmospheric carbon dioxide has been absorbed since the 1980s, acidifying the ocean.^[4]

Temperature increases faster over the land and in northern latitudes, so for every 1 °C of global warming temperatures in North America increase by almost 1.6 °C (3 °F).^[5] Climate change has already contributed to land degradation through changes to average conditions and increases in extreme weather.^[6] Changes to average conditions include raised temperatures and depleted soil moisture, increasing wildfire risk and impacting agriculture.^[6] Weather extremes that have increased in frequency and intensity include heat waves, drought, and storms. These changes are most acutely felt in natural ecosystems and developing countries where adaptation is most difficult. This can result in environmental migration for people that are dependent on land for food, feed, fibre, timber and energy.^[7][8]

The future impact of climate change depends on how much nations reduce greenhouse gas emissions and adapt to climate change.^[9][10] Policy decisions made in the next few decades will have profound impacts on the global climate, ecosystems and human societies, not just for this century, but for the next millennia, as near-term climate change policies significantly affect long-term climate change impacts.^[9][11][12] Stringent mitigation policies might be able to limit global warming (in 2100) to around 2 °C or below, relative to pre-industrial levels.^[13][14] Without mitigation, increased energy demand and the extensive use of fossil fuels^[15] may lead to global warming of around 4 °C.^[16][17] With higher magnitudes of global warming, societies and ecosystems will likely encounter limits to how much they can adapt.^[18] This will also threaten sustainability.^[19][20]

Global warming is the ongoing rise in the average temperature of the Earth's climate system. It is a major aspect of climate change, and has been demonstrated by the instrumental temperature record which shows global warming of around 1 °C since the pre-industrial period.^[22] Compared to other historical climate variability, current warming is also more globally coherent, affecting 98% of the planet.^[21][23] The impact on the environment, ecosystems, the animal kingdom, society and humanity depends on how much more the Earth warms.^[24]

Global surface temperatures in 2016 had increased about 1.0 °C since 1901.^[25] The linear trend for the past 50 years of 0.13 °C (plus or minus 0.03 °C) per decade is nearly twice that for the past 100 years.^{[needs update]} The warming has not been globally uniform. The recent warmth has been greatest over North America and Eurasia between 40 and 70°N.^[26] Of the warmest years on record, 16 out of 17 occurred in the 21st century.^[25] Winter temperatures are rising faster than summer temperatures and nights warm faster than days.^[25]

The range in temperature projections partly reflects the choice of emissions scenario, and the degree of "climate sensitivity".^[29] The projected magnitude of warming by 2100 is closely related to the level of cumulative emissions over the 21st century (i.e. total emissions between 2000 and 2100).^[30] The higher the cumulative emissions over this time period, the greater the level of warming is projected to occur.^[30] Climate sensitivity reflects uncertainty in the response of the climate system to past and future GHG emissions.^[29] Higher estimates of climate sensitivity lead to greater projected warming, while lower estimates lead to less projected warming.^[31]

The IPCC's Fifth Report, stated in 2013 that relative to the average from year 1850 to 1900, global surface temperature change by the end of the 21st century is likely to exceed 1.5 °C and may well exceed 2 °C for all Representative Concentration Pathway (RCP) scenarios except RCP2.6. It is likely to exceed 2 °C for RCP6.0 and RCP8.5, and more likely than not to 2 °C for RCP4.5. The pathway with the highest greenhouse gas emissions, RCP8.5, will lead to a temperature increase of about 4.3˚C by 2100.^[32] Warming will continue beyond 2100 under all RCP scenarios except RCP2.6.^[33] Even if emissions were drastically reduced overnight, the warming process is irreversible because CO₂ takes hundreds of years to break down, and global temperatures will remain close to their highest level for at least the next 1,000 years.^[34][35]

Mitigation policies currently in place will result in about 2.9 °C warming above pre-industrial levels. If all unconditional pledges and targets already made by governments will be achieved the temperature will rise by 2.4 °C. If all the 131 countries that actually adopted or only consider to adopt net – zero target will achieve it the temperature will rise by 2.0 °C. However, if current plans are not actually implemented, global warming is expected to reach 4.1 °C to 4.8 °C by 2100. There is a substantial gap between national plans and commitments and actual actions so far taken by governments around the world.^[36]

According to the World Meteorological Organization report from 2021, there is a 44% chance that the global temperature will temporarily pass the 1.5 limit already in the years 2021 – 2026.^[37]

One of the methods scientists use to predict the effects of human-caused climate change, is to investigate past natural changes in climate.^[38] Scientists have used various "proxy" data to assess changes in Earth's past climate or paleoclimate.^[39] Sources of proxy data include historical records such as tree rings, ice cores, corals, and ocean and lake sediments.^[39] The data shows that recent warming has surpassed anything in the last 2,000 years.^[40]

By the end of the 21st century, temperatures may increase to a level not experienced since the mid-Pliocene, around 3 million years ago.^[41] At that time, mean global temperatures were about 2–4 °C warmer than pre-industrial temperatures, and the global mean sea level was up to 25 meters higher than it is today.^[42]

Climate change causes a variety of physical impacts on the climate system. The physical impacts of climate change foremost include globally rising temperatures of the lower atmosphere, the land, and oceans. Temperature rise is not uniform, with land masses and the Arctic region warming faster than the global average. Effects on weather encompass increased heavy precipitation, reduced amounts of cold days, increase in heat waves and various effects on tropical cyclones. The enhanced greenhouse effect causes the higher part of the atmosphere, the stratosphere, to cool. Geochemical cycles are also impacted, with absorption of CO₂ causing ocean acidification, and rising ocean water decreasing the ocean's ability to absorb further carbon dioxide. Annual snow cover has decreased, sea ice is declining and widespread melting of glaciers is underway. Thermal expansion and glacial retreat cause sea levels to increase. Retreat of ice mass may impact various geological processes as well, such as volcanism and earthquakes. Increased temperatures and other human interference with the climate system can lead to tipping points to be crossed such as the collapse of the thermohaline circulation or the Amazon rainforest. Some of these physical impacts also affect social and economic systems.

A broad range of evidence shows that the climate system has warmed.^[47] Evidence of global warming is shown in the graphs (below right) from the US National Oceanic and Atmospheric Administration (NOAA). Some of the graphs show a positive trend, e.g., increasing temperature over land and the ocean, and sea level rise. Other graphs show a negative trend, such as decreased snow cover in the Northern Hemisphere, and declining Arctic sea ice, both of which are indicative of global warming. Evidence of warming is also apparent in living (biological) systems such as changes in distribution of flora and fauna towards the poles.^[48]

Human-induced warming could lead to large-scale, abrupt and/or irreversible changes in physical systems.^[49][50] An example of this is the melting of ice sheets, which contributes to sea level rise and will continue for thousands of years.^[51] The probability of warming having unforeseen consequences increases with the rate, magnitude, and duration of climate change.^[52]

Increasing temperature is likely to lead to increasing precipitation^[53][54] but the effects on storms are less clear. Extratropical storms partly depend on the temperature gradient, which is predicted to weaken in the northern hemisphere as the polar region warms more than the rest of the hemisphere.^[55] It is possible that the Polar and Ferrel cells in one or both hemispheres will weaken and eventually disappear, which would cause the Hadley cell to cover the whole planet.^[56] This would greatly decrease the temperature gradient between the arctic and the tropics, and cause the earth to flip to a hothouse state.^[56]

Global warming leads to an increase in extreme weather events such as heat waves, droughts, cyclones, blizzards and rainstorms.^[57] Such events will continue to occur more often and with greater intensity.^[58] Scientists have not only determined that climate change is responsible for trends in weather patterns, some individual extreme weather events have also directly be attributed to climate change.^[59]

The IPCC Sixth Assessment Report in 2021 summarized the impact of warming on some types of extreme weather as follows:^[60]

Historically (i.e., over the 20th century), subtropical land regions have been mostly semi-arid, while most subpolar regions have had an excess of precipitation over evaporation. Future global warming is expected to be accompanied by a reduction in rainfall in the subtropics and an increase in precipitation in subpolar latitudes and some equatorial regions. In other words, regions which are dry at present will generally become even drier, while regions that are currently wet will generally become even wetter. This projection does not apply to every locale, and in some cases can be modified by local conditions. Drying is projected to be strongest near the poleward margins of the subtropics (for example, South Africa, southern Australia, the Mediterranean, and the south-western U.S.), a pattern that can be described as a poleward expansion of these semi-arid zones.^[62]

This large-scale pattern of change is a robust feature present in nearly all of the simulations conducted by the world's climate modeling groups for the 4th Assessment of the Intergovernmental Panel on Climate Change (IPCC), and is also evident in observed 20th century precipitation trends.^[62]

Changes in regional climate are expected to include greater warming over land, with most warming at high northern latitudes, and least warming over the Southern Ocean and parts of the North Atlantic Ocean.^[63]

Future changes in precipitation are expected to follow existing trends, with reduced precipitation over subtropical land areas, and increased precipitation at subpolar latitudes and some equatorial regions.^[64]

A 2015 study published in Nature Climate Change, states:

About 18% of the moderate daily precipitation extremes over land are attributable to the observed temperature increase since pre-industrial times, which in turn primarily results from human influence. For 2 °C of warming the fraction of precipitation extremes attributable to human influence rises to about 40%. Likewise, today about 75% of the moderate daily hot extremes over land are attributable to warming. It is the most rare and extreme events for which the largest fraction is anthropogenic, and that contribution increases nonlinearly with further warming.^[65][66]

Higher temperatures lead to increased evaporation and surface drying. As the air warms, its water-holding capacity also increases, particularly over the oceans. In general the air can hold about 7% more moisture for every 1 °C of temperature rise.^[29] In the tropics, there's more than a 10% increase in precipitation for a 1 °C increase in temperature.^[67] Changes have already been observed in the amount, intensity, frequency, and type of precipitation. Widespread increases in heavy precipitation have occurred even in places where total rain amounts have decreased.^[68]

Projections of changes in precipitation show increases in the global average, with substantial shifts in location and pattern of rainfall.^[29] Projections suggest a reduction in rainfall in the subtropics, and an increase in precipitation in subpolar latitudes and some equatorial regions. In other words, regions which are dry at present will in general become even drier, while regions that are currently wet will in general become even wetter.^[69] Although increased rainfall will not occur everywhere, models suggest most of the world will have a 16–24% increase in heavy precipitation intensity by 2100.^[70]

Warming has increased contrasts in precipitation amounts between wet and dry seasons and weather regimes over tropical lands.^[71]

As described in the first section, global temperatures have risen by 1 °C and are expected to rise further in the future.^[22][33] Over most land areas since the 1950s, it is very likely that at all times of year both days and nights have become warmer due to human activities.^[72] Night-time temperatures have increased faster than daytime temperatures.^[73] In the U.S. since 1999, two warm weather records have been set or broken for every cold one.^[74][75]

Future climate change will include more very hot days and fewer very cold days.^[72] The frequency, length and intensity of heat waves will very likely increase over most land areas.^[72] Higher growth in anthropogenic GHG emissions would cause more frequent and severe temperature extremes.^[76]

Global warming boosts the probability of extreme weather events such as heat waves^[77][78] where the daily maximum temperature exceeds the average maximum temperature by 5 °C (9 °F) for more than five consecutive days.^[79]

In the last 30–40 years, heat waves with high humidity have become more frequent and severe. Extremely hot nights have doubled in frequency. The area in which extremely hot summers are observed has increased 50–100 fold. These changes are not explained by natural variability, and are attributed by climate scientists to the influence of anthropogenic climate change. Heat waves with high humidity pose a big risk to human health while heat waves with low humidity lead to dry conditions that increase wildfires. The mortality from extreme heat is larger than the mortality from hurricanes, lightning, tornadoes, floods, and earthquakes together.^[80]

In some cases, climate change may also lead to more frequent extremely cold winter weather across parts of Eurasia^[81] and North America.^{[82][83][84][85]} However, before the study, some researchers stated that warming will make such events less likely. Conclusions that link climate change to cold waves are considered to still be highly controversial.^{[86][additional citation(s) needed]} The JRC PESETA IV project concluded in 2020 that overall climate change will result in a decline in the intensity and frequency of extreme cold spells, with milder winters reducing fatalities from extreme cold,^{[87][additional citation(s) needed]} even if individual cold extreme weather may sometimes be caused by changes due to climate change and possibly even become more frequent in some regions.

Fire is a major agent for conversion of biomass and soil organic matter to CO₂ (Denman et al., 2007:527).^[88] There is a large potential for future alteration in the terrestrial carbon balance through altered fire regimes. With high confidence, Schneider et al. (2007:789) projected that:^[89]

• An increase in global mean temperature of about 0 to 2 °C by 2100 relative to the 1990–2000 period would result in increased fire frequency and intensity in many areas.
• An increase in the region of 2 °C or above would lead to increased frequency and intensity of fires. Sensitivity to fires in areas that were already vulnerable has been steadily increasing. In high altitude temperate areas, increased temperature is causing snow pack to melt sooner and in greater quantities. The number of days that of higher stream flow caused by snowmelt in the Mississippi, Missouri, and Ohio rivers has been increasing in recent years.^[90] The substantial amount of snow that remains atop mountains year around is also disappearing. This leads to the surrounding densely forested areas becoming more dry and staying dry for longer periods of time. In the 1970s, the length of a fire season, which is the period of the year fires are most likely to occur, was about five months. Today, the period is usually seven months, extending into the springtime mud season.^[91] In addition, many areas are experiencing higher than normal droughts. Between 2011 and 2014, California experienced the driest period in its recorded history^[92] and more than 100 million trees died in the drought, creating areas of dead, dry wood.^[93] The decrease in rainfall is also going to increase the risk of wildfire by allowing the fire access to drier fuels. Dry foliage is more susceptible to a wildfire trigger. Wildfire specialists use foliar moisture content to determine how susceptible an area is to a wildfire.^[94] In the United States, 2015 was the most destructive year on record for wildfires, with a total of 10,125,149 total acres destroyed by fires. 2017 was the second worst year on record with 10,026,086 acres destroyed.^[95] The Thomas Fire occurred in 2017 and was the largest fire in California's history.^[96]

The increasing frequency of wildfires as a result of climate change will also lead to an increase in the amount of CO₂ in the atmosphere. This will, in turn, increase the temperature and the frequency of hot days, which will further increase fire danger. It was forecasted that double levels of CO_2, would bring a greater risk of wildfires to Australia, especially the Australian outback. All of the eight sites tested projected an increase in fire danger as a result of CO₂ level increase and all but one projected a longer fire season. The largest population center said to be affected is Alice Springs, a city deep in the Outback.^[97]

IPCC (2007a:8) projected that in the future, over most land areas, the frequency of warm spells or heat waves would very likely increase.^[99] Other likely changes are listed below:

• Increased areas will be affected by drought^[100]
• There will be increased intense tropical cyclone activity^[100]
• There will be increased incidences of extreme high sea level (excluding tsunamis)^[100]

Global warming not only causes changes in tropical cyclones, it may also make some impacts from them worse via sea level rise. The intensity of tropical cyclones (hurricanes, typhoons, etc.) is projected to increase globally, with the proportion of Category 4 and 5 tropical cyclones increasing. Furthermore, the rate of rainfall is projected to increase, but trends in the future frequency on a global scale are not yet clear.^[101][102] Changes in tropical cyclones will probably vary by region.^[101]

Storm strength leading to extreme weather is increasing, such as the power dissipation index of hurricane intensity.^[103] Kerry Emanuel writes that hurricane power dissipation is highly correlated with temperature, reflecting global warming.^[104] However, a further study by Emanuel using current model output concluded that the increase in power dissipation in recent decades cannot be completely attributed to global warming.^[105] Hurricane modeling has produced similar results, finding that hurricanes, simulated under warmer, high-CO₂ conditions, are more intense, however, hurricane frequency will be reduced.^[106] Worldwide, the proportion of hurricanes reaching categories 4 or 5 – with wind speeds above 56 metres per second – has risen from 20% in the 1970s to 35% in the 1990s.^[107] Precipitation hitting the US from hurricanes has increased by 7% over the 20th century.^{[108][109][110]} The extent to which this is due to global warming as opposed to the Atlantic Multidecadal Oscillation is unclear. Some studies have found that the increase in sea surface temperature may be offset by an increase in wind shear, leading to little or no change in hurricane activity.^[111] Hoyos et al. (2006) have linked the increasing trend in number of category 4 and 5 hurricanes for the period 1970–2004 directly to the trend in sea surface temperatures.^[112]

An international team of scientists stated in 2016 that highly destructive category four and five storms have increased in most ocean basins, among which the North Atlantic.^[113][114] In 2008, Knutson et al. found that Atlantic hurricane and tropical storm frequencies could reduce under future greenhouse-gas-induced warming.^[106] Vecchi and Soden find that wind shear, the increase of which acts to inhibit tropical cyclones, also changes in model-projections of global warming. There are projected increases of wind shear in the tropical Atlantic and East Pacific associated with the deceleration of the Walker circulation, as well as decreases of wind shear in the western and central Pacific.^[115] The study does not make claims about the net effect on Atlantic and East Pacific hurricanes of the warming and moistening atmospheres, and the model-projected increases in Atlantic wind shear.^[116]

The World Meteorological Organization explains that "though there is evidence both for and against the existence of a detectable anthropogenic signal in the tropical cyclone climate record to date, no firm conclusion can be made on this point."^[117] They also clarified that "no individual tropical cyclone can be directly attributed to climate change."^[117]

A substantially higher risk of extreme weather does not necessarily mean a noticeably greater risk of slightly-above-average weather.^[118] However, the evidence is clear that severe weather and moderate rainfall are also increasing. Increases in temperature are expected to produce more intense convection over land and a higher frequency of the most severe storms.^[119]

Using the Palmer Drought Severity Index, a 2010 study by the National Center for Atmospheric Research projects increasingly dry conditions across much of the globe in the next 30 years, possibly reaching a scale in some regions by the end of the century that has rarely, if ever, been observed in modern times.^[120]

It was estimated in 2013 that global warming had increased the probability of local record-breaking monthly temperatures worldwide by a factor of 5.^[121] This was compared to a baseline climate in which no global warming had occurred. Using a medium global warming scenario, they project that by 2040, the number of monthly heat records globally could be more than 12 times greater than that of a scenario with no long-term warming.

Over the course of the 20th century, evaporation rates have reduced worldwide;^[126] this is thought by many to be explained by global dimming. As the climate grows warmer and the causes of global dimming are reduced, evaporation will increase due to warmer oceans. Because the world is a closed system this will cause heavier rainfall, with more erosion. This erosion, in turn, can in vulnerable tropical areas (especially in Africa) lead to desertification. On the other hand, in other areas, increased rainfall lead to growth of forests in dry desert areas.

Scientists have found evidence that increased evaporation could result in more extreme weather as global warming progresses. The IPCC Third Annual Report says: "...global average water vapor concentration and precipitation are projected to increase during the 21st century. By the second half of the 21st century, it is likely that precipitation will have increased over northern mid- to high latitudes and Antarctica in winter. At low latitudes there are both regional increases and decreases over land areas. Larger year-to-year variations in precipitation are very likely over most areas where an increase in mean precipitation is projected."^[53][127]

Dust from the Sahara Desert typically blows across the Atlantic Ocean. In June 2020, the Saharan dust plume was the most dense it had been in 25 years. It is uncertain whether climate change affects this.^[128]

Research based on satellite observations, published in October 2010, shows an increase in the flow of freshwater into the world's oceans, partly from melting ice and partly from increased precipitation driven by an increase in global ocean evaporation. The increase in global freshwater flow, based on data from 1994 to 2006, was about 18%. Much of the increase is in areas which already experience high rainfall. One effect, as perhaps experienced in the 2010 Pakistan floods, is to overwhelm flood control infrastructure.^[129]

The lower and middle atmosphere are heating due to the enhanced greenhouse effect. Increased greenhouse gases cause the higher parts of the atmosphere, the stratosphere to cool. This has been observed by a set of satellites since 1979 (the Microwave sounding unit) and radiosonde data. Satellites can not measure each height of the atmosphere separately, but instead measure a set of bands that slightly overlap. The overlap between the cooling stratosphere in the measurements of tropospheric warming may cause the latter to be underestimated slightly.^[130] The heated atmosphere contains more water vapour, which is itselfs also a greenhouse gas and acts as an self-reinforcing feedback.^[131]

A contraction of the thermosphere has been observed as a possible result in part due to increased carbon dioxide concentrations, the strongest cooling and contraction occurring in that layer during solar minimum. The most recent contraction in 2008–2009 was the largest such since at least 1967.^{[132][133][134]}

Climate change can have an effect on the carbon cycle in an interactive "feedback" process . A feedback exists where an initial process triggers changes in a second process that in turn influences the initial process. A positive feedback intensifies the original process, and a negative feedback reduces it (IPCC, 2007d:78).^[135] Models suggest that the interaction of the climate system and the carbon cycle is one where the feedback effect is positive (Schneider et al., 2007:792).^[89]

Using the A2 SRES emissions scenario, Schneider et al. (2007:789) found that this effect led to additional warming by 2100, relative to the 1990–2000 period, of 0.1 to 1.5 °C. This estimate was made with high confidence. The climate projections made in the IPCC Fourth Assessment Report of 1.1 to 6.4 °C account for this feedback effect. On the other hand, with medium confidence, Schneider et al. (2007) commented that additional releases of GHGs were possible from permafrost, peat lands, wetlands, and large stores of marine hydrates at high latitudes.

Gas hydrates are ice-like deposits containing a mixture of water and gas, the most common gas of which is methane (Maslin, 2004:1).^[136] Gas hydrates are stable under high pressures and at relatively low temperatures and are found underneath the oceans and permafrost regions. Future warming at intermediate depths in the world's oceans, as predicted by climate models, will tend to destabilize gas hydrates resulting in the release of large quantities of methane. On the other hand, projected rapid sea level rise in the coming centuries associated with global warming will tend to stabilize marine gas hydrate deposits.

Models have been used to assess the effect that climate change will have on the carbon cycle (Meehl et al., 2007:789-790).^[137] In the Coupled Climate-Carbon Cycle Model Intercomparison Project, eleven climate models were used. Observed emissions were used in the models and future emission projections were based on the IPCC SRES A2 emissions scenario.

Unanimous agreement was found among the models that future climate change will reduce the efficiency of the land and ocean carbon cycle to absorb human-induced CO₂. As a result, a larger fraction of human-induced CO₂ will stay airborne if climate change controls the carbon cycle. By the end of the 21st century, this additional CO₂ in the atmosphere varied between 20 and 220 ppm for the two extreme models, with most models lying between 50 and 100 ppm. This additional CO₂ led to a projected increase in warming of between 0.1 and 1.5 °C.

Global warming is projected to have a number of effects on the oceans. Ongoing effects include rising sea levels due to thermal expansion and melting of glaciers and ice sheets, and warming of the ocean surface, leading to increased temperature stratification.^[138] Other possible effects include large-scale changes in ocean currents. The oceans also serve as a sink for carbon dioxide, taking up much that would otherwise remain in the atmosphere, but increased levels of CO
₂ have led to ocean acidification. Furthermore, as the temperature of the oceans increases, they become less able to absorb excess CO
₂. The oceans have also acted as a sink in absorbing extra heat from the atmosphere.^[139]: 4

According to a Special Report on the Ocean and Cryosphere in a Changing Climate published by the Intergovernmental Panel on Climate Change, climate change has different impacts on the oceans, including an increase in marine heatwaves, shift in species distribution, ocean deoxygenation.^[140]

The decline in mixing of the ocean layers piles up warm water near the surface while reducing cold, deep water circulation. The reduced up and down mixing enhanced global warming. Furthermore, energy available for tropical cyclones and other storms is expected to increase, nutrients for fish in the upper ocean layers are set to decrease, as well as the capacity of the oceans to store carbon.^[141]

The oceans serve as a sink for carbon dioxide, taking up much that would otherwise remain in the atmosphere, but increased levels of CO₂ have led to ocean acidification. Furthermore, as the temperature of the oceans increases, they become less able to absorb excess CO₂. Global warming is projected to have a number of effects on the oceans. Ongoing effects include rising sea levels due to thermal expansion and melting of glaciers and ice sheets, and warming of the ocean surface, leading to increased temperature stratification. Other possible effects include large-scale changes in ocean currents.

From 1961 to 2003, the global ocean temperature has risen by 0.10 °C from the surface to a depth of 700 m.^[143] For example, the temperature of the Antarctic Southern Ocean rose by 0.17 °C (0.31 °F) between the 1950s and the 1980s, nearly twice the rate for the world's oceans as a whole.^[144] There is variability both year-to-year and over longer time scales, with global ocean heat content observations showing high rates of warming for 1991 to 2003, but some cooling from 2003 to 2007.^[143] Nevertheless, there is a strong trend during the period of reliable measurements.^[142] Increasing heat content in the ocean is also consistent with sea level rise, which is occurring mostly as a result of thermal expansion of the ocean water as it warms.^[142]

While the full implications of elevated CO₂ on marine ecosystems are still being documented, there is a substantial body of research showing that a combination of ocean acidification and elevated ocean temperature, driven mainly by CO₂ and other greenhouse gas emissions, have a compounded effect on marine life and the ocean environment. This effect far exceeds the individual harmful impact of either.^{[145][146][147]} In addition, ocean warming exacerbates ocean deoxygenation, which is an additional stressor on marine organisms, by increasing ocean stratification, through density and solubility effects, thus limiting nutrients,^[148][149]

Ocean acidification is an effect of rising concentrations of CO₂ in the atmosphere, and is not a direct consequence of global warming. The oceans soak up much of the CO₂ produced by living organisms, either as dissolved gas, or in the skeletons of tiny marine creatures that fall to the bottom to become chalk or limestone. Oceans currently absorb about one tonne of CO₂ per person per year. It is estimated that the oceans have absorbed around half of all CO₂ generated by human activities since 1800 (118 ± 19 petagrams of carbon from 1800 to 1994).^[150]

In water, CO₂ becomes a weak carbonic acid, and the increase in the greenhouse gas since the Industrial Revolution has already lowered the average pH (the laboratory measure of acidity) of seawater by 0.1 units, to 8.2. Predicted emissions could lower the pH by a further 0.5 by 2100, to a level probably not seen for hundreds of millennia and, critically, at a rate of change probably 100 times greater than at any time over this period.^[151][152]

There are concerns that increasing acidification could have a particularly detrimental effect on corals^[153] (16% of the world's coral reefs have died from bleaching caused by warm water in 1998,^[154] which coincidentally was, at the time, the warmest year ever recorded) and other marine organisms with calcium carbonate shells.^[155]

In November 2009 an article in Science by scientists at Canada's Department of Fisheries and Oceans reported they had found very low levels of the building blocks for the calcium chloride that forms plankton shells in the Beaufort Sea.^[156] Fiona McLaughlin, one of the DFO authors, asserted that the increasing acidification of the Arctic Ocean was close to the point it would start dissolving the walls of existing plankton: "[the] Arctic ecosystem may be risk. In actual fact, they'll dissolve the shells." Because cold water absorbs CO₂ more readily than warmer water the acidification is more severe in the polar regions. McLaughlin predicted the acidified water would travel to the North Atlantic within the next ten years.

There is some speculation that global warming could, via a shutdown or slowdown of the thermohaline circulation, trigger localized cooling in the North Atlantic and lead to cooling, or lesser warming, in that region.^[157] This would affect in particular areas like Scandinavia and Britain that are warmed by the North Atlantic drift.

The chances of this near-term collapse of the circulation, which was fictionally portrayed in the 2004 film Day After Tomorrow, are unclear. Lenton et al. found that "simulations clearly pass a THC tipping point this century".^[157]

IPCC (2007b:17) concluded that a slowing of the Meridional Overturning Circulation would very likely occur this century.^[158] Due to global warming, temperatures across the Atlantic and Europe were still projected to increase.

The amount of oxygen dissolved in the oceans may decline, with adverse consequences for ocean life.^[159][160]

Warmer water cannot contain as much oxygen as cold water, so heating is expected to lead to less oxygen in the ocean. Other processes also play a role: stratification may lead to increases in respiration rates of organic matter, further decreasing oxygen content. The ocean has already lost oxygen, throughout the entire water column and oxygen minimum zones are expanding worldwide.^[138] This has adverse consequences for ocean life.^[161][162]

Oceans have taken up over 90% of the excess heat accumulated on Earth due to global warming.^[163] The warming rate varies with depth: at a depth of a thousand metres the warming occurs at a rate of almost 0.4 °C per century (data from 1981 to 2019), whereas the warming rate at two kilometres depth is only half.^[164] The increase in ocean heat content is much larger than any other store of energy in the Earth's heat balance and accounts for more than 90% of the increase in heat content of the Earth system, and has accelerated in the 1993–2017 period compared to 1969–1993.^[165] In 2019 a paper published in the journal Science found the oceans are heating 40% faster than the fifth IPCC assessment report predicted just five years before.^[166][167]

As well as having effects on ecosystems (e.g. by melting sea ice affecting algae that grow on its underside), warming reduces the ocean's ability to absorb CO
₂.^[168] It is likely that the oceans warmed faster between 1993 and 2017 compared to the period starting in 1969.^[169]

Changes in climate indicators over several decades. Each of the different colored lines in each panel represents an independently analyzed set of data. The data come from many different technologies including weather stations, satellites, weather balloons, ships and buoys.^[175]

In 2019 the Intergovernmental Panel on Climate Change issued a Special Report on Climate Change and Land. The main statements of the report include:^[176][177]

• Humans affect 70% of the ice free land, that play a key role in supplying the needs of humans and in the climate system.
• The global food supply have raised what increased GHG emission, but 25% – 30% of the food is lost, 2 billion adults suffer from being overweight while 821 million people suffer from hunger.
• The rate of soil erosion is 10 – 20 times higher than the rate of soil accumulation in agricultural areas that use no-till farming. In areas with tilling it is 100 times higher. Climate Change increases land degradation and desertification.
• In the years 1960 – 2013 the area of drylands in drought, increased by 1% per year.
• In the year 2015 around 500 million people lived in areas that was impacted by desertification in the years 1980s – 2000s.
• People who live in the areas affected by land degradation and desertification are "increasingly negatively affected by climate change".

Climate change will also cause soils to warm. In turn, this could cause the soil microbe population size to dramatically increase 40–150%. Warmer conditions would favor growth of certain bacteria species, shifting the bacterial community composition. Elevated carbon dioxide would increase the growth rates of plants and soil microbes, slowing the soil carbon cycle and favoring oligotrophs, which are slower-growing and more resource efficient than copiotrophs.^[178]

Warmer air holds more water vapor. The value is 7% more water in its gas phase (vapour) per degree Celsius of warming.^[180] When this turns to rain, it tends to come in heavy downpours potentially leading to more severe flood hazards.^[180]

A 2017 study found that peak precipitation is increasing between 5 and 10% for every one degree Celsius increase.^[181] In the United States and many other parts of the world there has been a marked increase in intense rainfall events which have resulted in more severe flooding.^[182] Estimates of the number of people at risk of coastal flooding from climate-driven sea-level rise varies from 190 million,^[183] to 300 million or even 640 million in a worst-case scenario related to the instability of the Antarctic ice sheet.^[184][185] the Greenland ice sheet is estimated to have reached a point of no return, continuing to melt even if warming stopped. Over time that would submerge many of the world's coastal cities including low-lying islands, especially combined with storm surges and high tides.^[186]

Climate change affects multiple factors associated with droughts, such as how much rain falls and how fast the rain evaporates again. Warming over land drives an increase in atmospheric evaporative demand which will increase the severity and frequency of droughts around much of the world.^[187][188] Due to limitations on how much data is available about drought in the past, it is often impossible to confidently attribute droughts to human-induced climate change. Some areas however, such as the Mediterranean and California, already show a clear human signature.^[189] Their impacts are aggravated because of increased water demand, population growth, urban expansion, and environmental protection efforts in many areas.^[190]

Warm and dry temperatures driven by climate change increase the chance of wildfires.^[192] Prolonged periods of warmer temperatures typically cause soil and underbrush to be drier for longer periods, increasing the risk of wildfires. Hot, dry conditions increase the likelihood that wildfires will be more intense and burn for longer once they start.^[193] In California, summer air temperature have increased by over 3.5 °F such that the fire season has lengthened by 75 days over previous decades. As a result, since the 1980s, both the size and ferocity of fires in California have increased. Since the 1970s, the size of the area burned has increased fivefold.^[194]

In Australia, the annual number of hot days (above 35 °C) and very hot days (above 40 °C) has increased significantly in many areas of the country since 1950. The country has always had bushfires but in 2019, the extent and ferocity of these fires increased dramatically.^[195] For the first time catastrophic bushfire conditions were declared for Greater Sydney. New South Wales and Queensland declared a state of emergency but fires were also burning in South Australia and Western Australia.^[196]

The cryosphere is made up of those parts of the planet which are so cold, they are frozen and covered by snow or ice. This includes ice and snow on land such as the continental ice sheets in Greenland and Antarctica, as well as glaciers and areas of snow and permafrost; and ice found on water including frozen parts of the ocean, such as the waters surrounding Antarctica and the Arctic.^[198] The cryosphere, especially the polar regions, is extremely sensitive to changes in global climate.^[199]

The Intergovernmental Panel on Climate Change issued a Special Report on the Ocean and Cryosphere in a Changing Climate. According to the report climate change caused a massive melting of glaciers, ice sheets, snow and permafrost with generally negative effects on ecosystems and humans. Indigenous knowledge helped to adapt to those effects.^[140]

Since the beginning of the twentieth century, there has also been a widespread retreat of alpine glaciers,^[200] and snow cover in the Northern Hemisphere.^[201] The sensitivity to warming of the "1981–2010 Northern hemisphere snow cover extent" is about minus 1.9 million km² per degrees Celsius throughout the snow season.^[202] During the 21st century, glaciers and snow cover are projected to continue their retreat in almost all regions.^[203] The melting of the Greenland and West Antarctic ice sheets will continue to contribute to sea level rise over long time-scales.^[204]

Northern Hemisphere average annual snow cover has declined in recent decades. This pattern is consistent with warmer global temperatures. Some of the largest declines have been observed in the spring and summer months.^[142]

Sea ice reflects 50% to 70% of the incoming solar radiation, while 6% of the incoming solar engery is reflected by the ocean. With less solar energy, the sea ice absorbs and holds the surface colder, which can be a positive feedback toward climate change.^[205]

As the climate warms, snow cover and sea ice extent decrease. Large-scale measurements of sea-ice have only been possible since the satellite era, but through looking at a number of different satellite estimates, it has been determined that September Arctic sea ice has decreased between 1973 and 2007 at a rate of about -10% +/- 0.3% per decade. Sea ice extent for September for 2012 was by far the lowest on record at 3.29 million square kilometers, eclipsing the previous record low sea ice extent of 2007 by 18%. The age of the sea ice is also an important feature of the state of the sea ice cover, and for the month of March 2012, older ice (4 years and older) has decreased from 26% of the ice cover in 1988 to 7% in 2012.^[206] Sea ice in the Antarctic has shown very little trend over the same period, or even a slight increase since 1979. Though extending the Antarctic sea-ice record back in time is more difficult due to the lack of direct observations in this part of the world.^[26]

In a literature assessment, Meehl et al. (2007:750) found that model projections for the 21st century showed a reduction of sea ice in both the Arctic and Antarctic.^[137] The range of model responses was large. Projected reductions were accelerated in the Arctic. Using the high-emission A2 SRES scenario, some models projected that summer sea ice cover in the Arctic would disappear entirely by the latter part of the 21st century.

Warming temperatures lead to the melting of glaciers and ice sheets.^[142] IPCC (2007a:5) found that, on average, mountain glaciers and snow cover had decreased in both the northern and southern hemispheres.^[99] This widespread decrease in glaciers and ice caps has contributed to observed sea level rise.

As stated above, the total volume of glaciers on Earth is declining sharply. Glaciers have been retreating worldwide for at least the last century; the rate of retreat has increased in the past decade. Only a few glaciers are actually advancing (in locations that were well below freezing, and where increased precipitation has outpaced melting). The progressive disappearance of glaciers has implications not only for a rising global sea level, but also for water supplies in certain regions of Asia and South America.^[142]

With very high or high confidence, IPCC (2007d:11) made a number of projections related to future changes in glaciers:^[135]

• Mountainous areas in Europe will face glacier retreat
• In Latin America, changes in precipitation patterns and the disappearance of glaciers will significantly affect water availability for human consumption, agriculture, and energy production
• In Polar regions, there will be reductions in glacier extent and the thickness of glaciers.

In historic times, glaciers grew during a cool period from about 1550 to 1850 known as the Little Ice Age. Subsequently, until about 1940, glaciers around the world retreated as the climate warmed. Glacier retreat declined and reversed in many cases from 1950 to 1980 as a slight global cooling occurred. Since 1980, glacier retreat has become increasingly rapid and ubiquitous, and has threatened the existence of many of the glaciers of the world. This process has increased markedly since 1995.^[207] Excluding the ice caps and ice sheets of the Arctic and Antarctic, the total surface area of glaciers worldwide has decreased by 50% since the end of the 19th century.^[208] Currently glacier retreat rates and mass balance losses have been increasing in the Andes, Alps, Pyrenees, Himalayas, Rocky Mountains and North Cascades.

The loss of glaciers not only directly causes landslides, flash floods and glacial lake overflow,^[209] but also increases annual variation in water flows in rivers. Glacier runoff declines in the summer as glaciers decrease in size, this decline is already observable in several regions.^[210] Glaciers retain water on mountains in high precipitation years, since the snow cover accumulating on glaciers protects the ice from melting. In warmer and drier years, glaciers offset the lower precipitation amounts with a higher meltwater input.^[208] Some world regions, such as the French Alps, already show signs of an increase in landslide frequency.^[211]

Of particular importance are the Hindu Kush and Himalayan glacial melts that comprise the principal dry-season water source of many of the major rivers of the Central, South, East and Southeast Asian mainland. Increased melting would cause greater flow for several decades, after which "some areas of the most populated regions on Earth are likely to 'run out of water'" as source glaciers are depleted.^[212] The Tibetan Plateau contains the world's third-largest store of ice. Temperatures there are rising four times faster than in the rest of China, and glacial retreat is at a high speed compared to elsewhere in the world.^[213]

According to a Reuters report, the Himalayan glaciers that are the sources of Asia's biggest rivers—Ganges, Indus, Brahmaputra, Yangtze, Mekong, Salween and Yellow—could diminish as temperatures rise.^[214] Approximately 2.4 billion people live in the drainage basin of the Himalayan rivers.^[215] India, China, Pakistan, Bangladesh, Nepal and Myanmar could experience floods followed by droughts in coming decades. The Indus, Ganges and Brahmaputra river basins support 700 million people in Asia.^[216] In India alone, the Ganges provides water for drinking and farming for more than 500 million people.^{[217][218][219]} It has to be acknowledged, however, that increased seasonal runoff of Himalayan glaciers led to increased agricultural production in northern India throughout the 20th century.^[220] Research studies suggest that climate change will have a marked effect on meltwater in the Indus Basin.^[216]

The recession of mountain glaciers, notably in Western North America, Franz-Josef Land, Asia, the Alps, the Pyrenees, Indonesia and Africa, and tropical and sub-tropical regions of South America, has been used to provide qualitative support to the rise in global temperatures since the late 19th century. Many glaciers are being lost to melting further raising concerns about future local water resources in these glaciated areas. In Western North America the 47 North Cascade glaciers observed all are retreating.^[210]

Despite their proximity and importance to human populations, the mountain and valley glaciers of temperate latitudes amount to a small fraction of glacial ice on the earth. About 99% is in the great ice sheets of polar and subpolar Antarctica and Greenland. These continuous continental-scale ice sheets, 3 kilometres (1.9 mi) or more in thickness, cap the polar and subpolar land masses. Like rivers flowing from an enormous lake, numerous outlet glaciers transport ice from the margins of the ice sheet to the ocean. Glacier retreat has been observed in these outlet glaciers, resulting in an increase of the ice flow rate. In Greenland the period since the year 2000 has brought retreat to several very large glaciers that had long been stable. Three glaciers that have been researched, Helheim, Jakobshavn Isbræ and Kangerdlugssuaq Glaciers, jointly drain more than 16% of the Greenland Ice Sheet. Satellite images and aerial photographs from the 1950s and 1970s show that the front of the glacier had remained in the same place for decades. But in 2001 it began retreating rapidly, retreating 7.2 km (4.5 mi) between 2001 and 2005. It has also accelerated from 20 m (66 ft)/day to 32 m (105 ft)/day.^[221] Jakobshavn Isbræ in western Greenland had been moving at speeds of over 24 m (79 ft)/day with a stable terminus since at least 1950. The glacier's ice tongue began to break apart in 2000, leading to almost complete disintegration in 2003, while the retreat rate increased to over 30 m (98 ft)/day.^[222]

Permanent ice cover on land is a result of a combination of low peak temperatures and sufficient precipitation. Some of the coldest places on Earth, such as the dry valleys of Antarctica, lack significant ice or snow coverage due to a lack of snow. Sea ice however maybe formed simply by low temperature, although precipitation may influence its stability by changing albedo, providing an insulating covering of snow and affecting heat transfer. Global warming has the capacity to alter both precipitation and temperature, resulting in significant changes to ice cover. Furthermore, the behaviour of ice sheets, ice caps and glaciers is altered by changes in temperature and precipitation, particularly as regards the behaviour of water flowing into and through the ice.

Arctic sea ice began to decline at the beginning of the twentieth century but the rate is accelerating. Since 1979, satellite records indicate the decline in summer sea ice coverage has been about 13% per decade.^[223][224] The thickness of sea ice has also decreased by 66% or 2.0 m over the last six decades with a shift from permanent ice to largely seasonal ice cover.^[225] Arctic sea ice area will likely drop below 1 million km² in at least some Septembers before 2050.^[226]

While ice-free summers are expected to be rare at 1.5 °C degrees of warming, they are set to occur at least once every decade at a warming level of 2.0 °C.^[227]

Example of a mountain glacier retreat: White Chuck Glacier, Washington

White Chuck Glacier in the United States in 1973

Same vantage point in 2006, at the same time of the year. The glacier retreated 1.9 kilometres (1.2 mi) in 33 years.

The retreat of glaciers since 1850 is well documented and is one of the effects of climate change. The retreat of mountain glaciers provide evidence for the rise in global temperatures since the late 19th century. Examples include mountain glaciers in western North America, Asia, the Alps in central Europe and tropical and subtropical regions of South America and Africa. Since glacial mass is affected by long-term climatic changes, e.g., precipitation, mean temperature, and cloud cover, glacial mass changes are one of the most sensitive indicators of climate change. Retreat of glaciers is also a major reason for sea level rise. Excluding peripheral glaciers of ice sheets, the total cumulated global glacial losses over the 26-year period from 1993 to 2018 were likely 5500 gigatons, or 210 gigatons per yr.^{[243]: 1275}

Thawing of permafrost soils releases methane. Methane has 25 times the warming potential of carbon dioxide. Recent methane emissions of the world's soils were estimated between 150 and 250 million metric tons (2008).^[244] Estimated annual net methane emission rates at the end of the 20th century for the northern region was 51 million metric tons. Net methane emissions from northern permafrost regions included 64% from Russia, 11% from Canada and 7% from Alaska (2004). More recently, during 2019, 360 million tons of methane were released globally from anthropogenic activities, and 230 million tons were released from natural sources.^[245] The business-as-usual scenarios estimate the Arctic methane emissions from permafrost thawing and rising temperatures to range from 54 to 105 million metric tons of methane per year (2006).^[244]

Recent warming has strongly affected natural biological systems.^[48] Species worldwide are moving poleward to colder areas. On land, species move to higher elevations, whereas marine species find colder water at greater depths.^[247] Of the drivers with the biggest global impact on nature, climate change ranks third over the five decades before 2020, with only change in land use and sea use, and direct exploitation of organisms having a greater impact.^[248]

The impacts of climate change in nature and nature's contributions to humans are projected to become more pronounced in the next few decades.^[249] Examples of climatic disruptions include fire, drought, pest infestation, invasion of species, storms, and coral bleaching events. The stresses caused by climate change, added to other stresses on ecological systems (e.g. land conversion, land degradation, harvesting, and pollution), threaten substantial damage to or complete loss of some unique ecosystems, and extinction of some critically endangered species.^[250] Key interactions between species within ecosystems are often disrupted because species from one location do not move to colder habitats at the same rate, giving rise to rapid changes in the functioning of the ecosystem.^[247]

Impacts include changes in regional rainfall patterns, earlier leafing of trees and plants over many regions; movements of species to higher latitudes and altitudes in the Northern Hemisphere; changes in bird migrations in Europe, North America and Australia; and shifting of the oceans' plankton and fish from cold- to warm-adapted communities.^[251]

Climate change has been estimated to be a major driver of biodiversity loss in cool conifer forests, savannas, mediterranean-climate systems, tropical forests, and the Arctic tundra.^[253] In other ecosystems, land-use change may be a stronger driver of biodiversity loss, at least in the near-term.^[253] Beyond the year 2050, climate change may be the major driver for biodiversity loss globally.^[253] Climate change interacts with other pressures such as habitat modification, pollution and invasive species. Interacting with these pressures, climate change increases extinction risk for a large fraction of terrestrial and freshwater species.^[254] Between 1% and 50% of species in different groups were assessed to be at substantially higher risk of extinction due to climate change.^[255]

Warm water coral reefs are very sensitive to global warming and ocean acidification. Coral reefs provide a habitat for thousands of species and ecosystem services such as coastal protection and food. The resilience of reefs can be improved by curbing local pollution and overfishing, but 70–90% of today's warm water coral reefs will disappear even if warming is kept to 1.5 °C.^[256] Coral reefs are not the only framework organisms, organisms that build physical structures that form habitats for other sea creatures, affected by climate change: mangroves and seagrass are considered to be at moderate risk for lower levels of global warming according to a literature assessment in the Special Report on the Ocean and Cryosphere in a Changing Climate.^[257] Marine heatwaves have seen an increased frequency and have widespread impacts on life in the oceans, such as mass dying events.^[258] Harmful algae blooms have increased in response to warming waters, ocean deoxygenation and eutrophication.^[259] Between one-quarter and one-third of our fossil fuel emissions are consumed by the earth's oceans and are now 30 percent more acidic than they were in pre-industrial times. This acidification poses a serious threat to aquatic life, particularly creatures such as oysters, clams, and coral with calcified shells or skeletons.^[252]

The effects of climate change, in combination with the sustained increases in greenhouse gas emissions, have led scientists to characterize it as a climate emergency.^{[260][261][262]} Some climate researchers^[263][264] and activists^[265] have called it an existential threat to civilization. Some areas may become too hot for humans to live in^[266][267] while people in some areas may experience internal or long-distance displacement triggered by flooding and other climate change related disasters.^[268]

The vulnerability and exposure of humans to climate change varies from one economic sector to another and will have different impacts in different countries. Wealthy industrialised countries, which have emitted the most CO₂, have more resources and so are the least vulnerable to global warming.^[269] Economic sectors that are likely to be affected include agriculture, human health, fisheries, forestry, energy, insurance, financial services, tourism, and recreation.^[270] The quality and quantity of freshwater will likely be affected almost everywhere. Some people may be particularly at risk from climate change, such as the poor, young children and the elderly.^[271][272]

According to the World Health Organization, between 2030 and 2050, "climate change is expected to cause about 250,000 additional deaths per year, from malnutrition, malaria, diarrhoea and heat stress."^[273] As global temperatures increase, so does the number of heat stress, heatstroke, and cardiovascular and kidney disease deaths and illnesses.^[252] Air pollution generated by fossil fuel combustion is both a major driver of global warming and – in parallel and for comparison – the cause (or at least a substantially contributing factor) of a large number of annual deaths with some estimates as high as A review of this and a more nuanced assessment of mortality impacts in terms of contribution to death, rather than number of deceased, may be needed excess deaths during 2018.^[274][275] It may be difficult to predict or attribute deaths to anthropogenic global warming or its particular drivers as many effects – such as possibly contributing to human conflict and socioeconomic disruptions – and their mortality impacts could be highly indirect or hard to evaluate.

The effects of climate change are often interlinked and can mutually and synergistically^[276] exacerbate each other as well as existing vulnerabilities^{[277][278][279]} and other related environmental disruptions and pressures such as pollution and biodiversity loss.^[280][281]

Climate change will impact agriculture and food production around the world due to the effects of elevated CO₂ in the atmosphere; higher temperatures; altered precipitation and transpiration regimes; increased frequency of extreme events; and modified weed, pest, and pathogen pressure.^[282] Climate change is projected to negatively affect all four pillars of food security: not only how much food is available, but also how easy food is to access (prices), food quality and how stable the food system is.^[283]

As of 2019, negative impacts have been observed for some crops in low-latitudes (maize and wheat), while positive impacts of climate change have been observed in some crops in high-latitudes (maize, wheat, and sugar beets).^[284] Using different methods to project future crop yields, a consistent picture emerges of global decreases in yield. Maize and soybean decrease with any warming, whereas rice and wheat production might peak at 3 °C of warming.^[285]

In many areas, fisheries have already seen their catch decrease because of global warming and changes in biochemical cycles. In combination with overfishing, warming waters decrease the maximum catch potential.^[286] Global catch potential is projected to reduce further in 2050 by less than 4% if emissions are reduced strongly, and by about 8% for very high future emissions, with growth in the Arctic Ocean.^[287]

Climate change impacts depend strongly on projected future social and economic development. As of 2019^[update], an estimated 831 million people are undernourished.^[288] Under a high emission scenario (RCP6.0), cereals are projected to become 1–29% more expensive in 2050 depending on the socioeconomic pathway, particularly affecting low-income consumers.^[288] Compared to a no climate change scenario, this would put between 1–181 million extra people at risk of hunger.^[288]

While CO₂ is expected to be good for crop productivity at lower temperatures, it does reduce the nutritional values of crops, with for instance wheat having less protein and less of some minerals.^[289] It is difficult to project the impact of climate change on utilization (protecting food against spoilage, being healthy enough to absorb nutrients, etc.) and on volatility of food prices. Most models projecting the future do indicate that prices will become more volatile.^[290]

Droughts result in crop failures and the loss of pasture for livestock.^[291]

A number of climate-related trends have been observed that affect water resources. These include changes in precipitation, the cryosphere and surface waters (e.g., changes in river flows).^[292] Observed and projected impacts of climate change on freshwater systems and their management are mainly due to changes in temperature, sea level and precipitation variability.^[293] Changes in temperature are correlated with variability in precipitation because the water cycle is reactive to temperature.^[294] Temperature increases change precipitation patterns. Excessive precipitation leads to excessive sediment deposition, nutrient pollution, and concentration of minerals in aquifers.

The rising global temperature will cause sea level rise and will extend areas of salinization of groundwater and estuaries, resulting in a decrease in freshwater availability for humans and ecosystems in coastal areas. The rising sea level will push the salt gradient into freshwater deposits and will eventually pollute freshwater sources. The 2014 fifth IPCC assessment report concluded that:

• Water resources are projected to decrease in most dry subtropical regions and mid-latitudes, but increase in high latitudes. As streamflow becomes more variable, even regions with increased water resources can experience additional short-term shortages.^[295]
• Climate change is projected to reduce water quality before treatment. Even after conventional treatments, risks remain. The quality reduction is a consequence of higher temperatures, more intense rainfall, droughts and disruption of treatment facilities during floods.^[296]
• Droughts that stress water supply are expected to increase in southern Europe and the Mediterranean region, central Europe, central and southern North America, Central America, northeast Brazil, and southern Africa.^[297]

Humans are exposed to climate change through changing weather patterns (temperature, precipitation, sea-level rise and more frequent extreme events) and indirectly through changes in water, air and food quality and changes in ecosystems, agriculture, industry and settlements and the economy.^[298] Air pollution, wildfires, and heat waves caused by global warming have significantly affected human health,^[299] and in 2007, the World Health Organization estimated 150,000 people were being killed by climate-change-related issues every year.^[300]

A study by the World Health Organization^[301] concluded that climate change was responsible for 3% of diarrhoea, 3% of malaria, and 3.8% of dengue fever deaths worldwide in 2004. Total attributable mortality was about 0.2% of deaths in 2004; of these, 85% were child deaths. The effects of more frequent and extreme storms were excluded from this study.

A 2014 study by the World Health Organization^[302] estimated the effect of climate change on human health, but not all of the effects of climate change were included in their estimates. For example, the effects of more frequent and extreme storms were excluded. The report further assumed continued progress in health and growth. Even so, climate change was projected to cause an additional 250,000 deaths per year between 2030 and 2050.^[303]

The human impacts include both the direct effects of extreme weather, leading to injury and loss of life,^[304] as well as indirect effects, such as undernutrition brought on by crop failures or lack of access safe drinking water.^[305] For instance, having to emigrate due to an extreme weather event can lead to increased rates of physical illnesses and psychological distress.^[306] Higher levels of indoor and outdoor CO₂ levels may impair human cognition.^{[307][308][309]} Various infectious diseases are more easily transmitted in a warmer climate, such as dengue fever, which affects children most severely, and malaria. For instance, invasive mosquitoes have already become widely established across Europe and e.g. Malaria has re-emerged in Greece.^[310] A minor further effect are increases of pollen season lengths and concentrations in some regions of the world.^{[311][312][252]} Young children are the most vulnerable to food shortages, and together with older people, to extreme heat.^[313]

According to a report from the United Nations Environment Programme and International Livestock Research Institute, climate change can facilitate outbreaks of zoonoses, i.e. diseases that pass from animals to humans. One example of such outbreaks is the COVID-19 pandemic.^[314] Mitigation of climate change and related environmental issues such as deforestation could help prevent future pandemics and outbreaks.^{[315][316][317][318][319]}

It has been projected with high confidence that climate change will bring some benefits in temperate areas, such as fewer deaths from cold exposure, and some mixed effects such as changes in range and transmission potential of malaria in Africa.^[320]: 48 Benefits were projected to be outweighed by negative health effects of rising temperatures, especially in developing countries.

Warming above 1.5 degrees can make tropical regions uninhabitable, because the threshold of 35 degrees of wet bulb temperature (the limit of human adaptation to heat and humidity), will be passed. 43% of the human population live in the tropics.^[321]

A 2013 study found that significant climatic changes were associated with a higher risk of conflict worldwide, and predicted that "amplified rates of human conflict could represent a large and critical social impact of anthropogenic climate change in both low- and high-income countries."^[322] Similarly, a 2014 study found that higher temperatures were associated with a greater likelihood of violent crime, and predicted that global warming would cause millions of such crimes in the United States alone during the 21st century.^[323] Climate change can worsen conflicts by exacerbating tensions over limited resources like drinking water. Climate change has the potential to cause large population dislocations and migration, which can also lead to increased tensions and conflict.^[324][325]

However, a 2018 study in the journal Nature Climate Change found that previous studies on the relationship between climate change and conflict suffered from sampling bias and other methodological problems.^[326] Factors other than climate change are judged to be substantially more important in affecting conflict (based on expert elicitation). These factors include intergroup inequality and low socio-economic development.^[327]

Despite these issues, military planners are concerned that global warming is a "threat multiplier". "Whether it is poverty, food and water scarcity, diseases, economic instability, or threat of natural disasters, the broad range of changing climatic conditions may be far reaching. These challenges may threaten stability in much of the world".^[328] For example, the onset of the Arab Spring in 2010 was partly the result of a spike in wheat prices following crop losses from the 2010 Russian heat wave.^[329][330]

The United Nations said several times that climate change is already increasing conflicts in different regions of the world.^[331][332]

Economic forecasts of the impact of global warming vary considerably. Researchers have warned that current economic modelling may seriously underestimate the impact of potentially catastrophic climate change, and point to the need for new models that give a more accurate picture of potential damages. Nevertheless, one recent study has found that potential global economic gains if countries implement mitigation strategies to comply with the 2 °C target set at the Paris Agreement are in the vicinity of US$17 trillion per year up to 2100 compared to a very high emission scenario.^[333]

Global losses reveal rapidly rising costs due to extreme weather events since the 1970s.^[57] Socio-economic factors have contributed to the observed trend of global losses, such as population growth and increased wealth.^[334] Part of the growth is also related to regional climatic factors, e.g., changes in precipitation and flooding events. It is difficult to quantify the relative impact of socio-economic factors and climate change on the observed trend.^[335] The trend does, however, suggest increasing vulnerability of social systems to climate change.^[335]

A 2019 modelling study found that climate change had contributed towards global economic inequality. Wealthy countries in colder regions had either felt little overall economic impact from climate change, or possibly benefited, whereas poor hotter countries very likely grew less than if global warming had not occurred.^[336]

The total economic impacts from climate change are difficult to estimate, but increase for higher temperature changes.^[337] For instance, total damages are estimated to be 90% less if global warming is limited to 1.5 °C compared to 3.66 °C, a warming level chosen to represent no mitigation.^[338] One study found a 3.5% reduction in global GDP by the end of the century if warming is limited to 3 °C, excluding the potential effect of tipping points. Another study noted that global economic impact is underestimated by a factor of two to eight when tipping points are excluded from consideration.^[338] In the Oxford Economics high emission scenario, a temperature rise of 2 degrees by the year 2050 would reduce global GDP by 2.5% – 7.5%. By the year 2100 in this case, the temperature would rise by 4 degrees, which could reduce the global GDP by 30% in the worst case.^[339]

Self-reinforcing feedbacks amplify and accelerate climate change.^[340] The climate system exhibits threshold behaviour or tipping points when these feedbacks lead parts of the Earth system into a new state, such as the runaway loss of ice sheets or the destruction of too many forests.^[341][342] Tipping points are studied using data from Earth's distant past and by physical modelling.^[341] There is already moderate risk of global tipping points at 1 °C above pre-industrial temperatures, and that risk becomes high at 2.5 °C.^[343]

Tipping points are "perhaps the most 'dangerous' aspect of future climate changes", leading to irreversible impacts on society.^[344] Many tipping points are interlinked, so that triggering one may lead to a cascade of effects,^[345] even well below 2 degrees of warming.^[346] A 2018 study states that 45% of environmental problems, including those caused by climate change are interconnected and make the risk of a domino effect bigger.^[347][348]

Rainfall that falls on the Amazon rainforest is recycled when it evaporates back into the atmosphere instead of running off away from the rainforest. This water is essential for sustaining the rainforest. Due to deforestation the rainforest is losing this ability, exacerbated by climate change which brings more frequent droughts to the area. The higher frequency of droughts seen in the first two decades of the 21st century, as well as other data, signal that a tipping point from rainforest to savanna might be close. One study concluded that this ecosystem could enter a mode of a 50-years-long collapse to a savanna around 2021, after which it would become increasingly and disproportionally more difficult to prevent or reverse this shift.^{[349][350][351]}

Future melt of the West Antarctic ice sheet is potentially abrupt under a high emission scenario, as a consequence of a partial collapse.^[352] Part of the ice sheet is grounded on bedrock below sea level, making it possibly vulnerable to the self-enhancing process of marine ice sheet instability. A further hypothesis is that marine ice cliff instability would also contribute to a partial collapse, but limited evidence is available for its importance.^[353] A partial collapse of the ice sheet would lead to rapid sea level rise and a local decrease in ocean salinity. It would be irreversible on a timescale between decades and millennia.^[352]

In contrast to the West Antarctic ice sheet, melt of the Greenland ice sheet is projected to be taking place more gradually over millennia.^[352] Sustained warming between 1 °C (low confidence) and 4 °C (medium confidence) would lead to a complete loss of the ice sheet, contributing 7 m to sea levels globally.^[354] The ice loss could become irreversible due to a further self-enhancing feedback: the elevation-surface mass balance feedback. When ice melts on top of the ice sheet, the elevation drops. As air temperature is higher at lower altitude, this promotes further melt.^[355]

The Atlantic Meridional Overturning Circulation (AMOC), an important component of the Earth's climate system, is a northward flow of warm, salty water in the upper layers of the Atlantic and a southward flow of colder water in the deep Atlantic.^[357]: 5 Potential impacts associated with AMOC changes include reduced warming or (in the case of abrupt change) absolute cooling of northern high-latitude areas near Greenland and north-western Europe, an increased warming of Southern Hemisphere high-latitudes, tropical drying, as well as changes to marine ecosystems, terrestrial vegetation, oceanic CO
₂ uptake, oceanic oxygen concentrations, and shifts in fisheries.^[358]

According to a 2019 assessment in the IPCC's Special Report on the Ocean and Cryosphere in a Changing Climate it is very likely (greater than 90% probability, based on expert judgement) that the strength of the AMOC will decrease further over the course of the 21st century.^[359] Warming is still expected to occur over most of the European region downstream of the North Atlantic Current in response to increasing GHGs, as well as over North America. With medium confidence, the IPCC report stated that it is very unlikely (less than 10% probability) that the AMOC will collapse in the 21st century.^[359] The potential consequences of such a collapse could be severe.^[357]: 5

If emissions of CO₂ were to be abruptly stopped and no negative emission technologies deployed, the Earth's climate would not start moving back to its pre-industrial state. Instead, temperatures would stay elevated at the same level for several centuries. After about a thousand years, 20% to 30% of human-emitted CO₂ will remain in the atmosphere, not taken up by the ocean or the land, committing the climate to warming long after emissions have stopped.^[360] Pathways that keep global warming under 1.5 °C often rely on large-scale removal of CO₂, which feasibility is uncertain and has clear risks.^[361]

There are a number of examples of climate change impacts on the environment that may be irreversible, at least over the timescale of many human generations.^[362] These include the large-scale singularities such as the melting of the Greenland and West Antarctic ice sheets, and changes to the AMOC.^[362] In biological systems, the extinction of species would be an irreversible impact.^[362] In social systems, unique cultures may be lost due to climate change.^[362] For example, humans living on atoll islands face risks due to sea level rise, sea surface warming, and increased frequency and intensity of extreme weather events.^[363]

When the global temperature changes, the changes in climate are not expected to be uniform across the Earth. In particular, land areas change more quickly than oceans, and northern high latitudes change more quickly than the tropics, and the margins of biome regions change faster than do their cores.

Regional effects of climate change vary in nature. Some are the result of a generalised global change, such as rising temperature, resulting in local effects, such as melting ice. In other cases, a change may be related to a change in a particular ocean current or weather system. In such cases, the regional effect may be disproportionate and will not necessarily follow the global trend.

There are three major ways in which global warming will make changes to regional climate: melting or forming ice, changing the hydrological cycle (of evaporation and precipitation) and changing currents in the oceans and air flows in the atmosphere. The coast can also be considered a region, and will suffer severe impacts from sea level rise. The global sea level is predicted to keep rising for several centuries.^[365]

Developed countries are also vulnerable to climate change. For example, developed countries will be negatively affected by increases in the severity and frequency of some extreme weather events, such as heat waves.^[366] In 2021, observers noted that climate change is now obviously a problem to rich countries due to immediate issues of climate change exacerbated natural disasters.^[367][368]

Projections of climate changes at the regional scale do not hold as high a level of scientific confidence as projections made at the global scale.^[369] It is, however, expected that future warming will follow a similar geographical pattern to that seen already, with the greatest warming over land and high northern latitudes, and least over the Southern Ocean and parts of the North Atlantic Ocean.^{[370][needs update]} Land areas warm faster than ocean, and this feature is even stronger for extreme temperatures. For hot extremes, regions with the most warming include Central and Southern Europe and Western and Central Asia.^[371]

The Arctic, Africa, small islands, Asian megadeltas and the Middle East are regions that are likely to be especially affected by climate change.^[372][373] Low-latitude, less-developed regions are at most risk of experiencing negative impacts due to climate change.^[271]

The ten countries of the Association of Southeast Asian Nations (ASEAN) are among the most vulnerable in the world to the negative effects of climate change, however, ASEAN's climate mitigation efforts are not commensurate with the climate change threats the region faces.^[374] Africa is one of the most vulnerable continents to climate variability and change because of multiple existing stresses and low adaptive capacity. Climate change is projected to decrease freshwater availability in central, south, east and southeast Asia, particularly in large river basins. With population growth and increasing demand from higher standards of living, this decrease could adversely affect more than a billion people by the 2050s. Small islands, whether located in the tropics or higher latitudes, are already exposed to extreme weather events and changes in sea level. This existing exposure will likely make these areas sensitive to the effects of climate change.

Global sea level is currently rising due to the thermal expansion of water in the oceans and the addition of water from ice sheets.^[365] Because of this, there low-lying coastal areas, many of which are heavily populated, are at risk of flooding.

Areas threatened by current sea level rise include Tuvalu and the Maldives.^[375] Regions that are prone to storm surges, such as London, are also threatened.

In 2007 it was projected with very high confidence that by the 2080s, many millions more people would experience floods every year due to sea level rise.^[376] The numbers affected were projected to be largest in the densely populated and low-lying megadeltas of Asia and Africa. Small islands were judged to be especially vulnerable.

In the northern hemisphere, the southern part of the Arctic region (home to 4,000,000 people) has experienced a temperature rise of 1 °C to 3 °C (1.8 °F to 5.4 °F) over the last 50 years.^[377] Canada, Alaska and Russia are experiencing initial melting of permafrost. This may disrupt ecosystems and by increasing bacterial activity in the soil lead to these areas becoming carbon sources instead of carbon sinks.^[378] A study (published in Science) of changes to eastern Siberia's permafrost suggests that it is gradually disappearing in the southern regions, leading to the loss of nearly 11% of Siberia's nearly 11,000 lakes since 1971.^[379] At the same time, western Siberia is at the initial stage where melting permafrost is creating new lakes, which will eventually start disappearing as in the east. Furthermore, permafrost melting will eventually cause methane release from melting permafrost peat bogs.

Anisimov et al. (2007) assessed the literature on impacts of climate change in Polar regions.^[380] Model projections showed that Arctic terrestrial ecosystems and the active layer (the top layer of soil or rock in permafrost that is subjected to seasonal freezing and thawing) would be a small sink for carbon (i.e., net uptake of carbon) over this century (p. 662). These projections were viewed as being uncertain. It was judged that increased emissions of carbon from thawing of permafrost could occur. This would lead to an amplification of warming.

Arctic sea ice extent and area have declined every decade since the start of start of satellite observations in 1979: Greenland ice sheet had experienced a "massive melting event" in 2012, which reoccurred in 2019 and 2021; Satellite image of the extremely anomalous 2020 Siberian heatwave; Permafrost thaw is leading to severe erosion, like in this coastal location in Alaska

Due to climate change in the Arctic, this polar region is expected to become "profoundly different" by 2050.^{[381]: 2321} The speed of change is "among the highest in the world",^{[381]: 2321} with the rate of warming being 3-4 times faster than the global average.^{[382][383][384][385]} This warming has already resulted in the profound Arctic sea ice decline, the accelerating melting of the Greenland ice sheet and the thawing of the permafrost landscape.^{[381]: 2321 [386]} These ongoing transformations are expected to be irreversible for centuries or even millennia.^{[381]: 2321}

Small islands developing states are especially vulnerable to the effects of climate change. Harsh and extreme weather conditions is a part of everyday life however as the climate changes these small islands find it difficult to adapt to the rising scale and intensity of storm surges, salt water intrusion and coastal destruction.^[393]

The projected damage to small islands and atoll communities will be a consequence of climate change caused by developing countries that will disproportionately affect these developing nations.^[394] Sea-level rise and increased tropical cyclones are expected to place low-lying small islands in the Pacific, Indian, and Caribbean regions at risk of inundation and population displacement.^{[394][395][396]}

According to a study on the vulnerability of island countries in the South Pacific to sea level rise and climate change, financially burdened island populations living in the lowest-lying regions are most vulnerable to risks of inundation and displacement.^[396] On the islands of Fiji, Tonga and western Samoa for example, high concentrations of migrants that have moved from outer islands inhabit low and unsafe areas along the coasts.^[396]

Atoll nations, which include countries that are composed entirely of the smallest form of islands, called motus, are at risk of entire population displacement.^[397][394] These nations include Kiribati, Maldives, the Marshall Islands, Tokelau, and Tuvalu.^[394][395] According to a study on climate dangers to atoll countries, characteristics of atoll islands that make them vulnerable to sea level rise and other climate change impacts include their small size, their isolation from other land, their low income resources, and their lack of protective infrastructure.^[394]

A study that engaged the experiences of residents in atoll communities found that the cultural identities of these populations are strongly tied to these lands.^[398] The risk of losing these lands therefore threatens the national sovereignty, or right to self-determination, of Atoll nations.^[394] Human rights activists argue that the potential loss of entire atoll countries, and consequently the loss of cultures and indigenous lifeways cannot be compensated with financial means.^[397][394] Some researchers suggest that the focus of international dialogues on these issues should shift from ways to relocate entire communities to strategies that instead allow for these communities to remain on their lands.^[397][398]

The region of Middle East is one of the most vulnerable to climate change. The impacts include increase in drought conditions, aridity, heatwaves, sea level rise. If greenhouse gas emissions are not reduced, the region can become uninhabitable before the year 2100.^[406][407]

• Anthropocene
• Effects of climate change on the water cycle
• Global catastrophic risk
• Intertropical Convergence Zone (ITCZ)
• Temperature-dependent sex determination