A global warming conspiracy theory invokes claims that the scientific consensus on global warming is based on conspiracies to produce manipulated data or suppress dissent. It is one of a number of tactics used in climate change denial to attempt to legitimize political and public controversy disputing this consensus. Conspiracy theorists typically allege that, through worldwide acts of professional and criminal misconduct, the science behind global warming has been invented or distorted for ideological or financial reasons.
As stated by the Intergovernmental Panel on Climate Change (IPCC), the largest contributor to global warming is the increase in atmospheric carbon dioxide (CO2) since 1750, particularly from fossil fuel combustion, cement production, and land use changes such as deforestation. The IPCC's Fifth Assessment Report (AR5) states:
Human influence has been detected in warming of the atmosphere and the ocean, in changes in the global water cycle, in reductions in snow and ice, in global mean sea level rise, and in changes in some climate extremes. This evidence for human influence has grown since AR4. It is extremely likely (95–100%) that human influence has been the dominant cause of the observed warming since the mid-20th century
The evidence for global warming due to human influence has been recognized by the national science academies of all the major industrialized countries. No scientific body of national or international standing maintains a formal opinion dissenting from the summary conclusions of the IPCC.
Despite this scientific consensus on climate change, allegations have been made that scientists and institutions involved in global warming research are part of a global scientific conspiracy or engaged in a manipulative hoax. There have been allegations of malpractice, most notably in the Climatic Research Unit email controversy ("ClimateGate"). Eight committees investigated these allegations and published reports, each finding no evidence of fraud or scientific misconduct. The Muir Russell report stated that the scientists' "rigor and honesty as scientists are not in doubt," that the investigators "did not find any evidence of behavior that might undermine the conclusions of the IPCC assessments," but that there had been "a consistent pattern of failing to display the proper degree of openness." The scientific consensus that global warming is occurring as a result of human activity remained unchanged at the end of the investigations.
Climate change conspiracy theories have resulted in poor action or no action at all to effectively mitigate the damage done by global warming. In some countries like the United States of America, 40% of Americans believe that climate change is a hoax  in spite of the fact that there is a 100% consensus among climate scientists that it is not according to a report in 2019.  President Donald Trump previously even pulled the United States out of the Paris Agreement, which was set up in the hopes of reducing global warming. There may be an ideology of climate change denial in some regions of the world, which would lead to disagreements over how to handle climate change and what should be done in the face of it.
Steve Connor links the terms "hoax" and "conspiracy," saying, "Reading through the technical summary of this draft (IPCC) report, it is clear that no one could go away with the impression that climate change is some conspiratorial hoax by the science establishment, as some would have us believe."
The documentary The Great Global Warming Swindle received criticism from several experts. George Monbiot described it as "the same old conspiracy theory that we’ve been hearing from the denial industry for the past ten years". Similarly, in response to James Delingpole, Monbiot stated that his Spectator article was "the usual conspiracy theories [...] working to suppress the truth, which presumably now includes virtually the entire scientific community and everyone from Shell to Greenpeace and The Sun to Science." Some Australian meteorologists also weighed in, saying that the film made no attempt to offer a "critical deconstruction of climate science orthodoxies", but instead used various other means to suggest that climate scientists are guilty of lying or are seriously misguided. Although the film's publicist's asserted that "global warming is 'the biggest scam of modern times'", these meteorologists concluded that the film was "not scientifically sound and presents a flawed and very misleading interpretation of the science".
Former UK Secretary of State for Environment, Food and Rural Affairs David Miliband presented a rebuttal of the main points of the film and stated "There will always be people with conspiracy theories trying to do down the scientific consensus, and that is part of scientific and democratic debate, but the science of climate change looks like fact to me."
National Geographic fact-checked 6 persistent scientific conspiracy theories. Regarding the persistent belief in a global warming hoax they note that the Earth is continuing to warm and the rate of warming is increasing as documented in numerous scientific studies. The rise in global temperature and its rate of increase coincides with the rise of greenhouse gases in the atmosphere due to human activity. Moreover, global warming is causing Arctic sea ice to thaw at historic rates, many species of plants are blooming earlier than expected, and the migration routes of many birds, fish, mammals, and insects are changing.
There is evidence that some of those alleging such conspiracies are part of well-funded misinformation campaigns designed to manufacture controversy, undermine the scientific consensus on climate change and downplay the projected effects of global warming. Individuals and organisations kept the global warming debate alive long after most scientists had reached their conclusions. These doubts have influenced policymakers in both Canada and the US, and have helped to form government policies.
Since the late 1980s, this well-coordinated, well-funded campaign by contrarian scientists, free-market think tanks and industry has created a paralyzing fog of doubt around climate change. Through advertisements, op-eds, lobbying and media attention, greenhouse doubters (they hate being called deniers) argued first that the world is not warming; measurements indicating otherwise are flawed, they said. Then they claimed that any warming is natural, not caused by human activities. Now they contend that the looming warming will be minuscule and harmless. "They patterned what they did after the tobacco industry," says former senator Tim Wirth, who spearheaded environmental issues as an under secretary of State in the Clinton administration. "Both figured, sow enough doubt, call the science uncertain and in dispute. That's had a huge impact on both the public and Congress."
Greenpeace presented evidence of the energy industry funding climate change denial in their 'Exxon Secrets' project. An analysis conducted by The Carbon Brief in 2011 found that 9 out of 10 of the most prolific authors who cast doubt on climate change or speak against it had ties to ExxonMobil. Greenpeace have said that Koch industries invested more than US$50 million in the past 50 years on spreading doubts about climate change. ExxonMobil announced in 2008 that it would cut its funding to many of the groups that "divert attention" from the need to find new sources of clean energy, although in 2008 still funded over "two dozen other organisations who question the science of global warming or attack policies to solve the crisis." A survey carried out by the UK Royal Society found that in 2005 ExxonMobil distributed US$2.9 million to 39 groups that "misrepresented the science of climate change by outright denial of the evidence".
The novel State of Fear by Michael Crichton, published in December 2004, describes a conspiracy by scientists and others to create public panic about global warming. The novel includes 20 pages of footnotes, described by Crichton as providing a factual basis for the non-plotline elements of the story. In a Senate speech on 4 January 2005, Inhofe mistakenly described Crichton as a "scientist", and said the book's fictional depiction of environmental organizations primarily "focused on raising money, principally by scaring potential contributors with bogus scientific claims and predictions of a global apocalypse" was an example of "art imitating life."
In a piece headed Crichton's conspiracy theory, Harold Evans described Crichton's theory as being "in the paranoid political style identified by the renowned historian Richard Hofstadter," and went on to suggest that "if you happen to be in the market for a conspiracy theory today, there's a rather more credible one documented by the pressure group Greenpeace," namely the funding by ExxonMobil of groups opposed to the theory of global warming.
(p1) ... there is a strong, credible body of evidence, based on multiple lines of research, documenting that climate is changing and that these changes are in large part caused by human activities. While much remains to be learned, the core phenomenon, scientific questions, and hypotheses have been examined and found wanting, lacking consistent support in the face of serious scientific debate and careful evaluation of alternative explanations. * * * (p21-22) Some scientific conclusions or theories have been so thoroughly examined and tested, and supported by so many independent observations and results, that their likelihood of subsequently being found to be wrong is vanishingly small. Such conclusions and theories are then regarded as settled facts. This is the case for the conclusions that the Earth system is warming and that much of this warming is very likely due to human activities.
It is likely that most of the warming in recent decades can be attributed to human activities (IPCC 2001). This warming has already led to changes in the Earth's climate.
The AAPG stands alone among scientific societies in its denial of human-induced effects on global warming.