There is scientific consensus that the Earth has been consistently warming since the start of the Industrial Revolution, that the rate of recent warming is largely unprecedented, and that this warming is mainly the result of a rapid increase in atmospheric carbon dioxide (CO2) caused by human activities. The human activities causing this warming include fossil fuel combustion, cement production, and land use changes such as deforestation,"Total radiative forcing is positive and has led to an uptake of energy by the climate system. The largest contribution to total radiative forcing is caused by the increase in the atmospheric concentration of CO2 since 1750." and "From 1750 to 2011, CO2 emissions from fossil fuel combustion and cement production have released 375 [345 to 405] GtC to the atmosphere, while deforestation and other land-use change are estimated to have released 180 [100 to 260] GtC." In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. with a significant supporting role from the other greenhouse gases such as methane and nitrous oxide. This human role in climate change is considered "unequivocal" and "incontrovertible".

Nearly all actively publishing climate scientists say humans are causing climate change. Surveys of the scientific literature are another way to measure scientific consensus. A 2019 review of scientific papers found the consensus on the cause of climate change to be at 100%, and a 2021 study concluded that over 99% of scientific papers agree on the human cause of climate change. The small percentage of papers that disagreed with the consensus often contained errors or could not be replicated.

The evidence for global warming due to human influence has been recognized by the national science academies of all the major industrialized countries. In the scientific literature, there is a very strong consensus that global surface temperatures have increased in recent decades and that the trend is caused by human-induced emissions of greenhouse gases. No scientific body of national or international standing disagrees with this view. A few organizations with members in extractive industries hold non-committal positions, and some have tried to persuade the public that climate change is not happening, or if the climate is changing it is not because of human influence, attempting to sow doubt in the scientific consensus.

Existence of a scientific consensus

Studies of the scientific opinion on climate change have been undertaken since the 1970s, and they have been establishing widespread consensus since the 1990s, with the level of agreement increasing over time. Individual scientists, universities, and laboratories contribute to the scientific opinion on climate change via their peer-reviewed publications, while the scientific bodies of national or international standing summarise the areas of collective agreement and relative certainty in synthesis reports.

Examples of such reports include or the 2004 Arctic Climate Impact Assessment from the International Arctic Science Committee and the governments of the Arctic Council, or the United States' National Climate Assessment, which has been released periodically since 2000 under the auspices of the United States Global Change Research Program. The fourth NCA, released in 2017, involved the efforts of thirteen federal agencies, led by the National Oceanic and Atmospheric Administration (NOAA), and around "1,000 people, including 300 leading scientists, roughly half from outside the government."

The Intergovernmental Panel on Climate Change (IPCC) had been formed by the United Nations in 1988, and it presents reports summarizing the strength and extent of consensus on climate change and its numerous aspects to the member states of the United Nations, with the major reports released at 5-to-7-year intervals starting from 1990.

In 2001, science academies from 17 countries (Australia, Belgium, Brazil, Canada, China, France, Germany, India, Indonesia, Ireland, Italy, Malaysia, New Zealand, Sweden, Trinidad, Turkey and the United Kingdom) made a joint statement endorsing the work of IPCC. They concurred that the temperatures are rising and will continue to rise due to human activities, and also stressed the importance of cutting greenhouse gas emissions, concluding that "Business as usual is no longer a viable option". It is also notable for being one of the first statements to explicitly use the term "consensus". In 2005, another joint statement from the science academies of major countries (Brazil, Canada, China, France, Germany, India, Italy, Japan, Russia, United Kingdom and the United States) referred to the conclusions of the IPCC as "the international scientific consensus", and urged prompt action on both climate change mitigation and climate change adaptation. Elsewhere around the world, other organizations to have referred to the scientific consensus include Network of African Science Academies in 2007, and the International Union for Quaternary Research in 2008.

In 2013, a study which found that out of over 4,000 peer-reviewed papers on climate science published since 1990, 97% agree, explicitly or implicitly, that global warming is happening and is human-caused. Surveys of scientists' views on climate change – with a focus on human caused climate change – have been undertaken since the 1970s. A 2016 reanalysis confirmed that "the finding of 97% consensus [that humans are causing recent global warming] in published climate research is robust and consistent with other surveys of climate scientists and peer-reviewed studies."048002 A 2019 study found scientific consensus to be at 100%, and a 2021 study found that consensus exceeded 99%.

Consensus points

The scientific consensus regarding causes and mechanisms of climate change, its effects and what should be done about it (climate action) is that:

It is "unequivocal" and "incontrovertible" that the greenhouse gas emissions from human activities have caused warming on land, in oceans and in the troposphere. There are no natural processes which can provide an alternate explanation. The atmospheric levels of carbon dioxide are the highest they have been in at least 2 million years, if not 3.2 million years. The atmospheric levels of two other major greenhouse gases, methane and nitrous oxide, are the highest they have been in at least the past 800,000 years. The record of the past 800,000 years also shows that the increases in their concentrations seen since 1750 would take millennia to be caused by natural processes. The decade of 2010s has been warmer than the late 19th century, and the warmest since the start of a consistent instrumental temperature record. The warming of the past 50 years has occurred faster than any other warming over the past 2,000 years, if not longer. Precipitation appears to have been increasing since 1950, but the rainfall patterns have also been shifting, and there is more evidence for increases in heavy precipitation which causes flash floods. Global sea level has increased by since 1900, with half of that increase occurring since 1980. This sea level rise has been the fastest in "at least the last 3000 years", which is very likely to have been caused by human activity. As the recent warming heats the ocean, its water expands in volume. This causes half of the recent sea level rise, with the rest due to the warming melting the ice sheets and glaciers. While there have always been severe and extreme weather events (e.g. tropical cyclones, thunderstorms, tornados, droughts, heat waves, precipitation extremes), climate change has made many of them more severe, more frequent, or more likely to co-occur, in every part of the globe. The dangers of extreme weather events will continue increasing unless there is a rapid decrease in greenhouse gas emissions needed to curb further warming. Increased warming will lead to worse impacts. The extent of human-caused emissions will be the main cause of future warming.

Statements by major scientific organizations about climate change

Many of the major scientific organizations about climate change have issued formal statements of opinion. The vast majority of these statements concur with the IPCC view, some very few are non-committal, or dissent from it. The California Governor's Office website lists nearly 200 worldwide scientific organizations who hold the position that climate change has been caused by human action.

Surveys of scientists' views on climate change

1970s

In 1978, the National Defense University of the United States had surveyed 24 experts about the near-term climate change and its effects on agriculture. The majority of respondents had expected some warming to occur between 1970 and 2000, and described human emissions of carbon dioxide as the primary cause, but there was a disagreement on the extent, and a few had thought that an increase in volcanic activity would offset carbon dioxide emissions by elevating atmospheric sulfate concentrations (which have a reflective effect, also associated with global dimming, and with some solar geoengineering proposals) and result in overall cooling. When NDU had combined their predictions, they estimated a 10% likelihood of large (~) cooling occurring by 2000, a 25% likelihood of smaller cooling around , a 30% likelihood of limited change, with around warming, a 25% likelihood of "moderate" warming of ~, and a 10% likelihood of large warming of around . Subsequently, about had occurred between 1950 and 2000, with about since 1970, largely matching the survey's "moderate global warming" scenario.

1980s In 1989, David H. Slade had surveyed 21 climate scientists, of whom 17 had expressed "a strong belief" in "the reality of a significant climate change".

1990s In March 1990, Cutter Information Corporation (now known as Cutter Consortium) sent questionnaires to 1,500 researchers who were on the attendance lists of climate change conferences, and received 331 responses from 41 countries. The survey revealed widespread agreement that global warming is already happening, that it will result in negative impacts such as sea level rise, and that reducing carbon dioxide emissions and halting deforestation is an appropriate response to it. Only 1.9% of respondents predicted that there would be an overall cooling across the next 100 years. There was more disagreement on the strength of future warming: i.e. around 30% believed that there was a less than 50% chance that the warming would reach or exceed over the next 100 years, while a larger fraction (almost 40%) thought such temperatures were at least 75% likely.