Last updated on: 2/19/2021 | Author:

History of Climate Change Debate

Click for an Encyclopaedia Britannica video about the impact of greenhouse-gas emissions on Earth’s temperature

Average surface temperatures on earth have risen more than 2°F over the past 100 years. [205] During this time period, atmospheric levels of greenhouse gases such as carbon dioxide (CO2) and methane (CH4) have notably increased. [10][11] This site explores the debate on whether climate change is caused by humans (also known as anthropogenic climate change).

The pro side argues rising levels of atmospheric greenhouse gases are a direct result of human activities such as burning fossil fuels, and that these increases are causing significant and increasingly severe climate changes including global warming, loss of sea ice, sea level rise, stronger storms, and more droughts. They contend that immediate international action to reduce greenhouse gas emissions is necessary to prevent dire climate changes.

The con side argues human-generated greenhouse gas emissions are too small to substantially change the earth’s climate and that the planet is capable of absorbing those increases. They contend that warming over the 20th century resulted primarily from natural processes such as fluctuations in the sun’s heat and ocean currents. They say the theory of human-caused global climate change is based on questionable measurements, faulty climate models, and misleading science.

Early Science on Greenhouse Gasses and Climate Change

Carbon dioxide (CO2) is released and absorbed in the global carbon cycle.
Source: United States Department of Energy “Simplified Global Carbon Cycle,” (accessed June 2, 2010)

Scientists have known of the heating potential (greenhouse effect) of gases such as CO2 since at least 1859, when Irish physicist John Tyndall first began experiments leading to the discovery that CO2 in the atmosphere absorbs the sun’s heat. [126]

On Feb. 16, 1938, engineer Guy S. Callendar published an influential study suggesting increased atmospheric CO2 from fossil fuel combustion was causing global warming. [127] Many scientists at that time were skeptical of Callendar’s conclusion, arguing that that natural fluctuations and atmospheric circulation changes determined the climate, not CO2 emissions. [158]

In Mar. 1958, US climate scientist Charles Keeling began measuring atmospheric CO2 at the Mauna Loa observatory in Hawaii for use in climate modeling. [128] Using these measurements, Keeling became the first scientist to confirm that atmospheric CO2 levels were rising rather than being fully absorbed by forests and oceans (carbon sinks). [129] When Keeling began his measurements, atmospheric CO2 levels stood at 315 parts per million (ppm). [10]

The US National Academy of Sciences issued a 1977 report titled “Energy and Climate” that concluded the burning of fossil fuels was increasing atmospheric CO2, and that increased CO2 was associated with a rise in global temperatures. [130]

Image of protesters at the Sep. 20, 2019 Climate Strike in Sydney, Australia. Photo by Jenny Evans/Getty.
Source: Laura Parker, “Listen and Help Us: Kids Worldwide Are on Strike for the Climate,”, Sep. 22, 2019

On June 23, 1988 National Aeronautics and Space Administration (NASA) scientist James Hansen presented testimony to the US Senate stating that increases in CO2 were warming the planet and “changing our climate.” [131][132] At the time, MIT meteorologist Richard Lindzen criticized these findings, arguing that computerized climate models were unreliable and that natural processes would balance out any warming caused by increased CO2. [133]

Formation of the IPCC and the UN Framework Convention on Climate Change

The Intergovernmental Panel on Climate Change (IPCC) was created in 1988 by the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP) to review research on global climate change (as of Mar. 2020, there were 195 IPCC member countries). [136] The IPCC issued its first assessment report in 1990 stating that “emissions resulting from human activities are substantially increasing the atmospheric concentrations of the greenhouse gases,” resulting in “an additional warming of the Earth’s surface.” [135]

The United Nations Framework Convention on Climate Change (UNFCCC) was signed by US President George H.W. Bush on Oct. 13, 1992. [137] The goal of the convention was the “stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system.” [138] The UNFCCC became the parent treaty for the 1997 Kyoto Protocol and the 2015 Paris Agreement. [159]

Cartoon satirizing Vice President Al Gore and his views on global warming.
Source: (accessed June 11, 2010)

Kyoto Protocol, Paris Agreement, and Other International Conferences on Climate Change

Over 161 nations met in Kyoto, Japan, in Dec. 1997 to negotiate a treaty to limit greenhouse gas emissions and work toward the objectives of the UNFCCC. The resulting Kyoto Protocol, [139] signed by President Bill Clinton, set binding targets for 37 industrialized countries and the European Union to reduce greenhouse gas emissions roughly 5% below 1990 levels by 2012. [140]

President George W. Bush withdrew the United States from the Kyoto Protocol in Mar. 2001 due to Senate opposition and concerns that limiting greenhouse gas emissions would harm the US economy. From July 16-27, 2001 the COP 6 conference (conference of signatory parties to the UNFCCC) took place in Bonn, Germany, and the final amendments to the Kyoto Protocol were made. 179 countries reached a binding agreement without US participation. [141]

Click for an Encyclopaedia Britannica video about the effects of ocean temperatures on penguins.

On Mar. 2, 2008 the Heartland Institute sought to challenge the idea that human activity was causing climate change by holding its own international conference on climate change. At the conference, 98 speakers including PhD climate scientists from major universities argued that global warming was most likely a natural event. [148]

In Dec. 2009 the COP 15 conference took place in Copenhagen, Denmark. The resulting Copenhagen Accord, signed by 114 nations including the United States and China, called for “deep cuts” in human greenhouse gas emissions in order to make sure that earth’s temperature rises no more than 1.5°C above pre-industrial levels. [142]

In Dec. 2015, the COP 21 met in Paris where 195 countries, including the United States, adopted the Paris Agreement. [176][178] The agreement’s central aim was to prevent global temperatures from rising more than 1.5°C – 2°C above pre-industrial levels. Under the agreement, all countries were required to create a national plan to reduce greenhouse gas emissions and report regularly on their individual progress towards meeting their emission reduction goals. [177] President Obama, still in office at the time, called the agreement a “turning point for the world” that “establishes the enduring framework the world needs to solve the climate crisis.” [179]

On June 1, 2017, President Trump announced his intention to withdraw the United States from the Paris Agreement and ordered the federal government to “cease all implementation” of the agreement. President Trump said the Paris Agreement had imposed “draconian financial and economic burdens” on the United States and created “serious obstacles” to energy development. [180][181] On Nov. 7, 2017, during the COP 23 UN climate talks in Bonn, Germany, Syria announced that it would sign the Paris agreement on climate change, leaving the United States as the only country that has rejected the global pact. [182] The United States officially left the Paris Climate Agreement on Nov. 4, 2020. [207]

On inauguration day (Jan. 20, 2021), President Joe Biden released a statement rejoining the Paris Climate Agreement. The United States officially rejoined the agreement 30 days later on Feb. 19, 2021. [208] [209] [210]

Graph showing that rising CO2 levels correlate with higher global temperatures.
Source: National Oceanic and Atmospheric Administration, “Global Climate Change Indicators,” (accessed Mar 12, 2015)

US Debate over Climate Change Heats Up

Al Gore’s documentary An Inconvenient Truth premiered in 2006 and was seen by over 5 million people worldwide. The film warned that human-caused climate change was real, and that without immediate reductions in greenhouse gas emissions, catastrophic climate changes would severely disrupt human societies, leading to a possible collapse of industrial civilization. [145]

A IPCC assessment report stated that climate change was accelerating, which could lead to more war and conflict around the world, and the report called for urgent counter-measures to be implemented. [170] The IPCC and Al Gore jointly received the 2007 Noble Peace Prize “for their efforts to build up and disseminate greater knowledge about man-made climate change.” [146] In response to the IPCC findings, a group of scientists formed the Nongovernmental International Panel on Climate Change (NIPCC) to compile a report challenging the science behind man-made climate change. Their Mar. 2, 2008 report, “Nature, Not Human Activity, Rules the Climate,” was published by the Heartland Institute. [147]

Between 1998 and 2009, the United States allocated $99 billion to federal agencies for work related to climate change. During that period there was a big uptick in climate-related technology development while spending on climate science remained about the same. [151]

On Apr. 2, 2007, the US Supreme Court ruled (5-4) in Massachusetts v. EPA that greenhouse gases met the criteria to be considered pollutants under the Clean Air Act. [149] In response, the US EPA announced in 2009 that greenhouse gases “threaten public health” and are “the primary driver of climate change.” [150] In its June 23, 2014 decision in Utility Air Regulatory Group v. EPA, the US Supreme Court upheld the EPA’s authority to regulate greenhouse gas emissions from stationary sources such as power plants. [96]

Graph showing that Arctic air temperature (blue line) parallels natural solar activity (red line).
Source: Oregon Institute of Science and Medicine, “Environmental Effects of Increased Atmospheric Carbon Dioxide,” Journal of American Physicians and Surgeons, Fall 2007

On Sep. 21, 2014 the largest climate march to date took place in New York, NY, as over 400,000 people marched to demand that world governments take immediate action to reduce greenhouse gas emissions. [161] In Mar. 2019, as many as 1.4 million people worldwide participated in a school walk out to bring attention to climate change. [196] The student movement started by Swedish climate activist Greta Thunberg continued on Sep. 20, 2019 with an estimated four million demonstrators in at least 160 countries calling for action on climate change, an event that is thought to be the largest climate protest in history to that point. [197]

The Obama Administration enacted the strictest passenger vehicle fuel efficiency standards in US history as part of a plan to address climate change. The CO2 standards set in 2012 required an annual 5% increase in fuel efficiency to reach 54.5 miles per gallon by 2025. [200] On Mar. 31, 2020, the Trump Administration lowered the requirement to a 1.5% increase each year towards a goal of 40 miles per gallon on average by 2026. [201] An analysis by Rhodium Group predicted the lowered standards would result in about 20% of the reduction in greenhouse gas emissions that were expected under the Obama-era standards. [202]

How Will Climate Change Affect Us?

According to NOAA’s National Climatic Data Center, 2014 was the hottest year on record across the globe since 1880 when record keeping began. [94] The following five years were even warmer as 2016 set the record for hottest year ever and global average temperatures in 2019 were the second-hottest as of Mar. 2020. [198]

In 2019, CO2 levels were 415.3 ppm, up from 315.7 ppm when measurements began in 1958. [194] [10] These CO2 levels are reportedly higher than at any time in the last 650,000 years when levels fluctuated between 180 and 300 ppm. [102]

The Unites States makes up about 4% of the world’s population but was responsible for nearly one-third of historical global greenhouse gas emissions. [103] In 2018, global emissions of human-produced CO2 were about 37 billion tons. [199]

The greenhouse effect illustrated.
Source: US Environmental Protection Agency (EPA), “Frequently Asked Questions about Global Warming and Climate Change: Back to Basics,” (accessed Mar. 12, 2015)

Predictions about how climate changes would affect civilization ranged from a Department of Defense report [154] detailing catastrophic weather events and a “significant drop in the human carrying capacity of the Earth’s environment,” to an Oregon Institute of Science and Health report detailing “an increasingly lush environment of plants and animals.” [155]

The question of how climate change impacts extreme weather came to the forefront of public debate when wildfires raged across Australia for 240 days from 2019 through early 2020. A World Weather Attribution study found that climate change increased the likelihood of wildfires such as those in Australia by at least 30% since 1900. [203] William Reville, emeritus professor of biochemistry at University College Cork, noted that other factors also contributed to the fires, such as failing to clear undergrowth and leaves that fuel the fires, a shortage of skilled firefighters, population density, and arson. [204]

Ongoing IPCC Findings, National Climate Assessment, and Counterpoints

On Sep. 27, 2013 the IPCC announced that it is now “extremely likely [95% confidence] that human influence has been the dominant cause of the observed warming since the mid-20th century.” [156]

The Heartland Institute argued against human-caused global warming in its 2013 NIPCC report which said that global warming since 1860 is the result of natural “cycles driven by ocean-atmosphere oscillations, or by solar variations.” [67]

The US Global Change Research Program released the 2014 National Climate Assessment on May 6, 2014. The report called climate change “a global public health problem,” stated that climate change impacts are already “visible in every state,” and concluded that human-induced “climate change is happening now.” [16] The report was criticized by some members of Congress, including US Senator James Inhofe (R-OK), who stated that “we can all agree that natural variations in the climate are taking place, but man-made global warming still remains a theory.” [157]

In Nov. 2018, Volume II of the 4th National Climate Assessment was published. It concluded, in part, that “rising temperatures, extreme heat, drought, wildfire on rangelands, and heavy downpours” are expected to increase and that “[w]ithout adaptation, climate change will continue to degrade infrastructure performance over the rest of the century, with the potential for cascading impacts that threaten our economy, national security, essential services, and health and well-being.” [186] The Trump administration criticized the report, stating that “it’s not based on facts… It’s not data-driven. We’d like to see something that is more data-driven. It’s based on modeling, which is extremely hard to do when you’re talking about the climate.” [187]

US Public Opinion

A Jan. 22, 2019 report from the Yale Program on Climate Change Communication found that 73% of Americans think global warming is occurring, marking a ten percentage point increase over Mar. 2015; meanwhile, 14% of Americans deny climate change is happening. Six in ten surveyed (62%) believe that global warming is being caused by humans, while 23% attribute it to “natural changes in the environment.” [192]

The group’s 2018 report showed that 95% of liberal Democrats think global warming is happening and 84% think it is caused by humans. On the other end of the ideological spectrum, 40% of Republicans think global warming is happening and 26% think it is caused by humans. [193]

A 2017 Gallup poll found that 68% of Americans thought global warming was caused by human activity, up from 50% in 2010 and 61% in 2001, while 29% thought it was caused by natural causes, down from 46% in 2010 and 33% in 2001. [190]

A 2018 Pew Research Center poll found that 18% of Republicans in the Baby Boomer generation thought that “the earth is warming mostly due to human activity,” compared to 36% of millennial Republicans and 75% of all Democrats. [188] A July/Aug. 2019 Washington Post-Kaiser Family Foundation survey found that 86% of teenagers believe human activity is causing climate change, compared to 79% of adults. [195]