There is strong scientific evidence that the Earth's climate is changing as a result of human activity. Emissions of greenhouse gases, combined with human destruction of ecosystems that help remove greenhouse gases, is driving a sharp rise in greenhouse gas levels. This rise is creating an upward trend in global warming that can be distinguished clearly from Earth's natural cycles.
According to the contribution of the Working Group I to the Fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate Change (IPCC), a report compiled by leading scientists and released in 2014, "atmospheric concentrations of the greenhouse gases carbon dioxide, methane, and nitrous oxide have all increased since 1750 due to human activity." In 2011 the concentrations of these greenhouse gases exceeded pre-industrial levels by about 40 percent, 150 percent, and 20 percent, respectively, reaching the highest levels in at least 800,000 years. The World Meteorological Organization (WMO) reported that atmospheric carbon dioxide levels increased over a one-year period spanning 2012 and 2013 at the fastest rate since 1984. The WMO estimated that the globally averaged amount of carbon dioxide in the atmosphere in 2013 was 396 ppm.
The 2014 IPCC report further asserted that "continued emissions of greenhouse gases will cause further warming and changes in all components of the climate system," and "limiting climate change will require substantial and sustained reductions of greenhouse gas emissions." However, according to the IPPC's own summary, climate change will continue and "most aspects of climate change will persist for many centuries," even if there were a way to immediately stop CO2 emissions.
Consensus and controversy
There are, however, political activists and critics of the IPCC, the accumulated data supporting climate change, models used to predict change, and the need to undertake potentially economically destabilizing measures without additional study of the problem and solutions. Because of the potential impact on fundamental aspects of security, health, and the global economy, climate change is one of the most contentious issues facing humanity. The well-being or survival of hundreds of millions of people may be threatened by rising sea levels, disrupted food production, extreme weather, and emergent diseases.
There is an unprecedented amount of scientific data and consensus among experts concluding that climate change outside of Earth's natural cycles is already in progress. Moreover, there is a strong consensus in scientific data and conclusions that this change is driven by anthropogenic factors (i.e., those caused by humans).
According to peer reviewed publication in academic journals, the vast majority of climate scientists agree that, as the IPCC argues, "Human activities have been shown to have contributed to increased atmospheric greenhouse gas concentrations, rising sea level, higher ocean and atmospheric temperatures and reductions in ice and snow." Accordingly, "it is extremely likely that human influence has been the dominant cause of the observed warming since the mid-twentieth century."
In IPCC assessments and reports, virtually certain means a 99–100 percent probability; extremely likely means a 95–100 percent probability; very likely means a 90–100 percent probability; and more likely than not means a 50–100 percent probability.
Scientists responding to critics who claim global warming has stopped, argue that plateaus and minor reversions embedded within a strong upward trend in temperatures is expected. Many critics of global warming point to an extraordinary high global average temperature reached in 1998 to then argue warming has stopped or slowed. Such a "cherry picking" approach to data is invalid and is often deceptive of longer-term trends. Data collected from multiple sources show, that each of the last three decades has been successively warmer at the Earth's surface than any preceding decade since 1850. According to two independent analyses completed by scientists from National Aeronautics and Space Administration (NASA) and National Oceanic and Atmospheric Administration (NOAA), 2016 ranked as Earth's warmest on record since 1880. The record continued a trend of observations of a warming climate, with nine of the ten warmest years in the instrumental record occurring since the year 2000.
Although divergent data and opinions still exist, including whether recent predictions might need to be revised to reflect faster-than-expected changes in some areas but apparent slower rises in temperature over the last decade following a surge in temperatures in the 1990s. There is also intensified scientific interest in quantifying the climate sensitivity of carbon, and consequently the amount of global warming to be expected from unarguable and observed increases in atmospheric carbon dioxide levels.
Outside of the United States and Australia, worldwide public debate has largely turned from the question of whether climate change driven by anthropogenic global warming (AGW) is real to politically contentious questions of what can be done to mitigate (lessen the severity of) climate change and adapt to its impacts.
Climate is the average weather of a region over time. Temperature, winds, heat waves, cold snaps, rainfall, storms, and similar weather-related conditions are geographically narrower, short-term facets of broader, longer-term climate conditions. The average is determined after accounting for scientific error, seasonal effects, random vacillations of the jet-stream and weather, historical warming and cooling cycles, changes in solar irradiation, and such factors as fluctuations in El Niño and La Niña patterns.
Climate change itself is not new. Global and regional climates have been changing since Earth formed about 4.5 billion years ago. Ice ages and hot spells have gripped the planet for millions of years. Sudden warming and cooling periods have occurred in as little as a decade, ice caps and jungles have spread and shrunk, and seas have risen and fallen by many meters. However, the changes in global climate recorded in the last half-century are more drastic than any seen for more than 1,000 years. Moreover, instead of corresponding to natural cycles, the changes in global climate correspond to human alteration of atmospheric greenhouse gases and dramatic population increases since the Industrial Revolution began in the late eighteenth century.
Ice core data play an important part of assessing whether current greenhouse gas levels are truly exceptional or if they are simply at a high part of a natural cycle. According to the 2014 IPCC report, "Concentrations of carbon dioxide, methane, and nitrous oxide now substantially exceed the highest concentrations recorded in ice cores during the past 800,000 years. The mean rates of increase in atmospheric concentrations over the past century are, with very high confidence, unprecedented in the last 22,000 years."
The warming of Earth's climate, due mostly to increased trapping of heat by gases in the atmosphere, often is termed the greenhouse effect. Some degree of greenhouse effect is natural and normal. Without Earth's natural atmospheric greenhouse effect, the ocean surfaces would freeze and most life on Earth would perish. However, higher emissions of greenhouse gases have added greatly to the natural greenhouse effect, creating an upward trend in global warming that is not part of natural cycles and variations.
Global warming and climate change trigger a host of sometimes divergent consequences, including more rain and snow in some places and less in others, more floods and droughts, increased melting of mountain glaciers and ice caps in many areas but increased snowfall in others. This is why scientists prefer the phrase "global climate change" when speaking of the overall impacts of global warming. Parts of Antarctica, North America, and Europe may even become cooler or see more snow as the world gets warmer and humidity levels increase, but local exceptions or short-term cool spells do not contradict the upward trend of global warming. Neither do seemingly conflicting localized impacts contradict larger and more directional changes in rising seas and increasing extinctions of plants and animals.
In 2007, the IPCC released its Fourth Assessment Report on climate change. Prepared by a global array of more than 2,500 scientists and economists, the report declared that global warming was "unequivocal" (certain) and that there was at least a 90 percent probability that human beings were the cause. The report had an unprecedented impact on world opinion about climate change, creating a heightened sense of urgency to deal with the issue. The 2014 IPCC report reiterated the scientific consensus that humans are "extremely likely," representing a 95 to 100 percent likelihood, to be the driving force behind climate change.
According to the 2014 IPCC report, "Global surface temperatures are more likely than not to exceed 3.6 degrees Fahrenheit (2 degrees Celsius) compared to pre-industrial levels by 2100."
With regard to sea level rise and the perils of coastal flooding, between 1901 and 2010 (the last year for which there is coordinated global data), "global mean sea level rose by between 6.7 and 8.3 inches (0.17 and 0.21 meters)" and that "for the rest of the century, sea level rise is very likely to exceed that observed between 1971 and 2010."
There are also concerns that the most severe effects of climate change will have significant impact on the world's poorest countries, which have fewer resources and less technological capacity to adapt or mitigate damage. The United Nations University Institute for Environment and Human Security estimated that the number of environmental refugees could rise to 150 million by 2050, mostly as a result of climate change.
Biodiversity loss is also a concern. Animals and plants too will be endangered by changing climate. In 2004, out of a random sample including 1,103 animal and plant species, scientists calculated that 15 to 37 percent would become extinct as a result of climate change by 2050.
Most scientists agree that efforts to mitigate climate change by reducing greenhouse gas emissions—including international treaties and programs to limit greenhouse gas emissions—have largely failed. According to the International Energy Agency, global CO2 emissions continue to accelerate instead of decline.
The December 2009 climate conference in Copenhagen failed to achieve a strong and binding treaty, instead producing the Copenhagen Accord, a weaker agreement calling for voluntary emissions reductions that scientists argue will not prevent global warming and climate change. Nations participating in COP meetings following Copenhagen have yet to agree on a Kyoto-successor treaty, but the 2012 COP18 in Doha, Qatar, extended the deadline of the Kyoto Protocol to 2020. Subsequent COP meetings in Warsaw, Poland (COP19) in 2013 and Lima, Peru (COP20) in 2014 produced few concrete results towards a comprehensive, post-Kyoto climate change agreement.
In 2010, Richard A. Muller (1944–), a professor of physics at the University of California, Berkeley, founded the Berkeley Earth Surface Temperature Study to analyze climate data in an open and transparent manner. Several interest groups that promote or fund climate change skepticism, offered financial support to the organization. Climate change skeptics often cite Muller's earlier work, particularly his criticism of a hockey stick-like graph pattern showing a sharp spike in global temperatures in the twentieth century that correlates to increased carbon dioxide emissions. Muller acknowledged in July 2012 that he has changed his position on climate change, stating in an op-ed piece in the New York Times that his work at Berkeley Earth had provided him with sufficient data to lead him to accept the international scientific consensus that humans are affecting climate change. Muller notes that his findings go even further than the 2007 IPCC Assessment Report, which attributed temperature rises since the mid-twentieth century as "very likely" due to human activity.
A 2013 report by the scientists from the Arctic Monitoring and Assessment Program (AMAP) found that carbon dioxide emissions altered global ocean chemistry. Carbon emissions acidify ocean waters, altering the marine ecosystem. Researchers noted that the greatest changes occurred where cooler Arctic waters absorb CO2 faster and sea ice loss has exposed more sea-surface area to airborne emissions.
The Paris Climate Agreement, negotiated to limit carbon emissions to levels that would limit or prevent the most catastrophic perils predicted by climate change data and models, entered into force in November 2016. All countries except Syria and Nicaragua, signed the agreement.
U.S. President Donald Trump (1946–) announced the withdrawal of the United States from the Paris Climate Agreement on 2 June 2017, saying the accord disadvantaged U.S. businesses and workers. Trump did not rule out rejoining the Agreement after formulating a more favorable position for the U.S., but France, Italy, and the UK quickly responded that the Paris Agreement was not negotiable. China, India, and the European Union renewed their commitment to the Agreement, and several U.S. states pledged to generate legislation with greenhouse-gas emissions targets in line with the Paris accord. Chinese President Xi Jinping (1953–) urged Trump not to withdraw from the Agreement, and David Rank, acting U.S. Ambassador to China resigned rather than deliver the official statement to China that the U.S. had withdrawn from the climate accord. A Reuters poll conducted shortly after the announced withdrawal showed that 68 percent of Americans wanted the United States to lead the world in fighting climate change, half of participants thought participation in the Paris Agreement was necessary to achieve this, but only four percent identified the issue as more important than the U.S. economy or security.
There was widespread and strong international condemnation of President Trump's decision to withdraw the U.S. from the Paris Climate agreement from leaders of other countries, scientists, and major business leaders.