Scientific consensus on climate change



The  is that the Earth is warming and that this warming is mainly caused by human activities. This consensus is supported by various studies of scientists' opinions and by position statements of scientific organizations, many of which explicitly agree with the (IPCC) synthesis reports.

Nearly all actively publishing climate scientists (97–98%) support the consensus on anthropogenic climate change, and the remaining 3% of contrarian studies either cannot be replicated or contain errors.


 * Quotes are entered in reverse chronological order.

AR5 Synthesis Report: Climate Change (2014)

 * IPCC, 2014: Climate Change 2014: Synthesis Report. Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Core Writing Team, R.K. Pachauri and L.A. Meyer (eds.)]. IPCC, Geneva, Switzerland, 151 pp. source, alt.


 * [H]uman influence on the climate system is clear and growing, with impacts observed across all continents and oceans.


 * Many of the observed changes since the 1950s are unprecedented over decades to millennia.


 * The IPCC is now 95 percent certain that humans are the main cause of current global warming.


 * [T]he more human activities disrupt the climate, the greater the risks of severe, pervasive and irreversible impacts for people and ecosystems, and long-lasting changes in all components of the climate system.


 * [This report] calls for the urgent attention of both policymakers and citizens of the world to tackle this challenge.


 * Ignorance can no longer be an excuse for tergiversation.


 * This report distils, synthesizes and integrates the key findings of the three Working Group contributions—The Physical Science Basis, Impacts, Adaptation, and Vulnerability and Mitigation of Climate Change...


 *  Human influence on the climate system is clear, and recent anthropogenic emissions of greenhouse gases are the highest in history. 


 *  Recent climate changes have had widespread impacts on human and natural systems. 


 *  Warming of the climate system is unequivocal, and since the 1950s, many of the observed changes are unprecedented over decades to millennia. 


 *  The atmosphere and ocean have warmed, the amounts of snow and ice have diminished, and sea level has risen. 


 *  Anthropogenic greenhouse gas emissions have increased since the pre-industrial era, driven largely by economic and population growth, and are now higher than ever. This has led to atmospheric concentrations of carbon dioxide, methane and nitrous oxide that are unprecedented in at least the last 800,000 years. Their effects, together with those of other anthropogenic drivers, have been detected throughout the climate system and are extremely likely to have been the dominant cause of the observed warming since the mid-20th century. 


 *  In recent decades, changes in climate have caused impacts on natural and human systems on all continents and across the oceans. 


 * Many terrestrial,  freshwater  and  marine  species  have  shifted  their  geographic  ranges, seasonal activities, migration patterns, abundances and species interactions in response to ongoing climate change...


 * [N]egative impacts of climate change on crop yields have been more common than positive impacts...


 *  Changes in many extreme weather and climate events have been observed since about 1950. Some of these changes have been linked to human influences, including a decrease in cold temperature extremes, an increase in warm temperature extremes, an increase in extreme high sea levels and an increase in the number of heavy precipitation events in a number of regions. 


 * It is very likely that the number of cold days and nights has decreased and the number of warm days and nights has increased on the global scale.


 * It is very likely that human influence has contributed to the observed global scale changes in the frequency and intensity of daily temperature extremes since the mid-20th century.


 * Impacts from recent climate-related extremes, such as heat waves, droughts, floods, cyclones and wildfires, reveal significant vulnerability and exposure of some ecosystems and many human systems to current climate variability...


 *  Continued emission of greenhouse gases will cause further warming and long-lasting changes in all components of the climate system, increasing the likelihood of severe, pervasive and irreversible impacts for people and ecosystems. 


 *  Limiting climate change would require substantial and sustained reductions in greenhouse gas emissions which, together with adaptation, can limit climate change risks. 


 *  Cumulative emissions of CO2 largely determine global mean surface warming by the late 21st century and beyond. 


 * Multiple lines of evidence indicate a strong, consistent, almost linear relationship between cumulative CO2 emissions and projected global temperature change to the year 2100...


 *  Surface temperature is projected to rise over the 21st century under all assessed emission scenarios. It is very likely that heat waves will occur more often and last longer, and that extreme precipitation events will become more intense and frequent in many regions. The ocean will continue to warm and acidify, and global mean sea level to rise. 


 * It is virtually certain that there will be more frequent hot and fewer cold temperature extremes over most land areas on daily and seasonal timescales, as global mean surface temperature increases.


 * It is very likely that heat waves will occur with a higher frequency and longer duration. Occasional cold winter extremes will continue to occur.


 * Extreme precipitation events over most of the mid-latitude land masses and over wet tropical regions will very likely become more intense and more frequent.


 * The global ocean will continue to warm during the 21st century, with the strongest warming projected for the surface in tropical and Northern Hemisphere subtropical regions...


 *  Climate change will amplify existing risks and create new risks for natural and human systems. Risks are unevenly distributed and are generally greater for disadvantaged people and communities in countries at all levels of development. 


 * Most plant species cannot naturally shift their geographical ranges sufficiently fast to keep up with current and high projected rates of climate change in most landscapes; most small mammals and s will not be able to keep up at the rates projected... in flat landscapes in this century...


 * Marine organisms will face progressively lower oxygen levels and high rates and magnitudes of ...


 * Coral reefs and polar ecosystems are highly vulnerable.


 * Coastal systems and low-lying areas are at risk from sea level rise, which will continue for centuries even if the global mean temperature is stabilized...


 * Climate change is projected to undermine ...


 * Due to projected climate change by the mid-21st century and beyond, global marine species redistribution and marine biodiversity reduction in sensitive regions will challenge the sustained provision of fisheries productivity and other ecosystem services...


 * Global temperature increases of ~4°C or more above late 20th century levels, combined with increasing food demand, would pose large risks to food security globally...


 * Climate change is projected to reduce renewable surface water and groundwater resources in most dry subtropical regions... intensifying competition for water...


 * [C]limate change will impact human health mainly by exacerbating health problems that already exist... Throughout the 21st century, climate change is expected to lead to increases in ill-health in many regions and especially in developing countries with low income...


 * By 2100... the combination of high temperature and humidity in some areas for parts of the year is expected to compromise common human activities, including growing food and working outdoors...


 * In urban areas climate change is projected to increase risks for people, assets, economies and ecosystems, including risks from heat stress, storms and extreme precipitation, inland and coastal flooding, landslides, air pollution, drought, water scar-city, sea level rise and storm surges... These risks are amplified for those lacking essential infrastructure and services or living in exposed areas.


 * Rural areas are expected to experience major impacts on water availability and supply, food security, infrastructure and agricultural incomes...


 *  Many aspects of climate change and associated impacts will continue for centuries, even if anthropogenic emissions of greenhouse gases are stopped. The risks of abrupt or irreversible changes increase as the magnitude of the warming increases. 


 * Surface temperatures will remain approximately constant at elevated levels for many centuries after a complete cessation of net anthropogenic CO2 emissions. A large fraction of anthropogenic climate change resulting from CO2 emissions is irreversible on a multi-century to millennial timescale, except in the case of a large net removal of CO2 from the atmosphere over a sustained period.


 * Stabilization of global average surface temperature does not imply stabilization for all aspects of the climate system. Shifting, , s, ocean temperatures and associated all have their own intrinsic long timescales which will result in changes lasting hundreds to thousands of years after global surface temperature is stabilized.


 * [O]cean acidification will increase for centuries if CO2 emissions continue, and will strongly affect s.


 * [G]lobal mean sea level rise will continue for many centuries beyond 2100...


 * A reduction in extent is virtually certain with continued rise in global temperatures.


 *  Adaptation and mitigation are complementary strategies for reducing and managing the risks of climate change. Substantial emissions reductions over the next few decades can reduce climate risks in the 21st century and beyond, increase prospects for effective adaptation, reduce the costs and challenges of mitigation in the longer term and contribute to climate-resilient pathways for sustainable development. 


 * Many of those most vulnerable to climate change have contributed and contribute little to GHG emissions.


 * Climate change has the characteristics of a collective action problem at the global scale, because most GHGs accumulate over time and mix globally, and emissions by any agent (e.g., individual, community, company, country) affect other agents.


 * Effective mitigation will not be achieved if individual agents advance their own interests independently.


 * Cooperative responses, including international cooperation, are... required to effectively mitigate... and address other climate change...


 *  There are multiple mitigation pathways that are likely to limit warming to below 2°C relative to pre-industrial levels. These pathways would require substantial emissions reductions over the next few decades and near zero emissions of CO2 and other long-lived greenhouse gases by the end of the century. 


 * Delaying additional mitigation to 2030 will substantially increase the challenges associated with limiting warming over the 21st century to below 2°C...


 * In the absence or under limited availability of mitigation technologies (such as, CCS and their combination BECCS, nuclear, wind/solar), mitigation costs can increase substantially...


 *  (SRM) involves large-scale methods... is untested and is not included in any of the mitigation scenarios. If it were deployed, SRM would entail numerous uncertainties, side effects, risks and shortcomings... If it were terminated [after and if once begun], there is high confidence that surface temperatures would rise very rapidly impacting ecosystems...


 *  Many adaptation and mitigation options can help address climate change, but no single option is sufficient by itself. 


 *  Adaptation and mitigation responses are underpinned by common enabling factors. These include effective institutions and governance, innovation and investments in environmentally sound technologies and infrastructure, sustainable livelihoods and behavioural and lifestyle choices. 


 * Improving institutions as well as coordination and cooperation in governance can help overcome regional constraints associated with mitigation, adaptation and disaster risk reduction...


 *  Mitigation can be more cost-effective if using an integrated approach that combines measures to reduce energy use and the green-house gas intensity of end-use sectors, decarbonize energy supply, reduce net emissions and enhance carbon sinks... 


 * The most cost-effective mitigation options in forestry are, sustainable forest management and reducing ... and in agriculture, cropland management, grazing land management and restoration of organic soils...


 * Behaviour, lifestyle and culture have a considerable influence on energy use and associated emissions... Emissions can be substantially lowered through changes in consumption patterns, adoption of energy savings measures, dietary change and reduction in food wastes.


 * International cooperation is critical for effective mitigation...


 * The (UNFCCC) is the main multilateral forum focused on addressing climate change, with nearly universal participation.


 * The offers lessons towards achieving the ultimate objective of the UNFCCC...


 * International cooperation for supporting adaptation planning and implementation... has assisted in the creation of adaptation strategies, plans and actions...


 * Substantial reductions in emissions would require large changes in investment patterns...


 * Potential synergies between international finance for disaster risk management and adaptation have not yet been fully realized...


 *  Climate change is a threat to sustainable development. ...Successful implementation relies on relevant tools, suitable governance structures and enhanced capacity to respond ... 


 * Climate change exacerbates other threats to social and natural systems, placing additional burdens particularly on the poor...


 * Delaying global mitigation actions may reduce options...


 * Strategies and actions can be pursued now... while at the same time helping to improve livelihoods, social and economic well-being and effective environmental management.


 * Climate change exposes people, societies, economic sectors and ecosystems to risk. ...[H]igh risk can result not only from high probability outcomes but also from low probability outcomes with very severe consequences.


 *  Human influence on the climate system is clear, and recent anthropogenic emissions of greenhouse gases are the highest in history. Recent climate changes have had widespread impacts on human and natural systems. 


 *  Warming of the climate system is unequivocal, and since the 1950s, many of the observed changes are unprecedented over decades to millennia. The atmosphere and ocean have warmed, the amounts of snow and ice have diminished, and sea level has risen. 


 * Over the last two decades, the Greenland and Antarctic s have been losing mass... s have continued to shrink almost worldwide... Northern Hemisphere spring snow cover has continued to decrease in extent...


 * Glaciers have lost mass and contributed to sea level rise throughout the 20th century.


 * The rate of ice mass loss from the has very likely substantially increased over the period 1992 to 2011, resulting in a larger mass loss over 2002 to 2011...


 * Arctic sea ice extent has decreased in every season and in every successive decade since 1979...


 * There is very high confidence that the extent of Northern Hemisphere snow cover has decreased since the mid-20th century by 1.6% [0.8 to 2.4%] per decade for March and April, and 11.7% per decade for June, over the 1967 to 2012 period.


 * The rate of since the mid-19th century has been larger than the mean rate during the previous two millennia...


 * Since the early 1970s, glacier mass loss and ocean thermal expansion from warming together explain about 75% of the observed global mean sea level rise...


 *  Anthropogenic greenhouse gas emissions have increased since the pre-industrial era driven largely by economic and population growth. From 2000 to 2010 emissions were the highest in history. Historical emissions have driven atmospheric concentrations of carbon dioxide, methane and nitrous oxide to levels that are unprecedented in at least the last 800,000 years, leading to an uptake of energy by the climate system. 


 * Atmospheric concentrations of GHGs are at levels that are unprecedented in at least 800,000 years. Concentrations of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) have all shown large increases since 1750 (40%, 150% and 20%, respectively)...


 * The total anthropogenic over 1750–2011 is... 2.3 [1.1 to 3.3] W/m2... and it has increased more rapidly since 1970 than during prior decades. Carbon dioxide is the largest single contributor to radiative forcing...


 * About half of the cumulative anthropogenic CO2 emissions between 1750 and 2011 have occurred in the last 40 years...


 * About 40% of... anthropogenic CO2 emissions have remained in the atmosphere... since 1750.


 * Globally, economic and population growth continue to be the most important drivers of increases in CO2 emissions from fossil fuel combustion.


 *  Human influence has been detected in warming of the atmosphere and the ocean, in changes in the global, in reductions in snow and ice, and in global mean sea level rise; and it is extremely likely to have been the dominant cause of the observed warming since the mid-20th century. In recent decades, changes in climate have caused impacts on natural and human systems on all continents and across the oceans. 


 * It is extremely likely that more than half of the observed increase in global average surface temperature from 1951 to 2010 was caused by the anthropogenic increase in GHG concentrations and other anthropogenic forcings...


 * Anthropogenic influences have very likely contributed to Arctic sea ice loss since 1979...


 * It is very likely that anthropogenic forcings have made a substantial contribution to increases in global upper ocean heat content... observed since the 1970s...


 * It is very likely that there is a substantial anthropogenic contribution to the global mean sea level rise since the 1970s.


 * Glaciers continue to shrink almost worldwide due to climate change...


 * Climate change is causing permafrost warming and thawing in high-latitude regions and in high-elevation regions...


 *  Changes in many extreme weather and climate events have been observed since about 1950. Some of these changes have been linked to human influences, including a decrease in cold temperature extremes, an increase in warm temperature extremes, an increase in extreme high sea levels and an increase in the number of heavy precipitation events in a number of regions. 


 *  Continued emission of greenhouse gases will cause further warming and long-lasting changes in all components of the climate system, increasing the likelihood of severe, pervasive and irreversible impacts for people and ecosystems. Limiting climate change would require substantial and sustained reductions in greenhouse gas emissions which, together with adaptation, can limit climate change risks. 


 *  Surface temperature is projected to rise over the 21st century under all assessed emission scenarios. It is very likely that heat waves will occur more often and last longer, and that extreme precipitation events will become more intense and frequent in many regions. The ocean will continue to warm and acidify, and global mean sea level to rise. 


 * It is virtually certain that there will be more frequent hot and fewer cold temperature extremes over most land areas on daily and seasonal timescales, as global mean surface temperature increases.


 * Extreme precipitation events over most of the mid-latitude land masses and over wet tropical regions will very likely become more intense and more frequent as global mean surface temperature increases.

IPCC, 2007: Summary for Policymakers

 * In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.


 *  Global atmospheric concentrations of carbon dioxide, methane and nitrous oxide have increased markedly as a result of human activities since 1750 and now far exceed pre-industrial values determined from ice cores spanning many thousands of years... The global increases in carbon dioxide concentration are due primarily to fossil fuel use and land use change, while those of methane and nitrous oxide are primarily due to agriculture. 


 *  Warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global average sea level. 


 *  At continental, regional and ocean basin scales, numerous long-term changes in climate have been observed. These include changes in arctic temperatures and ice, widespread changes in precipitation amounts, ocean salinity, wind patterns and aspects of extreme weather including droughts, heavy precipitation, heat waves and the intensity of tropical cyclones. 


 * Average arctic temperatures increased at almost twice the global average rate in the past 100 years.


 *  Palaeoclimatic information supports the interpretation that the warmth of the last half century is unusual in at least the previous 1,300 years. The last time the polar regions were significantly warmer than present for an extended period (about 125,000 years ago), reductions in polar ice volume led to 4 to 6 m of sea level rise. 


 * Average Northern Hemisphere temperatures during the second half of the 20th century were very likely higher than during any other 50-year period in the last 500 years and likely the highest in at least the past 1,300 years.


 * Global average sea level in the last interglacial period (about 125,000 years ago) was likely 4 to 6 m higher than during the 20th century, mainly due to the retreat of polar ice. Ice core data indicate that average polar temperatures at that time were 3°C to 5°C higher than present, because of differences in the Earth’s orbit.


 *  Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations. 


 *  Discernible human influences now extend to other aspects of climate, including ocean warming, continental-average temperatures, temperature extremes and wind patterns... 


 * [I]t is extremely unlikely that global climate change of the past 50 years can be explained without external forcing, and very likely that it is not due to known natural causes alone...


 * Anthropogenic forcing is likely to have contributed to changes in wind patterns, affecting extratropical storm tracks and temperature patterns in both hemispheres.


 *  For the next two decades, a warming of about 0.2°C per decade is projected for a range of SRES emission scenarios. Even if the concentrations of all greenhouse gases and aerosols had been kept constant at year 2000 levels, a further warming of about 0.1°C per decade would be expected. 


 * Since IPCC’s first report in 1990, assessed projections have suggested global average temperature increases between about 0.15°C and 0.3°C per decade for 1990 to 2005. This can now be compared with observed values of about 0.2°C per decade, strengthening confidence in near-term projections...


 * Model experiments show that even if all radiative forcing agents were held constant at year 2000 levels, a further warming trend would occur in the next two decades at a rate of about 0.1°C per decade, due mainly to the slow response of the oceans. About twice as much warming (0.2°C per decade) would be expected if emissions are within the range of the SRES scenarios.


 * [D]ecadal average warming over each inhabited continent by 2030... is very likely to be at least twice as large as the corresponding model-estimated natural variability during the 20th century.


 *  Anthropogenic warming and sea level rise would continue for centuries due to the time scales associated with climate processes and feedbacks, even if greenhouse gas concentrations were to be stabilised. 


 * Both past and future anthropogenic carbon dioxide emissions will continue to contribute to warming and sea level rise for more than a millennium, due to the time scales required for removal of this gas from the atmosphere...

Climate Change Science: An Analysis of Some Key Questions (2001)

 * National Academy of Sciences, Committee on the Science of Climate Change, Division on Earth and Life Studies, National Research Council. Source.


 * es are accumulating in Earth’s atmosphere as a result of human activities, causing surface air temperatures and subsurface ocean temperatures to rise.


 * Human-induced warming and associated sea level rises are expected to continue through the 21st century.


 * [N]ational policy decisions made now and in the longer-term future will influence the extent of any damage suffered by vulnerable human populations and ecosystems later in this century.


 * Of the greenhouse gases that are directly influenced by human activity, the most important are carbon dioxide, methane, ozone, nitrous oxide, and chlorofluorocarbons (CFCs). s released by human activities are also capable of influencing climate.


 * Concentrations of (CO2) extracted from s drilled in Greenland and Antarctica have typically ranged from near 190 parts per million by volume (ppmv) during the ice ages to near 280 ppmv during the warmer "interglacial" periods like the present one that began around 10,000 years ago. Concentrations did not rise much above 280 ppmv until the Industrial Revolution. By 1958... they had reached 315 ppmv, and they are currently ~370 ppmv and rising at a rate of 1.5 ppmv per year... Human activities are responsible for the increase. The primary source, fossil fuel burning, has released roughly twice as much... as... required... for the observed increase. Tropical deforestation also has contributed to carbon dioxide releases during the past few decades. The excess... has been taken up by the oceans and land .


 * Like carbon dioxide,  (CH4) is more abundant in Earth’s atmosphere now than at any time during the 400,000 year long ice core record, which dates back over a number of glacial/interglacial cycles.


 * About two-thirds of the current emissions of methane are released by human activities such as rice growing, the raising of cattle, coal mining, use of land-fills, and natural gas handling, all of which have increased over the past 50 years.


 * Besides greenhouse gases, human activity also contributes to the atmospheric burden of s, which include both sulfate particles and (soot).


 * Recent "clean coal technologies" and use of low sulfur fuels have resulted in decreasing sulfate concentrations...


 * Black carbon aerosols are end-products of the incomplete combustion of fossil fuels and biomass burning (forest fires and land clearing).


 * Nitrous oxide (N2O) is formed by many microbial reactions in soils and waters, including those acting on the increasing amounts of nitrogen-containing fertilizers. Some synthetic chemical processes that release nitrous oxide have also been identified. Its concentration has increased approximately 13% in the past 200 years.


 * Atmospheric concentrations of CFCs rose steadily following their first synthesis in 1928 and peaked in the early 1990s. Many other industrially useful fluorinated compounds... have very long atmospheric lifetimes, which is of concern... s (HFCs), which are replacing CFCs, have a greenhouse effect, but it is much less pronounced...


 * The sensitivity and generality of modern analytical systems make it quite unlikely that any currently significant greenhouse gases remain to be discovered.


 * The IPCC’s conclusion that most of the observed warming of the last 50 years is likely to have been due to the increase in greenhouse gas concentrations accurately reflects the current thinking of the scientific community... Despite the uncertainties, there is general agreement that the observed warming is real and particularly strong within the past 20 years.


 * If a central estimate of climate sensitivity is used, about 40% of the predicted warming is due to the direct effects of greenhouse gases and aerosols. The other 60% is caused by s.


 * Water vapor feedback (the additional greenhouse effect accruing from increasing concentrations of atmospheric water vapor as the atmosphere warms)... is expected to increase the temperature response to increases in human induced greenhouse gas concentrations by a factor of 1.6. The ice-albedo feedback (the reduction in the fraction of incoming solar radiation reflected back to space as snow and ice cover recede) also is believed to be important. Together, these two feedbacks amplify the simulated climate response to the greenhouse gas forcing by a factor of 2.5.


 * With higher sea level, coastal regions could be subject to increased wind and flood damage...


 * [H]eat stress and smog induced respiratory illnesses in major urban areas would increase, if no adaptation occurred.


 * [A]ssessments that examine only the next 100 years may well underestimate the magnitude of the eventual impacts.


 * [R]isk increases with increases in both the rate and the magnitude of climate change.