The Overwhelming Consensus of Climate Scientists Worldwide

Overwhelmingly, scientific evidence – including millions of measurements and observations of Earth from land, air, sea, and space – is telling us that:

  • the global climate has been changing over the past several decades in a manner that is highly unusual compared to nature climatic variability;
  • human emissions of greenhouse gases are the dominant cause of these unusual changes;
  • these changes are already having significant adverse impacts on human wellbeing and on ecosystems; and
  • that this harm will continue to grow unless and unless the offending emissions are greatly reduced.

The extremely rigorous and thorough 2013 and 2014 working group reports of the Intergovernmental Panel on Climate Change (IPCC), each developed by hundreds of scientists and technical experts from around the world, as well as the massive third U.S. national Climate Assessment, which was developed by more than 300 of the Nation’s top climate scientists and released by the Obama Administration last month, have further strengthened these fundamental scientific understandings.

I encourage anyone who questions whether there is widespread agreement among scientists about the reality and causes of climate change to read the selections provided below from these recent reports and earlier studies and statements by scientific leaders from around the world.

May 2014: U.S. Global Change Research Program, Third U.S. National Climate Assessment, Climate Change Impacts in the United States.

Long-term, independent records from weather stations, satellites, ocean buoys, tide gauges, and many other data sources all confirm that our nation, like the rest of the world, is warming.  Precipitation patterns are changing, sea level is rising, the oceans are becoming more acidic, and the frequency and intensity of some extreme weather events are increasing.  Many lines of independent evidence demonstrate that the rapid warming of the past half-century is due primarily to human activities.

Human-induced climate change means much more than just hotter weather. Increases in ocean and freshwater temperatures, frost-free days, and heavy downpours have all been documented.  Global sea level has risen, and there have been large reductions in snow-cover extent, glaciers, and sea ice.  These changes and other climatic changes have affected and will continue to affect human health, water supply, agriculture, transportation, energy, coastal areas, and many other sectors of society, with increasingly adverse impacts on the American economy and quality of life.

April 2014: Intergovernmental Panel on Climate Change (IPCC), Contribution of Working Group III to the IPCC Fifth Assessment: Climate Change 2014: Impacts, Adaptation, and Vulnerability: Summary for Policy Makers.

Without additional efforts to reduce GHG emissions beyond those in place today, emissions growth is expected to persist driven by growth in global population and economic activities.

Baseline scenarios, those without additional mitigation, result in global mean surface temperature increases in 2100 from 3.7 to 4.8°C compared to preindustrial levels  (median values; the range is 2.5°C to 7.8°C when including climate uncertainty, see Table SPM.1).

March 2014: American Association for the Advancement of Science (AAAS, the largest general scientific society in the world and the publisher of the prestigious journal, SCIENCE), What We Know: The Reality, Risks, and Response to Climate Change.

The overwhelming evidence of human-caused climate change documents both current impacts with significant costs and extraordinary future risks to society and natural systems. The scientific community has convened conferences, published reports, spoken out at forums and proclaimed, through statements by virtually every national scientific academy and relevant major scientific organization — including the AAAS—that climate change puts the well-being of people of all nations at risk.

March 2014: U.N. World Meteorological Organization, WMO Statement on the Status of the Global Climate in 2013, WMO.

The year 2013 tied with 2007 as the sixth warm­est since global records began in 1850. … Thirteen of the fourteen warmest years on record, including 2013, have all occurred in the twenty-first century. …  While the rate at which surface air temperatures are rising has slowed in recent years, heat continues to be trapped in the Earth system, mostly as increased ocean heat content. About 93 per cent of the excess heat trapped in the Earth system between 1971 and 2010 was taken up by the ocean. From around 1980 to 2000, the ocean gained about 50 zettajoules (1021 joules) of heat. Between 2000 and 2013, it added about three times that amount.

March 2014: Intergovernmental Panel on Climate Change (IPCC), Contribution of Working Group II to the IPCC Fifth Assessment: Climate Change 2014: Impacts, Adaptation, and Vulnerability: Summary for Policy Makers.

Observed impacts of climate change are widespread and consequential. Recent changes in climate have caused impacts on natural and human systems on all continents and across the oceans.

February 2014: U.K. Royal Society and U.S. National Academy of Sciences (the two most prestigious science academies in the world), Climate Change: Evidence and Causes.

Earth’s lower atmosphere is becoming warmer and moister as a result of human-emitted greenhouse gases. This gives the potential for more energy for storms and certain severe weather events. Consistent with theoretical expectations, heavy rainfall and snowfall events (which increase the risk of flooding) and heat waves are generally becoming more frequent. … While changes in hurricane frequency remain uncertain, basic physical understanding and model results suggest that the strongest hurricanes (when they occur) are likely to become more intense and possibly larger in a warmer, moister atmosphere over the oceans. This is supported by available observational evidence in the North Atlantic.

September 2013: Intergovernmental Panel on Climate Change (IPCC), Contribution of Working Group I to the IPCC Fifth Assessment: Climate Science 2013: The Physical Science Basis: Summary for Policy Makers.

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, sea level has risen, and the concentrations of greenhouse gases have increased. …It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century. [Emphasis in original.  In IPCC terminology, “extremely likely” means the statement’s probability of being correct is between 95 and 99 percent.]

Fall 2010: Dr. Lonnie G. Thompson (Distinguished University Professor in the School of Earth Science at Ohio State University, winner of the National Medal of Science, member of the U.S. National Academy of Sciences, arguably the most distinguished glaciologist/paleoclimatologist in the world), “Climate Change: The Evidence and Our Options”, Byrd Polar Research Center Publication 1402. 

Climatologists, like other scientists, tend to be a stolid group. We are not given to theatrical rantings about falling skies. Most of us are far more comfortable in our laboratories or gathering data in the field than we are giving interviews to journalists or speaking before Congressional committees.  Why then are climatologists speaking out about the dangers of global warming?  The answer is that virtually all of us are now convinced that global warming poses a clear and present danger to civilization.

May 2010: Dr. Robert McCormick Adams (former Secretary of the Smithsonian Institution) and 254 other members of the U.S. National Academy of Sciences, “Climate Change and the Integrity of Science”, Letters to the Editor, SCIENCE, May 10, 2010

There is compelling, comprehensive, and consistent objective evidence that humans are changing the climate in ways that threaten our societies and the ecosystems on which we depend. … Natural causes always play a role in changing Earth's climate, but are now being overwhelmed by human-induced changes.

October 2009: Dr. Alan Leshner (Executive Director of the American Association for the Advancement of Science) and the Presidents or Executive Directors of 17 other U.S. scientific societies (including the American Chemical Society, the American Geophysical Union, the American Meteorological Society, the American Statistical Association, and the Ecological Society of America), Open Letter to Members of the U.S. Senate.

Observations throughout the world make it clear that climate change is occurring, and rigorous scientific research demonstrates that the greenhouse gases emitted by human activities are the primary driver.  These conclusions are based on multiple independent lines of evidence, and contrary assertions are inconsistent with an objective assessment of the vast body of peer-reviewed science. Moreover, there is strong evidence that ongoing climate change will have broad impacts on society, including the global economy, and on the environment. For the United States, climate change impacts include sea level rise for coastal states, greater threats of extreme weather events, and increased risk of regional water scarcity, urban heat waves, western wildfires, and the disturbance of biological systems throughout the country.

May 2009: Dr. Bruce Alberts (President of the U.S. National Academy of Sciences) and the presidents of all of the other national academies of science of the G8+5 countries (which include Russia, China, India, and Brazil), G8+5 Academies Statement: Climate Change and the Transformation of Energy Technologies for a Low-Carbon Future.

Climate change is happening even faster than previously estimated; global CO2 emissions since 2000 have been higher than even the highest predictions, Arctic sea ice has been melting at rates much faster than predicted, and the rise in the sea level has become more rapid. Feedbacks in the climate system might lead to much more rapid climate changes.  The need for urgent action to address climate change is now indisputable.


John P. Holdren is Assistant to the President for Science & Technology and Director of the White House Office of Science & Technology Policy.

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