Climate change is real. It’s caused by greenhouse-gas pollution released by human industrial activity. Its consequences can already be felt across every region and coastline of the United States—and, unless we stop emitting greenhouse gases soon, those consequences will almost certainly get worse.
Those are the headline findings of the Climate Science Special Report, a sweeping and more than 800-page examination of the evidence. The report was published Friday by four agencies of the U.S. government and academics from across the country.
Their conclusions form the first volume of the new National Climate Assessment, a report on the science and impacts of global warming that Congress requires agencies to complete every four years. A draft version of the second volume, on the human impacts of climate change, was also released Friday.
“This is the most comprehensive assessment of climate science currently available in the world, and it reaffirms what we’ve already known,” said Robert Kopp, one of the lead authors of the report and a professor of climate science at Rutgers University. “If we want to do something like stay under 2 degrees Celsius of warming, the window to do that is closing in the next couple decades.”
The two-degree limit is a rough target used by the United Nations to signal the point where dangerous climate change could begin. The report finds that the world can only continue to emit carbon for roughly another 23 years at current levels before it will have a more than two-thirds chance of going over the limit.
The Climate Science Special Report comes at an auspicious time in the history of global warming and the United States. The report’s conclusions do not deviate wildly from the last 20 years of consensus in climate science. They do not shock anyone who follows the field. And they don’t break new ground: The authors have synthesized the best available papers; they have not conducted new research for this report.
But simply by affirming the science of climate change, the authors—and the interagency bureaucrats who shepherded the writing of the document—provide a contrast to the actions and statements of political figures in the Trump administration. Scott Pruitt, the director of the Environmental Protection Agency, has cast doubt on the idea, fundamental to climate science, that the amount of carbon dioxide in the atmosphere significantly controls Earth’s climate.
President Trump and Pruitt have dismantled the aggressive policies advanced by President Barack Obama meant to reduce U.S. carbon pollution. In October, Pruitt repealed the Clean Power Plan, which would have reduced greenhouse-gas emissions from the power sector. In June, Trump withdrew the United States from the Paris Agreement—and, with it, pulled back from Obama’s decision to apply the full force of the U.S. foreign-policy apparatus to reducing carbon pollution worldwide.
The assessment is a scientific achievement by itself. The last major synthesis of climate science, as a field, was published in 2013 by the UN Intergovernmental Panel on Climate Change. Much has changed over those four years, including significant updates to how the field understands the interaction between global warming and hurricane strength.
It is also the end result of a colossal amount of work. The National Academy of Sciences and the U.S. Global-Change Research Program both convened expert panels to comb through the report line by line and subject it to meticulous comment and approval. During the writing process, scientists from four federal agencies—including NASA, the National Oceanic and Atmospheric Administration, the Department of Energy, and the Environmental Protection Agency—contributed significant writing and expertise. They were joined by researchers from the U.S. Army Corps of Engineers and 11 academic institutions, including Columbia University, Texas Tech University, and the Naval Postgraduate School. It is not an overstatement to say that a vast swath of the profession of American climate science played a hand in this report.
Its conclusions span the Earth system. Modern climate change is primarily caused by the release of carbon dioxide and other gases into the atmosphere, which prevent the sun’s heat from escaping back into space. Over the past several decades, this heat has accumulated in the atmosphere and oceans. Now, the United States sets many more extreme heat records than extreme cold records; it sees more intense heat waves and weaker, briefer cold snaps.
That heat is shrinking the ice sheets at the planet’s north and south poles and causing the oceans to expand. The report includes forecasts of how much the sea level will rise around the world. The sea level worldwide has risen by about seven or eight inches since 1900, with three of those inches coming in the last 25 years. This will intensify: The world ocean is almost certain to rise one to four feet by the end of this century. And if some of the fastest scenarios for the melting of the Antarctic come to pass, then the sea could rise as much as eight feet.
The report sketches out what that sea-level rise will do to different regions of the United States. The East Coast will likely experience even more of that sea-level rise than the world average. The West Coast will likely experience less—unless Antarctica begins to melt in its entirety, in which case it will also see above-average rise.
It also dives into “potential surprises” that the United States may encounter in a climate-changed world. In that chapter, which previous National Climate Assessments did not include, the authors warn of “compound extremes,” the risk of multiple unusual weather conditions coming to pass at the same time. For instance, scorching heat and a lack of rain during the same summer will intensify a drought (and increase the chance of wildfires) far more than either one would alone. If excess rain falls on waterlogged ground, then the chance of a devastating flood also rises. It’s hard for scientists to predict how the various extremes of climate change will overlap with each other.
The authors caution that current climate models are more likely to underestimate future warming than overestimate it. While climate models have accurately predicted the past few decades of warming, they struggle to describe warmer climates that occurred millions of years ago. Models suggest that these climates should be colder than archaeological and climatic evidence tells us that they were, which means that they may fail to capture how warm Earth can get. There may be tipping points in the climate system—difficult-to-predict points of no return—that researchers may not fully understand.
Environmentalists and some government scientists had worried that the Trump administration would try to suppress the release of the report. But some of the authors, speaking anonymously so as not to distract from the release, said they saw little evidence of political interference during the writing process.
Some references to the Paris Agreement were removed from the final report, as compared to a draft version leaked over the summer, sources indicated. Environmental groups plan to conduct a line-by-line comparison of the final report with the leaked drafts in the coming days.