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.