If you lived through historically deadly wildfires in California this fall, or catastrophic flooding and storm surges from Hurricanes Florence or Michael, or record-setting heat and rainfall that afflicted large swaths of the country for much of the year, then the main message of the U.S. government’s National Climate Assessment released the day after Thanksgiving may come as little surprise: “The assumption that current and future climate conditions will resemble the recent past is no longer valid.” Of course, the vast majority of the scientific community, along with a growing number of ordinary citizens, have been working their way toward this sober conclusion for some time now. Climate science can be complicated, as climate-change deniers are fond of pointing out. But a general trend is clear even to non-scientists. The past thirty years have included twenty of the hottest ever recorded; entire ecologies seem to be transforming before our eyes. The president can tweet ignorantly about a brief, localized cold snap or hold to the wholly uninformed opinion that the climate will “probably change back,” but the government’s report is a damning verdict on the damage that fossil-fuel emissions have wrought: “It is very likely that some physical and ecological impacts will be irreversible for thousands of years, while others will be permanent.”
The climate assessment—which is required by Congress—draws on the work of hundreds of scientists, synthesizes thousands of studies, and is endorsed by NASA, the Department of Defense, and eleven other federal agencies. In warning that drought and sea-level rise could cause severe humanitarian crises by the middle of this century, it echoes the dire findings of the U.N.’s October report on climate change. It also lays out the potential financial toll. Climate change could reduce projected U.S. gross domestic product by 10 percent before the end of the century, costing the economy twice as much as the 2008 recession. Heat-related deaths and infrastructure damage are likely to impose hundreds of billions of dollars in costs. Trade and agriculture are particularly at risk. Extreme weather events will damage factories and disrupt supply chains, leading to shortages in critical goods worldwide. Higher temperatures and heavier rainfall will reduce the yield and quality of crops and livestock. By 2050, the agricultural productivity of the Midwest will fall to 1980 levels. It is already too late to avert some of these effects, the report says; at best, we can try to mitigate them.