On Friday, the US presidential administration of Donald Trump released the Fourth National Climate Assessment (NCA4), a massive two-part congressionally mandated 1,656-page National Climate Assessment by the United States Global Change Research Program (USGCRP). The report, which focuses on the United States, warned consequences of climate change are increasing across the country. The concentration of carbon dioxide in the Earth's atmosphere has exceeded 400 parts per million. The last time this level was reached was approximately three million years ago.
NCA4 Volume 1, entitled "Climate Science Special Report" (CSSR), was released late last year, and Volume 2, entitled "Impacts, Risks, and Adaptation in the United States", on Friday. NCA4 is part of an ongoing series of NCA stand-alone reports updating the state of science on climate change and its potential physical impacts on the regions of the United States. The the third report was released in 2014.
Changes in the United States highlighted in the report include worsening and more frequent landfalling atmospheric rivers — bands of rain-laden air in the sky. These rivers of water vapor tend to be around 300 miles (500 km) wide, carrying something like 25 times the water volume of the Mississippi River. They can cause extreme precipitation events and flooding. NCA4 also reported on increasing wildfires, such as the record California wildfires, high temperature extremes, retreating glaciers, and melting snow cover, the decline of sea ice, rising sea levels with increasing ocean acidification and coastal flooding. It describes how climate change would reportedly impact the US economy and quality of life if effective action is not taken to address climate change. The report says by the end of the 21st century, the United States economy may face a roughly ten percent loss of GDP.
A federal advisory committee was mandated to use the NCA reports' scientific studies, findings, and data to make public policy recommendations states could implement. In August 2017, the Trump administration dissolved this committee. NCA reports do not include policy recommendations. In a Reuters interview, a White House spokeswoman dismissed the report as inaccurate.
The report was issued by thirteen federal agencies, including DOD and NASA, with NOAA as administrative lead. It was the result of two years' work. Three hundred scientists with a total of over a thousand people — half of which were non-governmental staff — worked on the report.