Last week, the U.S. Global Change Research Program (USGCRP) released a report entitled Climate Science Special Report: Fourth National Climate Assessment, Volume I (NCA4). USGCRP consists of thirteen federal departments and agencies, including the Department of Energy, EPA, the Department of Transportation, NASA, the Department of Defense, and the Department of State. It is required by the Global Change Research Act of 1990 to develop “a comprehensive and integrated United States research program which will assist the Nation and the world to understand, assess, predict, and respond to human-induced and natural processes of global change.” The last USGCRP assessment was released in May 2014, making NCA4 the first report issued under the Trump Administration.
NCA4 provides an updated analysis of how climate change is currently affecting the weather and climate of the United States as well as projections of the future impacts of climate change. The report concludes that “it is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century. For the warming over the last century, there is no convincing alternative explanation supported by the extent of the observational evidence.”
The report notes several climate trends that have occurred over the past century and that are expected to continue. Global average temperature has increased more than 1.2 degrees Celsius for the period of 1986-2016 compared to 1901-1960, and without significant reductions in the emissions of greenhouse gases, temperatures could increase to 5 degrees Celsius above pre-industrial levels by the end of this century. Temperature and precipitation extremes have also become more common in the past century, and this trend is expected to continue as global temperature increases. At the same time, oceans have become warmer and more acidic, and sea levels have risen by seven to eight inches since 1900.
NCA4 reports that stabilizing global temperatures to less than 2 degrees Celsius above pre-industrial levels will require substantial reductions in net CO2 emissions prior to 2040 and likely will require net emissions to become zero or even negative later this century. Although emission growth rates have slowed in recent years, this trend is not sufficient to limit the increase in global temperatures to below 2 degrees Celsius.