President Trump and many agency heads are trying to discredit a report their own administration released—the fourth National Climate Assessment (NCA)—that shows climate change is already taking a toll on the United States, with impacts occurring in every part of the country.
“The administration isn’t able to discount the science showing that climate change is a problem we’re grappling with right now in every part of the country,” said Brenda Ekwurzel, one of the NCA report authors and director of climate science at the Union of Concerned Scientists. “Instead of warning about the risks to U.S. residents and the economy, we have heard administration officials attempting to misrepresent the way in which the report assessed how climate change could unfold under different emission scenarios.”
In writing the report, scientists assessed the latest climate research, including studies that analyzed climate impacts under various representative concentration pathways (RCP) of heat-trapping gases. In focusing on future impacts, the previous administration directed NCA authors to primarily focus on 4.5 and 8.5 RCP scenarios—the standard scenarios that researchers analyze when studying climate change. The 4.5 RCP scenario correlates with a temperature increase ranging from 1.7 to 3.2 degrees Celsius and the 8.5 RCP scenario correlates with a temperature increase of 3.2 to 5.4 degrees Celsius.
“The report’s low and high scenarios essentially start with the goal of the Paris climate agreement and go up to the path the world is currently on,” said Ekwurzel. “While the Paris agreement calls for the global temperature to increase less than 2 degrees Celsius, the pledges that countries have made would likely cause temperatures to increase around 3 degree Celsius, and they are not even on track to meeting that.
“If emissions continue unabated, the RCP 8.5 scenario could well be our reality,” she said. “So, it’s entirely reasonable to use that as the high-end scenario in the NCA and compare it to the emissions reductions the Paris agreement calls for. The United States and other countries have a choice about which path to take.”
A memo to NCA4 authors encouraged them also to include information in the report about the even lower emissions scenario, RCP 2.6, which correlates with a 0.9 to 2.3 degree Celsius temperature increase, when studies were available that included that scenario and especially in assessing mitigation pathways.