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On November 23, 2018, the Trump administration released volume two of the Fourth National Climate Assessment (NCA4). Mandated by law, the assessment documents and projects the impacts of climate change on the environment, economy, and public health of the US. The first volume, which examines the physical science of climate change, appeared in November 2017.

Although there have been no indications the administration interfered with NCA4’s production, top officials have criticized its methods and conclusions. It is also widely suspected that the administration’s decision to release it ahead of schedule on the day after Thanksgiving was a deliberate attempt to minimize attention to it.

NCA4 concludes climate impacts are “intensifying” across the US and that the evidence of anthropogenic climate change is “overwhelming and continues to strengthen.” It also offers detailed accounts of localized impacts, along with accompanying resilience and mitigation planning tools, and includes a more systematic analysis of the economic impacts of climate change than its predecessors.

Asked by reporters about NCA4, President Trump replied, “I’ve seen it. I’ve read some of it, and it’s fine.” But when pressed on his views of its assessment of the economic damage climate change could cause to the U.S., he said, “I don’t believe it.”

Trump has famously said he believes climate change is a “hoax.” In more recent interviews, he has acknowledged the climate is changing but, rejecting the scientific consensus, he has also suggested it is the result of natural variability.

Acting Environmental Protection Agency (EPA) Administrator Andrew Wheeler offered the administration’s most detailed criticisms of NCA4 in a live interview with the Washington Post on November 28. While he thanked the federal employees who worked on the report, he argued it is based on overly pessimistic assumptions that do not take sufficient account of future technological improvements that would reduce greenhouse gas emissions.

Wheeler also suggested the Obama administration had specifically instructed the report’s authors to use a “worst-case scenario.” He said the next national assessment, due in four years, might assume more technological advances in reducing emissions.

Later that day, the EPA released a “fact check” pointing to a 2015 memorandum from the U.S. Global Change Research Program (USGCRP) as evidence the assessment team had been “pushed” into employing a high-emissions scenario called RCP8.5. A “representative concentration pathway” (RCP) is a particular emissions trajectory that can be consistent with a variety of scenarios for future energy demand, energy sources employed, and other factors.

The memo in question actually documents USGCRP’s decision to use both RCP8.5 and a moderate-emissions scenario called RCP4.5 as the “core scenarios” in its analysis. It states these scenarios are in line with the ranges of scenarios considered in the third NCA and the Intergovernmental Panel on Climate Change’s Fifth Assessment Report, which were both released in 2014.

NCA4 itself states that RCP8.5, which generally assumes high population growth and the use of carbon-intensive energy sources, is consistent with current global emissions trends. RCP4.5, it notes, is associated with lower population growth, more technological innovation, and the use of energy sources with a lower carbon intensity.

Justifying its use of the scenarios, the report states they “capture a range of potential greenhouse gas pathways and associated atmospheric concentration levels through 2100,” while noting it does not assess the “feasibility of the socioeconomic assumptions” underlying them. The report further states, “The resulting range of projections reflects, in part, the uncertainty that comes with quantifying future human activities and their influence on climate.”

Adria Schwarber is a Science Policy Analyst and William Thomas is Senior Science Policy Analyst with FYI at the American Institute of Physics.

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