The Trump administration has released the Fourth National Climate Assessment, which details current and potential climate change impacts on the U.S. While the report considers a range of scenarios, the administration has asserted it relies on an “extreme” scenario based on overly pessimistic assumptions.
On Nov. 23, the Trump administration released volume two of the Fourth National Climate Assessment (NCA), a congressionally mandated report that documents and projects the impacts of climate change on the environment, economy, and public health of the United States. Volume one of the assessment, examining the physical science of climate change, was released in November 2017.
Like the third NCA, released in 2014, the new report is largely built around two scenarios for future greenhouse gas emissions that establish a broad range of potential climate change impacts. The report also presents more localized analysis than its predecessors in an effort to improve guidance to decision-makers in preparing for and mitigating climate change.
The Trump administration has sought to distance itself from the report. In particular, administration officials have criticized the report’s use of a high-emissions scenario, asserting it is overly pessimistic.
NCA seeks to provide localized data to inform planning
The NCA is produced through the interagency U.S. Global Change Research Program (USGCRP). Volume two was assembled over a two-year period by 300 authors drawn from federal agencies, industry, universities, and other research institutions. In addition to undergoing a series of internal reviews, it was opened for public comment and externally reviewed by an expert committee convened by the National Academies.
The NCA identifies several ways it departs from its predecessor. In particular, it observes that the authors worked to make the NCA — encompassing both the report and associated “tools” — more useful to stakeholders, noting,
Hundreds of states, counties, cities, businesses, universities, and other entities are implementing actions that build resilience to climate-related impacts and risks, while also aiming to reduce greenhouse gas emissions.
USGCRP decided to release the report in two volumes for the first time so that the assessment of the underlying climate science could better inform the impact assessment. The report also states it has an “increased focus on local and regional information.” This includes two products developed by USGCRP that aim to enhance decision-makers’ ability to use NCA data: state-by-state climate impact summaries and the U.S. Climate Resilience Toolkit, which was first launched in 2014.
The report states this iteration of the NCA also includes “broader and more systematic quantification of climate change impacts in economic terms” than the prior assessment. It acknowledges, though, that assessing economic impacts is still an “emerging body of literature” that does not yet cover all geographic regions.
Identifying scientific advances since the last NCA, the report points to an increasing ability to identify impacts at more localized scales. It states that knowledge is improving around atmospheric circulation, ice loss, and the impacts of warming and acidification on oceans and coastal waters. And it observes there is improved understanding of how climate change affects extreme events such as floods, droughts, and wildfires, though it notes “the current state of the science does not yet permit detailed understanding.”
Among the NCA’s overarching conclusions are that the impacts of climate change are “intensifying” across the U.S. and that the evidence of human contributions to climate change is “overwhelming and continues to strengthen.” The report also stresses there remains a potential for future “surprises” that scientists have not identified but could have major consequences.
“The more the climate changes, the greater the potential for these surprises,” it notes.
Administration questions report’s conclusions
The Trump administration’s decision to release the NCA ahead of schedule and on the day after the Thanksgiving holiday raised widespread suspicions it was attempting to minimize its impact.
Then, following its release, the administration began to criticize the report. A White House spokesperson asserted it is “largely based on the most extreme scenario” and that the next NCA would provide an opportunity to implement a “more transparent and data-driven process that includes fuller information on the range of potential scenarios and outcomes.”
These criticisms elicited a rebuttal from Katharine Hayhoe, the lead author of a chapter of the report, who tweeted that the NCA "considered a very broad range of scenarios, from one where carbon emissions go negative to one where they continue to grow."
When first asked about the report, President Trump replied, “I’ve seen it. I’ve read some of it, and it’s fine.” But when pressed on his views of the NCA’s projections of the economic risks climate change presents to the U.S., he said, “I don’t believe it.”
Since the release of an Intergovernmental Panel on Climate Change (IPCC) special report in October, Trump has repeatedly said he believes that the climate is changing, but he has rejected the scientific consensus by suggesting it is the result of natural variability. In an interview with the Washington Post on Nov. 27, he stated, “As to whether or not it’s man-made and whether or not the effects that you’re talking about are there, I don’t see it.”
Criticisms centered on carbon emission scenarios
Acting Environmental Protection Agency Administrator Andrew Wheeler offered the administration’s most detailed criticisms of the NCA to date in an interview with the Washington Post on Nov. 28. While he thanked the federal employees who worked on the report, he said,
I think a lot of the criticism that you’ve seen from the Trump administration on the report is the emphasis from the media on the worst-case scenario in the report, which is based on — I believe it’s called the RCP8.5, which is the worst-case scenario. It’s a scenario that the U.N.’s IPCC is moving away from.
Any given “representative concentration pathway” (RCP) can be consistent with a variety of socio-economic scenarios entailing assumed changes in energy demand, energy sources employed, and other factors that result in a particular trajectory for future greenhouse gas emissions. The number represents the projected change in radiative forcing in the year 2100 relative to pre-industrial levels, measured in watts per square meter.
Asked about the more moderate scenario of RCP4.5 that the NCA also considered, Wheeler said that both RCP8.5 and 4.5 “downplay innovation and innovation that we’ve seen already in the marketplace.” He suggested specifically that the scenarios do not account for reductions in carbon dioxide emissions that have already been achieved.
Wheeler also suggested the Obama administration compelled the NCA to use RCP8.5 for its high-end scenario, saying, “I don’t know this for a fact, but I wouldn’t be surprised if the Obama administration told the report’s authors, ‘Take a look at the worst-case scenario for this report.’” He added that the next NCA might employ different assumptions, remarking, “I don’t think it’s appropriate to … project that there will be no further innovations in technology going forward.”
Later that day, EPA released a “fact check” pointing to a May 2015 USGCRP memorandum as evidence the Obama administration “pushed” the assessment team to employ the RCP8.5 scenario. The memo explains USGCRP’s decision to use RCP8.5 and 4.5 as its “core scenarios,” noting they are in line with the range of scenarios considered in the previous NCA and the IPCC’s Fifth Assessment Report, also published in 2014.
In justifying its use of the scenarios, the NCA states, “Current trends in annual greenhouse gas emissions, globally, are consistent with RCP8.5.” It observes that RCP4.5, which entails 85 percent lower greenhouse gas emissions than RCP8.5 by 2100, is “generally associated with lower population growth, more technological innovation, and lower carbon intensity in the global energy mix.”
The report notes it does not evaluate the “feasibility of the socioeconomic assumptions” underlying the scenarios. It explains the scenarios’ function is to “capture a range of potential greenhouse gas pathways and associated atmospheric concentration levels through 2100.”
It further states, “The resulting range of projections reflects, in part, the uncertainty that comes with quantifying future human activities and their influence on climate.”