US government climate report: Climate change is real and our fault

Information about the science and consequences of climate change has been removed from a number of federal agency websites since the Trump administration took over. But some agencies like NASA seem to have continued their work unhindered. And today saw the release of the fourth National Climate Assessment—an official summary of the current state of knowledge about climate change.

The heavily peer-reviewed report, following the last edition in 2014, is coordinated by NOAA, NASA, the Department of Energy, the Environmental Protection Agency, and the US Global Change Research Program. A group of US climate scientists volunteered to write the report, which gathers together the most recent peer-reviewed research into digestible conclusions about the causes and impacts of climate change.

A June 2017 draft was shared with The New York Times by someone who feared it might be censored by federal agencies during the final approval process. But in a call with media, NOAA’s David Fahey (one of three coordinating lead authors of the report) responded to questions about censorship by saying he was “quite confident” that there had been no political interference with the contents of the report. An initial review of the highlighted main points of the report’s “executive summary” shows only a few insignificant wording changes from the June 2017 draft.

Carl Mears, who helps run Remote Sensing System's satellite temperature dataset and was a lead author on the report, told Ars, "I didn't see any signs of political interference for the chapters I was involved in. The comments from the agencies appeared to be from scientists, and most if not all were directed toward making the report clearer and easier to understand."

Similar to the structure of the Intergovernmental Panel on Climate Change assessment reports, this first section of the Climate Assessment focuses on the physical science of climate change. The second half—which has not yet been finalized—will cover the impacts of climate change on the United States.

The conclusions are also largely similar to those of the last IPCC report, but some are stated a little more clearly—perhaps in response to the deliberate misrepresentation of some of those conclusions by politicians and “skeptics.”

The topline conclusion is obviously the degree to which observed global warming is human-caused, and the report pulls no punches: “Many lines of evidence demonstrate that it is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century. Over the last century, there are no convincing alternative explanations supported by the extent of the observational evidence.”

Specifically, the report quantifies the amount of human-caused warming in the period from 1951 to 2010: between 0.6-0.8 degrees Celsius (1.1-1.4 degrees Fahrenheit). Its best estimate of the total temperature change for that same time period is right within that range, at 0.65 degrees Celsius (1.2 degrees Fahrenheit). (Note that the warming influence of human activities can be larger than the actual change if natural factors would have caused cooling.)

The report also summarizes the evidence for changes in extreme weather of various types. Heat waves and intense rainfall events are up in most places, for example, while trends in tornadoes are unclear. Looking forward, it notes that the frequency and severity of “atmospheric river” weather patterns hitting the West Coast are expected to increase. Snowpack in the West (key to water supplies) is expected to shrink.

On the topic of wildfires and their relationship to weather patterns, the report finds that the “incidence of large forest fires in the western United States and Alaska has increased since the early 1980s (high confidence) and is projected to further increase in those regions as the climate warms.”

The report’s sea level rise projections are notably frank about the uncertainty surrounding the worst-case scenario. While it projects 0.3 to 1.3 meters (1 to 4.3 feet) of sea level rise over the 21st century, it notes that “eight feet by 2100 is physically possible, although the probability of such an extreme outcome cannot currently be assessed.”

If you’re wondering what could have changed since the last report in 2014, the new Assessment highlights a list of areas in which our understanding has improved. That includes the evaluation of the human contribution to individual extreme weather events, higher-resolution climate models producing better simulations of things like hurricanes, and studies of ice loss in Greenland and Antarctica that have bumped the worst-case sea level rise estimates upward.

While the report doesn’t speak to potential climate policies, it does explain that existing emissions pledges are not sufficient to limit global warming to 2 degrees Celsius (3.6 degrees Fahrenheit)—long an international goal. And it provides this sobering reminder: “The present-day emissions rate of nearly 10 [billion tons of carbon] per year suggests that there is no climate analog for this century any time in at least the last 50 million years.” In other words, if the planet has ever seen anything like this, it probably predates the end of the dinosaurs.

In a statement, the White House said, "The climate has changed and is always changing. As the Climate Science Special Report states, the magnitude of future climate change depends significantly on 'remaining uncertainty in the sensitivity of Earth's climate to [greenhouse gas] emissions.'"

This is true, but only to the extent that the projected warming between about 2000 and 2100 in the highest emissions scenario (for example) is 2.6 to 4.8 degrees Celsius, which includes that uncertainty. But that means we can be pretty certain it's going to get a lot warmer. As the report actually says, the majority of the uncertainty is in the future trajectory of our emissions: "Global climate is projected to continue to change over this century and beyond. The magnitude of climate change beyond the next few decades will depend primarily on the amount of greenhouse (heat-trapping) gases emitted globally and on the remaining uncertainty in the sensitivity of Earth’s climate to those emissions."

As the report makes clear, there is no reasonable doubt remaining that climate change is a story about human actions—not natural cycles.