The US Global Change Research Program (USGCRP) has released the Climate Science Special Report (CSSR or the Report). Published on November 3, 2017, this US-focused authoritative climate change science assessment serves as a foundation for assessing climate-related risks and informing decision-making.

As Volume I of the Fourth National Climate Assessment (NCA4), the CSSR provides:

  • Updated analyses of climate change findings
  • Executive summary and other materials providing the basis for discussion of climate science found in Volume II (forthcoming in 2018)
  • Foundational information and projections for climate change ranging from “likely” to “worst case” scenarios

A previous draft of the CSSR was leaked to the New York Times in August 2017, in what some commentators saw as a way to prevent the Trump Administration from drastically altering the final version’s findings . With the exception of wording changes intended to water down some of the report’s conclusions, the final CSSR’s substance and core discussion was not materially different from the August version — and reports indicate that there was no political interference with its drafting. For more information, please see Lisa Friedman and Glenn Thrush’s New York Times commentary, U.S. Report Says Humans Cause Climate Change, Contradicting Top Trump Officials. The CSSR comes on the heels of the Government Accountability Office’s climate change report, released on October 23, 2017 [GAO Releases Climate Change Report] which provides economic valuations, and policy recommendations associated with US climate change risks.

CSSR: Background and Overview

In accordance with the Global Change Research Act of 1990, the USGCRP is required to conduct national climate assessments (NCA) at regular intervals. The National Oceanic and Atmospheric Administration (NOAA) serves as the lead agency in preparing the Report, with assistance from the USGCRP’s 13 agencies and contributions from several US academic institution representatives. Since the 2014 Third National Climate Assessment (NCA3), the CSSR states that stronger evidence has emerged for “continuing, rapid, human-caused warming of the global atmosphere and ocean.” NCA4 includes stronger information regarding a variety of new climate effects, including:

  • Atmospheric circulation and extreme events
  • Higher-resolution global climate model simulations
  • Ocean acidification, warming, and oxygen loss
  • Geographic variation for sea level rise
  • Ice-sheet loss and low-sea ice areal extent
  • Potential climate related surprises
  • Important climate science aspects relevant to long-term temperature goals and different mitigation scenarios (including those implied by government announcements for the Paris Agreement)

The CSSR concludes, “it is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century.” The CSSR presents future climate condition projections by utilizing a range of “plausible future scenarios” generated by the Intergovernmental Panel on Climate Change (IPCC) . The CSSR’s worst-case scenario projection — representative concentration pathway (RCP) 8.5 — depicts a future with continued high emissions growth, while other RCPs represent varying pathways for mitigating future emissions. Each chapter of the Report includes key findings based on the authors’ expert judgment of the synthesis of the assessed literature . With each key finding, the CSSR provides its level of confidence ranging from “low” (inconclusive evidence, disagreement, or lack of opinions among experts) to “very high” (strong evidence, high consensus).

Global and US Temperatures Continue to Rise

The CSSR states that the “global, long-term and unambiguous warming trend has continued during recent years,” and, post-NCA3 , the years 2014-2016 successively have become the warmest on record. Recent data has only added to the weight of evidence for rapid global-scale warming, the dominance of human causes, and the expected continuation of increasing temperatures. The CSSR finds with very high confidence that annual average temperature in the contiguous US has risen by 1.8ºF (1ºC) from 1901-2016 — and is projected to continue rising . The CSSR projects that the magnitude of temperature increases in the coming decades depend on the amount of greenhouse gases emitted globally and the remaining uncertainty about the sensitivity of the Earth’s climate to those emissions.

With significant reductions in emissions, global temperature rise may possibly be limited to 3.6ºF (2ºC). However, without such major reductions, global temperature increases relative to preindustrial times could reach 9ºF (5ºC) by 2100. Furthermore, if emissions were stabilized at current levels, the existing concentration of greenhouse gases would highly likely increase global temperature by at least an additional 1.1ºF (6ºC) over this century (medium confidence in amount of temperature increase). Under all scenarios analyzed, the CSSR projects an increase of 2.5ºF (1.4ºC) in annual average temperatures for the contiguous US for the period of 2021-2050, with much larger increases likely for the latter half of the century. The CSSR also notes that, with temperature increases, the urban heat island effect is likely to exacerbate — further increasing temperatures across densely populated areas.

Many Temperature and Precipitation Extremes Are Increasing

The CSSR finds with very high confidence that the contiguous US has experienced “marked changes in temperature extremes.” Citing temperature and precipitation extremes’ effects on “water quality and supply, agricultural productivity, human health, vital infrastructure, iconic ecosystems and species, and the likelihood of disasters,” the CSSR concludes that such extremes have continuously increased — and likely will continue to do so. The CSSR finds that:

  • The number of high temperature records set in the past two decades far exceeds the number of low temperature records (very high confidence).
  • The frequency of cold waves has decreased while heat wave frequency has increased (very high confidence).
  • The intensity of heavy precipitation events is increasing across much of the globe (high confidence).
  • Heavy precipitation events in most parts of the US have increased in intensity and frequency since 1901 (high confidence).
  • The northeastern US likely will see the largest increase in heavy precipitation events (high confidence).

Furthermore, the CSSR finds that snow cover extent has declined and snowmelt has quickened across North America, while extreme snowfall events have proliferated. The CSSR projects that the Western US will see more rainfall than snowfall with correspondingly shallower snowpack, increased risk for long-duration drought, and increased risk of wildfires. The CSSR continues that hurricanes in the Atlantic, eastern North Pacific, and western North Pacific likely will continue to increase in precipitation rates and intensity. Additionally, the CSSR notes that tornado activity has become more variable in recent years. This section concludes that extreme temperatures in the contiguous US are likely to increase even more than average temperatures.

Oceans are Rising, Warming, and Becoming More Acidic

The CSSR finds with very high confidence that the “world’s oceans have absorbed about 93% of the excess heat caused by greenhouse gas warming since the mid-20th century, making them warmer and altering global and regional climate feedbacks.” Under higher emission scenarios, the CSSR projects that the global increase in average sea surface temperature could rise by as much as 4.9º ± 1.3º F by 2100. Additionally, the CSSR finds that global mean sea level has risen already by about seven to eight inches since 1900 (with three of those inches occurring since 1993), and that sea level is very likely to rise an additional 1-4.3 feet by 2100 — but an eight-feet increase or more by 2100 is physically possible.

Sea level rise in this century will vary along the US coastline due to variability in the Earth’s gravitational field, rotation from melting of land ice, changes in ocean circulation, and vertical land motion. The largest sea level increases likely will occur in the US Northeast and western Gulf of Mexico, though sea level rise is likely to be higher than the US global average outside of Alaska. Concurrent with sea level rise, the CSSR projects that coastal flooding rates will increase. Furthermore, the CSSR warns that the potential slowing of the Atlantic meridional overturning circulation — as a result of increasing ocean heat content and freshwater-driven buoyancy changes — could have “dramatic climate feedbacks” with difficult-to-predict consequences. Finally, the CSSR reports that the oceans have become significantly and increasingly more acidic and oxygen-depleted. Ocean acidification currently affects the oceans off the Pacific Northwest and northern New England most heavily.

Climate Change in Alaska and Across the Arctic Continues to Outpace Global Climate Change

The CSSR finds with very high confidence that Alaskan and Arctic air temperatures have increased at a rate more than twice the global average. Also, since the early 1980s, annual average arctic sea ice has decreased in extent, become thinner, and is melting on more days per year. Arctic sea ice loss is projected to continue throughout the 21st century, resulting in “nearly sea ice-free late summers by the 2040s.” The Report also cites that Alaskan permafrost has increasingly thawed, presenting uneasy permafrost-carbon feedback potential of uncertain magnitude and consequences. According to the CSSR, human activities have very likely contributed to “observed arctic surface temperature warming, sea ice loss, glacier mass loss, and northern hemisphere snow extent decline.”

Limiting Globally Averaged Warming to 2ºC (3.6ºF) will Require Major Emissions Reductions

Citing slowing emission growth rates in 2014 and 2015 as the global economy has become less carbon-intensive, the CSSR still finds with high confidence that “[e]ven if this slowing trend continues, however, it is not yet at a rate that would limit the increase in the global average temperature to well below the 3.6ºF (2ºC) above preindustrial levels.” The Report continues, “[r]educing net emissions of CO2 is necessary to limit near-term climate change and long-term warming,” adding that choices made today will determine the extent of climate change risks far into the future. The Report finds that limiting temperature increase through the latter half of this century may even require net negative emissions. Significantly, the CSSR addresses the Paris Agreement, stating:

“Achieving global greenhouse gas emissions reductions before 2030 consistent with targets and actions announced by governments in the lead up to the 2015 Paris climate conference would hold open the possibility of meeting the long-term temperature goal of limiting global warming to 3.6ºF (2ºC) above preindustrial levels, whereas there would be virtually no chance if net global emissions followed a pathway well above those implied by country announcements. Actions in the announcements are, by themselves, insufficient to meet a 3.6ºF (2ºC) goal; the likelihood of achieving that depends strongly on the magnitude of global emissions reductions after 2030.” (Finding: high confidence).

Unanticipated Changes Significantly Possible

Ominously, the CSSR finds that “unanticipated and difficult or impossible-to-manage changes in the climate system are possible throughout the next century as critical thresholds are crossed and/or multiple climate-related extreme events occur simultaneously.” The probability of these types of events increases as human influence on climate systems increases. The CSSR warns of the potential for positive feedbacks to accelerate human-induced climate change, and perhaps even shift the Earth’s climate system into new states that are “very different from those experienced in the recent past.” Finally, the Report suggests that that climate models are more likely to underestimate, rather than overestimate, the amount of long-term future changes in global climate and temperature.