4th National Climate Assessment, Volume I: Climate Science Special Report

The Climate Science Special Report is Volume I of the Fourth National Climate Assessment (NCA4). This technical document is the physical science foundation for the assessment, providing an updated analysis of climate change effects across the U.S. The National Climate Assessments are congressionally mandated to assess and inform the nation about the impacts of climate change. The third National Climate Assessment was released in 2014, and NCA4 builds upon this analysis with updated detail. 

The report discusses several of the most recent advancements in climate science, and concludes, as in prior editions, that human activities are largely responsible for global warming and climate change. Profound impacts across the continent are in motion, and will become more severe unless the world’s industrial nations significantly reduce emissions of greenhouse gases.

This report is a comprehensive and updated assessment of the current knowledge around climate change impacts on the U.S., including observed and future projected changes in temperatures, precipitation patterns, extreme-weather events (droughts, floods, wildfires and storms), sea-level rise, and ocean acidification. Climate science is translated also to describe the impacts of climate change on the Arctic, and these effects on Alaska and the rest of the U.S. Climate models that are utilized for these impact projections are explained in common language and detail. 

Some of the major findings include:

  • This period is now the warmest in the history of modern civilization; and the last three years have been the warmest years on record for the globe. 
  • Global average sea level has risen by about 7–8 inches since 1900, with almost half (about 3 inches) of that rise occurring since 1993.
  • The incidence of large forest fires in the western states and Alaska has increased since the early 1980s and is projected to further increase in those regions as the climate changes, with profound changes to regional ecosystems.
  • Annual trends toward earlier spring melt and reduced snowpack are already affecting water resources in the western U.S.

A chapter on “Potential Surprises: Compound Extremes and Tipping Elements” brings into perspective both the potential magnitude of compounding impacts, and the reality of uncertainty in predicting these extreme outcomes. Increasing the likelihood of compound extremes, in which multiple events occur simultaneously or in rapid sequence (such as paired extremes of droughts and intense rainfall), can be a surprise in that it is often not predicted by analyses that focus solely on one type of extreme. A second type of surprise discussed relates to self-reinforcing cycles, which can give rise to “tipping elements” - that occur for example with ice sheets, patterns of ocean circulation, and large-scale ecosystems like the Amazon rainforest.


Publication Date: November 3, 2017

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