DRAFT: DO NOT CITE, QUOTE, OR DISTRIBUTE Executive Summary
1 U.S. GLOBAL CHANGE RESEARCH PROGRAM
2 CLIMATE SCIENCE SPECIAL REPORT (CSSR)
3 Executive Summary
4 Introduction
5 New observations and new research have increased scientists’ understanding of past, current,
6 and future climate change since the Third U.S. National Climate Assessment (NCA3) was
7 published in May 2014. This Climate Science Special Report (CSSR) is designed to capture
8 that new information, build on the existing body of science, and summarize the current state
9 of knowledge.
10 Predicting how climate will change in future decades is a different scientific issue from
11 predicting weather a few weeks from now. Weather is what is happening in the atmosphere in
12 a given location at a particular time—temperature, humidity, winds, clouds, and precipitation.
13 Climate consists of the patterns exhibited by the weather—the averages and extremes of the
14 indicated weather phenomena and how those averages and extremes vary from month to
15 month over the course of a typical year—as observed over a period of decades. One can
16 sensibly speak of the climate of a specific location (for example, Chicago) or a region (for
17 example, the Midwest). Climate change means that these weather patterns—the averages and
18 extremes and their timing—are shifting in consistent directions from decade to decade.
19 The world has warmed (globally and annually averaged surface air temperature) by about
20 1.6°F (0.9°C) over the last 150 years (1865–2015), and the spatial and temporal non-
21 uniformity of the warming has triggered many other changes to the Earth’s climate. Evidence
22 for a changing climate abounds, from the top of the atmosphere to the depths of the oceans.
23 Thousands of studies conducted by tens of thousands of scientists around the world have
24 documented changes in surface, atmospheric, and oceanic temperatures; melting glaciers;
25 disappearing snow cover; shrinking sea ice; rising sea level; and an increase in atmospheric
26 water vapor. Many lines of evidence demonstrate that human activities, especially emissions
27 of greenhouse (heat-trapping) gases, are primarily responsible for recent observed climate
28 changes.
29 The last few years have also seen record-breaking, climate-related, weather extremes, as well
30 as the warmest years on record for the globe. Periodically taking stock of the current state of
31 knowledge about climate change and putting new weather extremes into context ensures that
32 rigorous, scientifically based information is available to inform dialogue and decisions at
33 every level.
34 Most of this special report is intended for those who have a technical background in climate
35 science and is also designed to provide input to the authors of the Fourth U.S. National
36 Climate Assessment (NCA4). In this executive summary, green boxes present highlights of
37 the main report followed by related bullet points and selected figures covering more scientific
