The Fourth National Climate Assessment: Updating the Science of Climate Change

For the 4^th National Climate Assessment (NCA4), the focus was on how scientific understanding of climate change had advanced since the previous 2013 international and 2014 national assessments. NCA4 discusses how new observations and new research have increased our understanding of past, current, and future climate change. NCA4 Volume I on climate change science was published in November 2017 and we have updated these findings on the science in Chapter 2 of Volume II. Our findings confirm prior assessments in concluding that the climate on our planet, including the United States, is changing, and changing rapidly. Observational evidence for a changing climate abounds, from the top of the atmosphere to the depths of the oceans. Documented changes include increasing surface, atmospheric, and oceanic temperatures; melting glaciers; disappearing snow cover; shrinking sea ice; and rising sea level. Many lines of evidence demonstrate that human activities have been the dominant cause of the observed warming since the mid-20th century. Changes in the characteristics of extreme events are particularly important for human safety, infrastructure, agriculture, water quality and quantity, and natural ecosystems. Some extremes have already become more frequent, intense, or of longer duration, and many extremes are expected to continue to increase or worsen, presenting substantial challenges. Heatwaves have become more frequent in the United States since the 1960s, while extreme cold temperatures and cold waves have become less frequent. Heavy rainfall is increasing in intensity and frequency across the United States and globally. These and other trends in severe weather are expected to continue. The Earth’s climate is projected to continue to change over this century and beyond. As a result, global average sea levels are expected to continue to rise. In our analyses for NCA4, we also developed a weighting system for global climate models, and not only applied this to the assessment, but also developed statistically downscaled products for temperature and precipitation at about 6 km resolution across the continental United States – these analyses were then made available for use in the regional and sector impacts analyses in NCA4 Volume II. This presentation provides an overview of the findings from NCA4, with a focus on what’s new in the science.