Here are the report’s three most chilling conclusions:

1. Extreme hot weather is getting more common, and cold weather more rare.

In its first chapter, the National Climate Assessment reports that heat-wave season has expanded by more than 40 days since the 1960s. In the bleakest scenario of unchecked climate change, Phoenix could have as many as 150 days per year above 100 degrees Fahrenheit by the end of the century.

Over the past two decades, the number of high temperature records recorded in the United States far exceeds the number of low temperature records. The length of the frost-free season, from the last freeze in spring to the first freeze of autumn, has increased for all regions since the early 1900s. The frequency of cold waves has decreased since the early 1900s, and the frequency of heat waves has increased since the mid-1960s. Over timescales shorter than a decade, the 1930s Dust Bowl remains the peak period for extreme heat in the United States for a variety of reasons, including exceptionally dry springs coupled with poor land management practices during that era.

Over the next few decades, annual average temperature over the contiguous United States is projected to increase by about 2.2°F (1.2°C) relative to 1986–2015, regardless of future scenario. As a result, recent record-setting hot years are projected to become common in the near future for the United States. Much larger increases are projected by late century: 2.3°–6.7°F (1.3°–3.7°C) under a lower scenario … and 5.4°–11.0°F (3.0°–6.1°C) under a higher scenario … relative to 1986–2015. Extreme high temperatures are projected to increase even more than average temperatures. Cold waves are projected to become less intense and heat waves more intense. The number of days below freezing is projected to decline, while the number of days above 90°F is projected to rise.

2. Climate change has doubled the devastation from wildfires in the Southwest.

According to the report, human-caused climate change has heated and dried out the American Southwest, leading to deaths, enormous costs, and lingering health consequences.

Climate change has led to an increase in the area burned by wildfire in the western United States. Analyses estimate that the area burned by wildfire from 1984 to 2015 was twice what would have burned had climate change not occurred. Furthermore, the area burned from 1916 to 2003 was more closely related to climate factors than to fire suppression, local fire management, or other non-climate factors.

national climate assessment

Climate change has driven the wildfire increase, particularly by drying forests and making them more susceptible to burning. Specifically, increased temperatures have intensified drought in California, contributed to drought in the Colorado River Basin, reduced snowpack, and caused spring-like temperatures to occur earlier in the year. In addition, historical fire suppression policies have caused unnatural accumulations of understory trees and coarse woody debris in many lower-elevation forest types, fueling more intense and extensive wildfires. Wildfire can threaten people and homes, particularly as building expands in fire-prone areas. Wildfires around Los Angeles from 1990 to 2009 caused $3.1 billion in damages (unadjusted for inflation). Respiratory illnesses and life disruptions from the Station Fire north of Los Angeles in 2009 cost an estimated $84 per person per day (in 2009 dollars). In addition, wildfires degraded drinking water upstream of Albuquerque with sediment, acidity, and nitrates and in Fort Collins, Colorado, with sediment and precursors of cancer-causing trihalomethane, necessitating a multi-month switch to alternative municipal water supplies.

3. Rising sea levels will necessitate mass migrations, and coastal cities aren’t doing enough.