Link to entire 477 page downloadable PDF
This period is now the warmest in the history of modern civilization.
Human activities, especially emissions of greenhouse gases, are the dominant cause of the observed warming since the mid-20th century
Thousands of studies conducted by researchers around the world have documented changes in surface, atmospheric, and oceanic temperatures; melting glaciers; diminishing snow cover; shrinking sea ice; rising sea levels; ocean acidification; and increasing atmospheric water vapor.
Global average sea level has risen by about 7–8 inches since 1900. The incidence of daily tidal flooding is accelerating in more than 25 Atlantic and Gulf Coast cities.
Global average sea levels are expected to continue to rise—by at least several inches in the next 15 years and by 1–4 feet by 2100. A rise of as much as 8 feet by 2100 cannot be ruled out.
Heavy rainfall is increasing in intensity and frequency across the United States and globally and is expected to continue to increase.
Heatwaves have become more frequent in the United States since the 1960s, while extreme cold temperatures and cold waves are less frequent.
Over the next few decades (2021–2050), annual average temperatures are expected to rise by about 2.5°F for the United States, relative to the recent past (average from 1976–2005), under all plausible future climate scenarios.
The incidence of large forest fires in the western United States and Alaska has increased since the early 1980s and is projected to further increase.
Annual trends toward earlier spring melt and reduced snowpack are already affecting water resources in the western United States.
Chronic, long-duration hydrological drought is increasingly possible before the end of this century.
The magnitude of climate change beyond the next few decades will depend primarily on the amount of greenhouse gases (especially carbon dioxide) emitted globally.
With significant reductions in emissions, the increase in annual average global temperature could be limited to 3.6°F (2°C) or less.
The global atmospheric carbon dioxide (CO2) concentration has now passed 400 parts per million (ppm), a level that last occurred about 3 million years ago, when both global average temperature and sea level were significantly higher than today.
In 2014 and 2015, emission growth rates slowed as economic growth became less carbon-intensive.