U.S. government report concludes human activities responsible for warming climate

The latest Climate Science Special Report, Fourth National Climate Assessment (NCA4), Volume I has been published. This report provides an authoritative assessment of climate change, with a focus on the United States by 13 federal agencies.

Human activities forcing climate change
Global annual average radiative forcing change from 1750 to 2011 due to human activities, changes in total solar irradiance, and volcanic emissions. Black bars indicate the uncertainty in each. Radiative forcing is a measure of the influence a factor (such as greenhouse gas emissions) has in changing the global balance of incoming and outgoing energy. Radiative forcings greater than zero (positive forcings) produce climate warming; forcings less than zero (negative forcings) produce climate cooling.

It finds that global annually averaged surface air temperature has increased by about 1.8°F (1.0°C) over the last 115 years (1901–2016) and concludes that, based on extensive evidence, it is extremely likely that human activities, especially emissions of greenhouse gases, are the dominant cause of the observed warming since the mid-20th century. For the warming over the last century, there is no convincing alternative explanation supported by the extent of the observational evidence.

In addition to warming, NCA4 presents evidence how many other aspects of global climate are changing, primarily in response to human activities. 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, with almost half (about 3 inches) of that rise occurring since 1993. The incidence of daily tidal flooding is accelerating in more than 25 Atlantic and Gulf Coast cities.
  • Heavy rainfall is increasing in intensity and frequency across the United States. The largest observed changes in the United States have occurred in the Northeast.
  • Heatwaves have become more frequent in the United States since the 1960s, while extreme cold temperatures and cold waves are less frequent.
  • The incidence of large forest fires in the western United States and Alaska has increased since the early 1980s.
  • Annual trends toward earlier spring melt and reduced snowpack are already affecting water resources in the western United States.
  • 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.

The National Oceanic and Atmospheric Administration (NOAA) serves as the administrative lead agency for the preparation of NCA4. The CSSR Federal Science Steering Committee has representatives from three NOAA, the National Aeronautics and Space Administration (NASA), and the Department of Energy (DOE), the U.S. Global Change Research Program (USGCRP) and three Coordinating Lead Authors, all of whom were Federal employees during the development of this report. 30 Lead Authors, all scientists from Federal agencies, national laboratories, universities, and the private sector were selected by the steering committee. An early draft was released for public comment 15 December 2016 – 3 February 2017. A draft was peer-reviewed by an expert panel at the National Academy of Sciences in April 2017. NOAA’s final draft was published on the website of The New York Times August 7, 2017.

USGCRP, 2017: Climate Science Special Report: Fourth National Climate Assessment, Volume I [Wuebbles, D.J., D.W. Fahey, K.A. Hibbard, D.J. Dokken, B.C. Stewart, and T.K. Maycock (eds.)]. U.S. Global Change Research Program, Washington, DC, USA, 470 pp, doi: 10.7930/J0J964J6.

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