US national climate science report finds 'no convincing alternative' to man-made global warm

The US Global Change Research Program has released the first volume of the Fourth National Climate Assessment, The Climate Science Special Report, concluding that 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”.

The hard-hitting report coincides with the opening of COP23 in Bonn, in which the US Government is not participating following President Trump's withdrawal from the Paris agreement.

With input from 13 federal agencies, the report states that the global average surface air temperature has increased by about 1.8°F (1.0°C) over the last 115 years (1901–2016), and this period is the warmest in the history of modern civilisation.

“For the warming over the last century, there is no convincing alternative explanation supported by the extent of the observational evidence.”

The 477-page report provides an overview of the state of science relating to climate change and its physical impacts, with a focus on the United States, designed to serve as the foundation for efforts to assess climate-related risks and inform decision-making about responses.

Its preparation was led by the National Oceanic and Atmospheric Administration (NOAA) with oversight by the CSSR Federal Science Steering Committee which has representatives from three agencies (NOAA, the National Aeronautics and Space Administration [NASA], and the Department of Energy [DOE]); USGCRP; and three Coordinating Lead Authors, all of whom were Federal employees during the development of the report.

The assessment conclude that In addition to warming, 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.”

It also noted that 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.

“Human-caused climate change has made a substantial contribution to this rise since 1900, contributing to a rate of rise that is greater than during any preceding century in at least 2,800 years. Global sea level rise has already affected the United States; the incidence of daily tidal flooding is accelerating in more than 25 Atlantic and Gulf Coast cities.”

Other key findings are:

  • 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

  • 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 i 2.5°F for the United States, relative to the recent past (average from 1976–2005), under all plausible future climate scenarios.

  • 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. Even if this slowing trend continues, however, it is not yet at a rate that would limit global average temperature change to well below 3.6°F (2°C) above preindustrial levels.

The Climate Science Special Report is available here

It is part of the quadrennial assessment mandated in the Global Change Research Act of 1990, and was accompanied by the release for public comment of the draft form of Volume II, Climate Change Impacts, Risks, and Adaptation in the United States, and the draft 2nd State of the Carbon Cycle Report (SOCCR-2) by the National Academies of Sciences, Engineering and Medicine. More information is at

Subscribe to The Environment Report