Last updated July 17, 2018

Global Warming

From 1880 to 2012, the Earth warmed by 1.5°F (0.85°C) dramatically reversing a long term millennial cooling trend. The physics behind the Earth’s temperature imbalance is well established. Scientists first determined in the 19th century that so-called greenhouse gases in the atmosphere hold extra heat in the atmosphere and have demonstrated since the late 1970s that greenhouse gas emissions by humans (mainly through burning of fossil fuels) are now increasing this effect, trapping more of the Earth’s infrared energy close to the surface. The atmospheric concentration of greenhouse gases has increased from 280 parts per million in preindustrial times to what is now over 400 ppm.

Climate science at a glance

  • Our planet is warming. Scientists know it is warming from temperature gauges, melting ice sheets, rising sea levels, changing seasons, and dozens more indicators.

  • Human activities, primarily burning fossil fuels and deforestation, load the atmosphere with CO2 and other greenhouse gas pollution. Multiple lines of evidence show that CO2 levels (the highest in millions of years) are driving global warming.

  • Greenhouse gases absorb and trap heat close to the Earth’s surface, much like adding a blanket around the planet. Once trapped in the atmosphere, CO2 pollution is essentially permanent on human timescales. As a result, the warming is also permanent unless the CO2 pollution is pulled back out of the atmosphere. Temperatures will otherwise continue to increase until there is zero CO2 pollution.

  • Global warming has already caused higher air and ocean temperatures, rising sea levels, changing seasons and an increase in the frequency and intensity of some extreme weather events like heat waves and heavy rainstorms.

US warming trends


  • U.S. average temperature has increased by 1.3°F to 1.9°F since 1895, and most of this increase has occurred since 1970.[1]

  • The 2000 to 2010 decade was the nation’s and the world’s hottest on record,[1] and 2012 and 2015 were the first and second hottest years on record in the continental United States.

Studies attribute warming in the US to human actions


  • (Diffenbaugh et al. 2017) formally identify the influence of anthropogenic warming in the observed trend of increasing heat extremes over the western US.[2]
  • (Lehner et al. 2018) find that warming in the southwestern US is largely due to greenhouse gas forcing.[3]
  • (Karl et al. 2008) conclude that anthropogenic greenhouse gas emissions have increased average temperature in North America.[4]
  • (Karoly et al. 2003) find that temperature changes from 1950 to 1999 in North America were unlikely to be due to natural climate variation alone.[5]

Global warming trends


  • Each of the last three decades has been successively warmer at the Earth’s surface than any preceding decade since 1850. The period from 1983 to 2012 was likely the warmest 30-year period of the last 1400 years in the Northern Hemisphere.[6]

  • The globally averaged combined land and ocean surface temperature a warming of 1.5°F (0.85°C) over the period 1880 to 2012.[6]

  • Warming of the climate system is unequivocal, and since the 1950s, many of the observed changes are unprecedented over decades to millennia. The atmosphere and ocean have warmed, the amounts of snow and ice have diminished, sea level has risen, and the concentrations of greenhouse gases have increased.[6]

Studies attribute global warming to human actions


  • (US NCA 4) The likely range of the human contribution to the global mean temperature increase over the period 1951–2010 is 1.1° to 1.4°F. This translates to a likely human contribution of 92 to 123 percent of the observed 1951–2010 change.[7]
  • (Gutowski et al. 2008) state that the global warming of the past 50 years is due primarily to human-induced increases in heat-trapping gases.[5]