In its latest report, the United Nation’s Intergovernmental Panel on Climate Change (IPCC) states that it is “extremely likely” that greenhouse gas (GHG) emissions caused by human activities are responsible for more than half the observed increase in global average surface temperature from 1951 to 2010.  While this assessment stops short of IPCC’s highest probability level, it represents the highest level of probability (95 percent to 100 percent) the IPCC has yet expressed on the significant impact of “anthropogenic forcings” on the earth’s climate.

The new human attribution estimate was included in IPCC’s 5th assessment report (Climate Change 2013:  The Physical Science Basis).  The previous IPCC report issued in 2007 found that human activities were very likely (90 percent to100 percent probability) causing more climate variability than natural causes.  The highest level, virtually certain, represents 99 percent to 100 percent probability while likely represents 66 percent to 100 percent probability. 

All continents, except Antarctica

Other statements in the 5th assessment regarding anthropogenic forcings include the following:

• GHGs contributed a global mean surface warming likely to be in the range of 0.5°C to 1.3°C between 1951 and 2010, with the contributions from other anthropogenic forcings, including the cooling effect of aerosols, likely to be in the range of -0.6°C to 0.1°C.  The contribution from natural forcings is likely in the range of -0.1°C to 0.1°C. 
• Over every continental region except Antarctica, anthropogenic forcings have likely made a substantial contribution to surface temperature increases since the mid-20th century.
• Anthropogenic influences have very likely contributed to Arctic sea ice loss since 1979.
• For Antarctica, large observational uncertainties result in low confidence that anthropogenic forcings have contributed to the observed warming.
• Anthropogenic influences likely contributed to the retreat of glaciers since the 1960s and to the increased surface mass loss of the Greenland ice sheet since 1993.
• It is very likely that anthropogenic forcings have made a substantial contribution to increases in global upper ocean heat content (0 to 700 meters) observed since the 1970s.
• It is likely that anthropogenic influences have affected the global water cycle since 1960.  Anthropogenic influences have contributed to observed increases in atmospheric moisture content in the atmosphere (medium confidence —about a5 out of 10 chance of being correct), to global-scale changes in precipitation patterns over land (medium confidence), to intensification of heavy precipitation over land regions where data are sufficient (medium confidence), and to changes in surface and subsurface ocean salinity (very likely).
• It is now very likely that human influence has contributed to observed global scale changes in the frequency and intensity of daily temperature extremes since the mid-20th century.
• It is very likely that there is a substantial anthropogenic contribution to the global mean sea level rise since the 1970s.

Second opinion

The 5th assessment was preceded by 1 week by a 1,000-page report from the Nongovernmental International Panel on Climate Change (NIPCC), a panel of scientists assembled to provide a second opinion on IPCC assessments.  The NIPCC report was published by the Heartland Institute, which states in a news release that the IPCC is a government agency whose “mission is to find a human impact on climate.”  Heartland emphasizes that the NIPCC comprises “independent scientists.”

Heartland continues: “The NIPCC report finds the human impact on climate is very small, and as a result, any warming that may be due to human greenhouse gas emissions is likely to be so small as to be invisible against a background of natural variability.  The authors of the NIPCC study do not believe man-made global warming is a crisis or that scientists know enough about how the climate works to make policy relevant recommendations to the world’s government leaders.”

Climate Change 2013:  The Physical Science Basis