James Hansen: 2012 9th warmest year in global record

Global Temperature Update Through 2012

by J. Hansen, M. Sato and R. Ruedy, January 15, 2013

Summary. Global surface temperature in 2012 was +0.56 °C (1 °F) warmer than the 1951-1980 base period average, despite much of the year being affected by a strong La Nina. Global temperature thus continues at a high level that is sufficient to cause a substantial increase in the frequency of extreme warm anomalies. The 5-year mean global temperature has been flat for a decade, which we interpret as a combination of natural variability and a slowdown in the growth rate of the net climate forcing.

An update through 2012 of our global analysis [1] (Fig. 1) reveals 2012 as having practically the same temperature as 2011, significantly lower than the maximum reached in 2010. These short-term global fluctuations are associated principally with natural oscillations of tropical Pacific sea surface temperatures summarized in the Nino index in the lower part of the figure. 2012 is nominally the 9th warmest year, but it is indistinguishable in rank with several other years, as shown by the error estimate for comparing nearby years. Note that the 10 warmest years in the record have all occurred since 1998.

The long-term warming trend, including continual warming since the mid-1970s, has been conclusively associated with the predominant global climate forcing, human-made greenhouse gases [2], which began to grow substantially early in the 20th century. The approximate stand-still of global temperature during 1940-1975 is generally attributed to an approximate balance of aerosol cooling and greenhouse gas warming during a period of rapid growth of fossil fuel use with little control on particulate air pollution, but satisfactory quantitative interpretation has been impossible because of the absence of adequate aerosol measurements [3,4].

Below we discuss the contributions to temperature change in the past decade from stochastic (unforced) climate variability and from climate forcings.

[I cannot post the figures, so please go to this link to read further:
http://www.columbia.edu/~jeh1/mailings/2013/20130115_Temperature2012.pdf]