A recent press release by the World Meteorological Organisation (WMO) described recent global temperature changes, and highlighted extreme weather in the 2001-2010 period. Much of the press release is good, but here I will examine the accuracy of two statements.
WMO Secretary-General Michel Jarraud: “WMO’s report shows that global warming was significant from 1971 to 2010 and that the decadal rate of increase between 1991-2000 and 2001-2010 was unprecedented.”
“The decadal rate of increase in the global temperature accelerated between 1971 and 2010.”
This first is not a very clear phrase. What does ‘significant’ mean, and what does a ‘decadal rate of increase’ mean? But, it suggests that the increase from the average of 1991-2000 to the average of 2001-2010 was unprecedented, and the second phrase suggests an acceleration in the rate of increase in global temperatures. These statements are misleading.
The figure below shows a similar bar graph to that used by the WMO showing averages of particular 10-year periods using HadCRUT4. The top panel shows the changes using the same definition as the WMO, with decades finishing with years ending in zero (i.e. 2001-2010, 1991-2000 etc). The largest change from decade to decade is indeed the last change, at +0.21K.
The bottom panel repeats the analysis but defining decades to end in a two (i.e. 2003-2012, 1993-2002 etc). Now, the largest change (or even second or third largest) change is not to the most recent decade. And, in fact, the largest observed decadal increase is actually from the average of 1987-1996 to the average of 1997-2006, at +0.24K.
Note firstly that different temperature datasets will give slightly different warming rates. However, 2001-2010 is the warmest 10-year period in the instrumental record. This is evidence enough of a warmer climate, but NOT of an accelerated warming rate.