Misleading temperature trends (1)


On page 5 of the AR4 WG1 SPM the IPCC states:

"Eleven of the last twelve years (1995–2006) rank among the 12 warmest years in the instrumental record of global surface temperature (since 1850). The updated 100-year linear trend (1906 to 2005) of 0.74°C [0.56°C to 0.92°C] is therefore larger than the corresponding trend for 1901 to 2000 given in the TAR of 0.6°C [0.4°C to 0.8°C]."

[TAR refers to the IPCC's Third Assessment Report, published in 2001]. Thus, the IPCC is claiming that the difference between these trends is caused by the recent warm years. But a glance at their own graph shows that this is a false and misleading claim by the IPCC:

(Ignore, if you can, the grossly deceptive straight lines that have already been discussed). Notice that there was a sharp decrease in temperatures from about 1900-1910.  So by starting the trend in 1905 when temperatures were significantly lower than in 1900, the IPCC is able to create a greater trend. The higher trend is primarily due to the lower starting point, not the recent warm years.

As proof of this, we can see that in the first draft of the SPM (the version that was scrutinised by scientists) the corresponding statement was the much more honest comparison

"For the period from 1901-2005 it is 0.65"

After the scientific review process, the IPCC replaced this with the misleading version given at the top of the page for the final draft. This final draft was only commented on by government representatives. Nevertheless, three of the governments picked up on the IPCC's misleading comparison:

Government of China: "Suggest to give the linear trend at the time of 1901-2005 in order to compare with the trend of the TAR".

Government of Belgium: "This is a peculiar way to report temperature changes".

Government of Russia: "The term 'therefore' is to be excluded". 

These three valid criticisms were completely ignored by the IPCC, who published the misleading version in their final report. However, it is interesting to note that in the SPM of the AR4 Synthesis Report, the same sentence appears without the misleading and incorrect word 'therefore'. Perhaps someone at the IPCC had a pang of conscience.

Summary: The IPCC claims that the greater trend from 1906-2005 compared with 1901-2000 is due to recent warm years, but in fact it is due to a sharp drop in temperatures from 1901-1906. The IPCC has erased a cooling period (1901-1906) from the record and replaced it with the essentially flat period from 2001-2006.

In fact, the IPCC claim is even more misleading than this, as we shall see in the next entry...