The UK Met Office seem determined to stand by their claim made in response to the David Rose article in the Mail on Sunday:
‘The linear trend from August 1997 (in the middle of an exceptionally strong El Nino) to August 2012 (coming at the tail end of a double-dip La Nina) is about 0.03°C/decade, amounting to a temperature increase of 0.05°C over that period.’
Several of us have been requesting statistical evidence via their blog that this trend is actually indistinguishable from flat. Dave Brittan has done a sterling job in replying on behalf of the Met Office, but he eventually crafted a complex answer as to whether the above statement made statistical sense.
“The first is measurement uncertainty associated with basic measurement error and uncertain biases in the observations. These are included in the HadCRUT4 ensemble, and when computing linear trends in global temperatures from August 1997 to August 2012 these give a trend of 0.034 ± 0.011 °C per decade (95% confidence interval) for the observed portion of the earth.”
I questioned this statement because I think their quoted error is actually about a factor 10 less than it should be. After waiting 36 hours with my post still in moderation, and with no other posts being accepted I am now presuming that this is their last word on the matter. Frustrated by the lack of response, I decided instead to do the analysis myself.
The result gives a trend of 0.03 ± 0.02 °C, where the error is one standard deviation (~60% confidence) – so essentially flat. Notice also that the correlation factor R of the data with a linear trend is anyway very poor – just 0.12, demonstrating just how large the non-random monthly variations exist in the data. I am now convinced that the trend is statistically consistent with zero. To emphasize this point, I now show exactly the same analysis done for HADCRUT3 which up to a few months ago was the flagship IPCC data as used in AR4.
The result is a “cooling” trend of -0.016 ± 0.02 °C. To state that the world has “cooled” over the last 16 years is just as meaningless as for the MET Office to claim it has “warmed”. The only rational scientific statement that can be made is that global temperatures have not significantly changed since August 1997. The MET Office should just acknowledge this fact and thereby maintain their scientific integrity.
See below for a comparison of Hadcrut3 & Handrut4. More discussion on their differences can be found here
P.S. The bottom line is that if your eye cannot distinguish any underlying trend – no amount of statistics will ever convince you otherwise. There has been evidence of past warming from the beginning of the 1970s until about 1995. However since then the data clearly show us that temperatures have since remained static.