The 'which years are the hottest?' non-conspiracy

Many climate change denialists argue that the concern of scientists about human induced climate change is a hoax or conspiracy.

Ironically, two of the institutions who map the world’s average temperature from year to year (and who you might think were part of the conspiracy) disagree as to which years, in recent times, have been the hottest and use slightly different methodologies to correct the incompleteness of the data sets to arrive at those figures.

The Director of the Goddard Institute for Space Studies, James Hansen, is a man the denialists love to hate. They call him a climate change alarmist. In a detailed paper written with three other authors, Professor Hansen discusses the global temperature results for 2009. The analysis by GISS scientists indicates that 2005 is the hottest year in the instrumental record which extends for over a hundred years. The analysis suggests that 2009 is the second warmest year. However, Professor Hansen states that the results for 1998, 2002, 2003, 2006 and 2007 are so close that they should be regarded as in a virtual tie as the second warmest year.

Hansen notes that the equally eminent UK research body, the joint Hadley Centre/University of East Anglia Climatic Research Unit (“Hadley”) finds 1998 to be the warmest year on record. This is the source of the denialist catch cry that the “global warming stopped in 1998” or “the earth has been cooling since 1998”.

Professor Hansen also explains why the different analyses have arrived at slightly different results. The earth stations which measure temperature do not cover every centimetre of the planet. Hadley leaves such unrepresented areas out of their averaging process. If, for example, the Arctic, which is poorly represented for measuring stations, is warming more quickly than other areas, the Hadley analysis would understate the process of warming. GISS uses a technique (which they have tested for accuracy) which allocates to unmeasured areas the reading of their neighbours. Hence, a different averaging and a different set of results ensue.

As it turns out, both centres consider the last decade to be the warmest on record. They also agree that continued warming is confirmed by taking five or 11-year running averages, a process that reduces the impact of the sorts of variations that occur on a year to year basis because of patterns such as the el Niño effect.

Professor Hansen’s paper is, in fact, entitled If It’s That Warm, How Come It’s So Damned Cold, a reference to recent cold snaps across Europe and North America. (Copenhagen was icy, remember.) The answer, according to the paper, concerns changes in the Arctic Oscillation which impede the jet stream and cause cold Arctic air to be blown south to higher latitudes. Part of the compensating effect is that warmer air leaks north such that parts of the Arctic have comparably warm temperatures for this time of year.

Disagreement, therefore between the leading climate change research institutes in the United States and the United Kingdom is neither unexpected nor fatal to the case that dangerous human caused climate change has already commenced. Some might say that it shows that good science is more complicated than lay people and politicians would like life to be.

Strange, though. You would think that conspirators would work harder to make their numbers match.