Global Warming Cycles

In the most recent edition of Nature, Anders Moberg and colleagues from the University of Stockholm, present a new analysis of global temperatures over the last 2000 years. Their analysis demonstrates that the poster-child of the Global Warming movement - the famous 'hockey stick' graph published by Michael Mann in Nature in 1998 is misleading.

As you can see below, the Mann graph suggests stable global temperatures over the last 1000 years, with the sharp rise since around 1900 strongly suggesting that anthropogenic emissions are altering the climate.



In contrast, the Moberg study (the colored lines in the graph below - the black line is the Mann study for comparison), emphasizes large variability in global temperatures compared across centuries. Global temperatures have fluctuated cyclically, and we are currently in a recovery period from a cold period that bottomed out around 1600.



So why the difference? The Swedish group emphasizes their use of a technique that combines measurements of average temperatures over different time-scales. However, the statistical techniques used to produce the original hockey-stick shaped graph have been questioned before ; Nature however rejected a submission rebutting the Mann results.

However this latest paper doesn't reject an anthropogenic cause of recent spikes in global temperature. In fact, the authors explicitly argue that "... the post-1990 warmth seen in the instrumental data ... appears to be unprecendented.". Their contribution is to show that we are currently in the 'upswing' phase of global temperature cycles - human emissions are likely causing temperature increases even beyond that.