Wednesday, December 06, 2006

Warmest European Autumn Since Records Began. So What?

Europe has just experienced its warmest Autumn since records began around 500 years ago. According to Nature:
Preliminary analysis shows that continental mean temperatures in September and October were 11°C — that's 1.8 °C higher than the long-term average for these months. November was 2.5 °C higher than the average. The results show that 2006 has beaten the 'hottest' autumns of 1772, 1938 and 2000 by about a degree.

Finding data to support seasonal trends can be tricky, however. The instrumental record doesn't date back much further than the onset of the twentieth century.

To get around this, Elena Xoplaki, a climate historian at the University of Bern in Switzerland, has looked at historic sources in Europe going back to the 1500s, such as weather observations recorded by monks, doctors and scholars.

She has now updated her reconstruction with the latest temperature data from NASA's Goddard Institute for Space Studies. The unpublished study reveals that the past three months have been uniquely warm in the context of the past half millennium, even when the uncertainties related to the historic data are taken into account.

Does this result confirm global warming?

No. It's only one year and it's regional. It certainly is consistent with AGW theory, but what is more important is if it is part of a trend of increasing temperatures. An excellent review of recent scientific literature by Michael Mann shows that this is indeed the case. Mann looked a the work of a multitude of scientists:
Building on pioneering earlier work (e.g., Lamb 1965, Fritts et al. 1971), a considerable body of more recent work (Bradley & Jones 1993; Hughes & Diaz 1994; Mann et al. 1995, 1998, 1999, 2003; Overpeck et al. 1997; Jones et al. 1998; Luterbacher et al. 1999; Crowley & Lowery 2000; Huang et al. 2000; Briffa et al. 2001; Folland et al. 2001; Esper et al. 2002; Mann & Jones 2003; Cook et al. 2004; Luterbacher et al. 2004; Moberg et al. 2005; Oerlemans 2005; Rutherford et al. 2005) has focused on reconstructing large-scale climate changes over the period of the past one to two millennia during which widespread high-resolution, generally well-dated proxy records are available for large regions of the Northern Hemisphere (NH), and some parts of the Southern Hemisphere [see Jones & Mann (2004) for a review]. A number of model simulation studies of this period have also recently been performed (Rind & Overpeck 1993; Crowley & Kim 1996; Cubasch et al. 1997; Free & Robock 1999; Crowley 2000; Delworth & Mann 2000; Shindell et al. 2001, 2003, 2004; Bertrand et al. 2002; Bauer et al. 2003; Braganza et al. 2003; Gerber et al. 2003; Bell et al. 2003; Gonzalez-Rouco et al. 2003; Crowley et al. 2003; Schmidt et al. 2004; Mann et al. 2005a).
The conclusions:
1. Proxy reconstructions and model simulations both suggest that late twentieth century warmth is anomalous in the context of the past 1000–2000 years.

2. Forced changes in large-scale atmospheric circulation, such as the NAO, and internal dynamics related to El Ni no may play an important role in explaining regional patterns of variability and change.

3. Important differences between estimates of extratropical and full (combined tropical and extratropical) hemispheric mean temperature changes in past centuries appear consistent with seasonally and spatially specific responses to climate forcing.

4. Tests with synthetic pseudoproxy networks derived from climate model simulations indicate that statistical methods used for reconstructing past climate from proxy data are likely to yield reliable reconstructions back at least 1000 years within estimated uncertainties, given the statistical properties estimated for actual proxy networks.
Basically, it is now warmer than at any time in the past 1000 years. Suprise, surprise!