September 25, 2007

Questioning 20th Century Warmth

In 2006, an article appeared in Science magazine reconstructing the temperature of the Northern Hemisphere back to 800 AD based on 14 smoothed and normalized temperature proxies (e.g., tree ring records). Osborn and Briffa proclaimed at the time that “the 20th century is the most anomalous interval in the entire analysis period, with highly significant occurrences of positive anomalies and positive extremes in the proxy records.” Obviously, concluding that the Northern Hemisphere has entered a period of unprecedented warmth is sure to make the news, and indeed, Osborn and Briffa’s work was carried in papers throughout the world and was loudly trumpeted by the American Association for the Advancement of Science (AAAS) that publishes the journal Science.

A recent issue of Science contains an article not likely to receive any press coverage at all. Gerd Bürger of Berlin’s Institut für Meteorologie decided to revisit the work of Osborn and Briffa, and his results raise serious questions about the claim that the 20th century has been unusually warm. Bürger argues that Osborn and Briffa did not apply the appropriate statistical tests that link the proxy records to observational data, and as such, Osborn and Briffa did not properly quantify the statistical uncertainties in their analyses. Bürger repeated all analyses with the appropriate adjustments and concluded “As a result, the ‘highly significant’ occurrences of positive anomalies during the 20th century disappear.” Further, he reports that “The 95th percentile is exceeded mostly in the early 20th century, but also about the year 1000.” Needless to say, Gerd Bürger is not going to win any awards from the champions of global warming – nothing is more sacred than 20th century warming!

The reconstruction of past temperatures is a science unto itself, and the library contains many journals dedicated to the field. We could easily locate an article a week presenting a temperature reconstruction from some part of the planet that would call into question the notion that the 20th century was a period of unusual warmth. You may recall many essays we presented over the past five years examining the “hockey stick” depiction of planetary temperature (little change for 900 years, and suddenly 100 years ago, the temperature shot up) so merrily adopted by Gore and many others.

A large and important article appeared recently in Earth-Science Reviews regarding a long-term reconstruction of temperatures from Russia’s Lake Baikal. In case you have forgotten your geography lessons, Lake Baikal is the world’s deepest lake, it contains the world’s largest volume of freshwater (20 percent of the global supply), and the lake has over 300 rivers flowing into it. Anson Mackay of University College London is the author of the article, and he notes that “the bottom sediments of the lake itself have never been directly been glaciated. Lake Baikal therefore, contains a potential uninterrupted paleoclimate archive consisting of over 7500 m of sedimentary deposits, extending back more than 20 million years.” If that is not perfect enough, the Lake “is perhaps best well known for its high degree of biodiversity; over 2500 plant and animal species have been documented in Baikal, most of which are believed to be endemic.” The Lake is a long way from the moderating effects of any ocean, and therefore, the Lake should experience large climatic fluctuations over long and short periods of time.

The trick to reconstructing temperatures here involves the shell remains of planktonic diatoms that have lived in the Lake for eons. During warm periods, some species of diatom phytoplankton flourish while during cold periods, some species flourish while most reduce production. Cores from the bottom of the Lake therefore contain a high-resolution temperature record for hundreds of thousands of years interpreted from biogenic silica left from the plankton.

Figure 1 below tells us an interesting story about the climate of the past 800,000 years including (a) the most recent 10,000 years have generally witnessed a warming (warmer conditions are towards the right, cooler towards the left), (b) cold periods dominate the last 800,000 years, (c) climate can change rapidly, and (d) there are many periods in the past much warmer than what we have there today.


Figure 1. Lake Baikal paleoclimate record from the past 800,000 years. Warmer conditions are towards the right, cooler ones towards the left (from Mackay, 2007).

Of greater interest to us is what Lake Baikal can tell us about the most recent thousand years, and in particular, we are interested in the warming of the last 1000 years. Mackay notes that “between c. A.D. 850 and 1200, S. acus dominated the assemblage, most likely due to prevailing warmer and wetter climate that occurred in Siberia at this time.” Well now, it certainly looks as if the Medieval Warm Period was noticed at the Lake. Next we learn that “Between c. A.D. 1200 and 1400, spring diatom crops growing under the ice decline in abundance, due in part to increased winter severity and snow cover on the lake, which is reflected in cooler early Siberian summers.” The Little Ice Age then hit hard as Mackay finds “The diatom-inferred snow model suggests significantly increased snow cover on the lake between A.D. 1200 and 1775, which mirrors for the large part increases in snow cover in China during AD 1400–1900.”

But here comes our favorite set of conclusions. Mackay writes “Diatom census data and reconstructions of snow accumulation suggest that changes in the influence of the Siberian High in the Lake Baikal region started as early as c. 1750 AD, with a shift from taxa that bloom during autumn overturn to assemblages that exhibit net growth in spring (after ice break-up). The data here mirror instrumental climate records from Fennoscandia for example, which also show over the last 250 years positive temperature trends and increasing early summer Siberian temperature reconstructions. Warming in the Lake Baikal region commenced before rapid increases in greenhouse gases, and at least initially, is therefore a response to other forcing factors such as insolation changes during this period of the most recent millennial cycle.”

The Lake Baikal study shows that warming has occurred in the most recent century, but it is certainly nothing out of the ordinary and possibly to some degree explained by non-greenhouse forcing. The Osborn and Briffa proclamation that the 20th century was somehow out of the ordinary is certainly not confirmed by the incredible reconstruction from Lake Baikal.

References:

Bürger, G., 2007. Comment on “The Spatial Extent of 20th-Century Warmth in the Context of the Past 1200 Years”. Science, 316, 1844a.

Mackay, A.W., 2007. The Paleoclimatology of Lake Baikal: A Diatom Synthesis and Prospectus. Earth-Science Reviews, 82, 181–215.

Osborn, T.J., and Briffa, K.R., 2006. The spatial extent of 20th-century warmth in the context of the past 1200 years. Science, 311, 841-844.




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