Timing and intensity of temperature deviation from pre-industrial levels. (source: IPCC, AR4, Chapter 6, p. 462)

Radiocarbon-dated macrofossils are used to document Holocene tree line history across northern Russia (including Siberia). Boreal forest development in this region commenced by 10,000 yr B.P. Over most of Russia, forest advanced to or near the current arctic coastline between 9000 and 7000 yr B.P. and retreated to its present position by between 4000 and 3000 yr B.P. Forest establishment and retreat was roughly synchronous across most of northern Russia. Tree line advance on the Kola Peninsula, however, appears to have occurred later than in other regions. During the period of maximum forest extension, the mean July temperatures along the northern coastline of Russia may have been 2.5° to 7.0°C warmer than modern. The development of forest and expansion of tree line likely reflects a number of complementary environmental conditions, including heightened summer insolation, the demise of Eurasian ice sheets, reduced sea-ice cover, greater continentality with eustatically lower sea level, and extreme Arctic penetration of warm North Atlantic waters. The late Holocene retreat of Eurasian tree line coincides with declining summer insolation, cooling arctic waters, and neoglaciation.

To summarize MacDonald’s results, he finds that for a period lasting somewhere around 5,000 years, the summer temperatures along the northern coastline of Russia may have been 2.5 to 7.0ºC warmer than present, and such a warming was associated with reduced sea ice, among other things.

If today’s level of Arctic warming, which has only lasted for about a decade or so, is pushing sea ice to shrink rapidly, then it would seem reasonable to think that a much warmer period lasting several thousands of years certainly did the same and even more so.

While MacDonald’s work is fairly recent, the idea that Arctic temperatures during the mid-Holocene were much warmer than our current era, and that this warming was accompanied by significant ice retreat is anything but new.

Noted early-20^th century British meteorologist and climate historian Dr. C.E.P. Brooks, wrote an article in 1949 for the Swedish scientific journal Geografiska Annaler entitled “Post-Glacial Climatic Changes in the Light of Recent Glaciological Research.” Dr. Brooks describes the early 20^th century glacial recession in an historical context:

We now [in 1949] seem to have entered this stage of unstable ice-sheet and glaciers. We know little about the extent of the floating ice-cap before the 19th Century, but the voyages of the Norsemen to Greenland in the early Middle Ages are strong evidence that up to about 1300 there was much less ice than at present in the East Greenland Current. The glaciological evidence shows that regions in Iceland and Norway are being laid bare which have been ice-covered for more than 600 years, but which were at one time cultivated. The retreat has evidently not yet reached the stage which it formerly maintained for several centuries, and it may be expected to continue until either the reduced polar ice-cap reaches a new position of stability, or until some meteorological “accident” reverses the trend and ushers in a new period of re-advance.

A still more advanced stage in the process of retreat was reached in the Post-glacial “Climatic Optimum” [~9,000 to 4,000 years ago], when the Arctic was so warm that peat-bogs could grow in Spitsbergen. It is not unlikely that during this period there was no permanent ice-cap in the Arctic; merely a winter ice-cap which largely disintegrated each summer [emphasis added].

And what effect did this multi-millennial warming have on the polar bears? Well, one thing is for sure, their existence today proves that they didn’t go extinct!

The most likely explanation is that they modified their behavior to adapt to the changing conditions, probably by spending more time on land foraging, hunting, and denning than they would during cooler, icier periods. There is evidence that these are precisely the kinds of adaptations that the bears are making to best cope with today’s warming climate. For instance, In May 2007, an AP article (http://www.tahoebonanza.com/article/20070502/Environment/105020027) reported that “More pregnant polar bears in Alaska are digging snow dens on land instead of sea ice, according to a federal study, and researchers say deteriorating sea ice due to climate warming is the likely reason.” So instead of perishing, the polar bears will adapt as best they can, as they always have.