November 8, 2007

Snow and Ice Surprises

A recent headline carried around the world certainly caught our eye as it proclaimed “Mont Blanc Growing Due to Global Warming.” When we first saw the piece, we fully expected to read about a process called isostatic rebound. If you are rusty in basic earth science principles, isostatic rebound occurs when weight is reduced from the earth’s crust (as is the case when a substantial amount of ice disappears from a region), the crust rebounds upward, and the land surface increases its elevation with respect to sea level. Oppositely, when ice accumulates over a region (e.g., Greenland, Antarctica), the crust is depressed, sometimes well below sea level. So when we first saw the piece about Mont Blanc, the highest peak in the Alps, we assumed the article would be about how melting of the mountain glaciers has now led to crustal rebounding and a higher summit. However, we were in for a huge surprise.

The article, in its various forms carried by many news services, states “The famous peak stood 4807 metres tall on the French-Italian border, but has been growing since 2003 at a faster rate in recent months. The volume of ice on Mont Blanc’s slopes has almost doubled since 2005 to reach 24,100 cubic metres this year, and snow has built up faster due to greater frequency of winds and higher temperatures in the summer, believed to be caused by global warming.” We learn that “Last month, surveyors from France’s Haute-Savoie region measured the Alps’ tallest peak at 4,810.90 metres (15,784 feet)” and that “The height as well as the volume of Mont Blanc has increased considerably, because the snow has massed on the summit over the last two years.” The article goes on to say this is unique to Mont Blanc given its height, but when all we hear is that mountain glaciers are retreating, we again find this interesting piece with evidence quite to the contrary.

No sooner than we digested the news from Mont Blanc, we discovered an article that will soon appear in Climate Dynamics dealing with temperature and sea ice extent in the Baltic Sea. Again, if your geography is as rusty as your basic earth science principles, the map below (Figure 1) shows the location of the Baltic Sea. The authors, Hansson and Omstedt, are with the oceanography group in the Earth Sciences Centre at Sweden’s Göteborg University. Obviously, people have been living in the Baltic area for a long time, and there is no end of instrumental and proxy records upon which reconstructions of temperatures and sea ice extent may be based. The authors claim that “We demonstrate that high-quality station data of the past century and gridded multi-proxy reconstructions for the past 500 years can be used with great success but with various levels of detail” in the attempt to analyze temperatures and sea ice extent in the Baltic Sea. They also used what they described as a “state-of-the-art global climate model” to aid in their efforts. The article is full of details on how they calibrated the regression equations using modern records of temperature and ice thereby allowing for the reconstruction over the past 500 years. Rather than worry about the details, let’s see the results.


Figure 1. The Baltic Sea and northern Europe. Names of the stations used throughout the study are provided (from Hansson and Omstedt, 2007)

Figure 2 shows the reconstructed water temperatures of the Baltic Sea from 1500 to 2001. In describing patterns in the data, several sentences seemed particularly important to us. First, Hansson and Omstedt note that “It appears that the late twentieth century warming in the Baltic Sea region cannot be determined to be unprecedented over the past 500 years, as the mid-eighteenth century warming is of comparable magnitude. Both these periods were exceptionally warm compared to the mean temperature of the considered period.” Second, they note that warming was observed from 1875 to 1935, “steadily marking the end of LIA” (Little Ice Age). And thirdly, “From 1935 to the present no statistically significant water temperature trend can be determined.” Recall that the Baltic is in a mid-to-high latitude location in the Northern Hemisphere – just where the warming should be most pronounced due to the buildup of greenhouse gases according to the models of climate.


Figure 2. (a) Anomalies of the annual and decadal moving average of the modelled Baltic Sea water temperature over the 1500–2001 period. The dotted horizontal lines are the standard deviations of the water temperature during the standard period 1900–1999 (from Hansson and Omstedt, 2007).

So what about the sea ice extent (note that they refer to maximum ice extent as MIB)? Well, as seen in Figure 3, there is great variability, but certainly no great recent decline that is out of the ordinary. To the probable amazement of the greenhouse advocates, they state “The 1730s is the decade with the least ice, followed by the 1740s and the 1930s.” They note that the winter of 1984 had one of the least areas covered in sea ice, but the winter of 1985 had one of the greatest areas covered in the past 500 years. If you think you detect a trend downward in the 20th century, the authors remind us “Bearing in mind that the LIA, with a generally colder climate, ended in the 1870’s” any reduction in sea ice in the century that followed would be expected.


Figure 3. Modelled sea-ice extent and reconstructed winter (DJF) air temperatures over the 1500–2001 period (from Hansson and Omstedt, 2007)

We will leave you with two further results from their study; 1)The authors conclude that the output data from the state-of-the-art climate model “are not suitable for modelling the Baltic Sea ocean climate” and 2) They finish the article with the sentence “Even though different climate forcing may operate on the climate system today compared to over the past 500 years, it is not possible to clearly state that the region is experiencing a climate change outside the natural limits of the past half millennium.”

Reference:

Hansson, D. and A. Omstedt. 2007. Modelling the Baltic Sea ocean climate on centennial time scale: temperature and sea ice. Climate Dynamics, DOI 10.1007/s00382-007-0321-2.




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