November 30, 2004

A long-term perspective

Filed under: Arctic, Climate History

The newly released Arctic Climate Impact Assessment’s alarmist claims of the unusual nature of the current state of the Arctic climate are inconsistent with long-term geological records of climate change in the region.

The newly released Arctic Climate Impact Assessment (ACIA) offers alarmist and highly questionable conclusions about the nature of Arctic climate and its variability. Consider the following statement, included in the Overview:

[The ACIA] is the largest, most comprehensive assessment of climate change in Arctic. The Arctic is experiencing some of the most rapid and severe climate change on earth. At least half the summer sea ice in the Arctic is projected to melt by the end of this century [as a result of increasing atmospheric CO2], along with a significant portion of the Greenland Ice Sheet, as the region is projected to warm an additional 4-7ºC (7 to 13ºF) by 2100.

We find serious problems with all three of these major sentiments.

First, the claim of the most rapid and severe climate change in the Arctic occurring during the 20th century requires a broader perspective and more accurate assessment from past climatic changes. Hu et al. (2000), for example, provided “the first quantitative temperature record that continuously spans the last 2000 years from Alaska” and found that:

Our SWT [surface water temperature] reconstruction at Farewell Lake indicated that although the 20th century, represented by the uppermost three samples, was among the warmest periods of the past two millennia, two earlier periods may have been comparably warm (A.D. 0-300 and A.D. 850-1200). … [B]oth the pronounced temperature minimum centered at A.D. 600 and the culmination of the Little Ice Age cooling at A.D. 1700 in the Farewell Lake region coincide with extensive glacial advances in the southern coasts and the Brooks Range of Alaska. The cooling events around A.D. 600 might have also caused the demise of the Kachemak culture in the northwestern Gulf of Alaska at this time.

Shiyatov (2003), as a second example, found that although large displacements of the upper treeline limit (mainly populated by the Siberian larch, Larix sibirica) had been detected in the region of the Polar Ural Mountains in the past 1150 years, the 20th century change has not been particularly exceptional or alarming (Figure 1).

Tree Line

Figure 1. Large displacement of the upper treeline limit in the Polar Ural Mountains region (66ºN-67ºN, 65ºE-66ºE) likely as a result of climatic and environmental conditionings. Note that although the instrumental thermometers recorded about a 1ºC warming of the June-July temperature in the 20th century, the treeline limit did not suggest anything exceptional nor unusual in relation to the change over the past 1150 years. [Adapted from Shiyatov, 2003]

On a wider coverage of the Arctic, the coarse temperature map sketched on NOAA’s Paleoclimatology web site (http://www.ngdc.noaa.gov/paleo/globalwarming/images/polarbigb.gif) suggests that the summer temperatures 6000 years ago may have been 2ºC to 4ºC warmer than present in the Eastern Hemisphere of the Arctic. Furthermore, Kaufman and 29 co-authors recently found that indeed there is clear evidence for warmer than present conditions during the Holocene at 120 out of the 140 sites (Figure 2) they identified across the Western Hemisphere of the Arctic. They estimated that, at the 16 terrestrial sites where quantitative data are available, the local Holocene Thermal Maximum summer temperatures were about 1.6 ± 0.8ºC higher than the average of the 20th century. Kaufman et al. estimated the history of sea-ice cover in the Canadian Arctic Archipelago, examining the distribution of more than 1,000 bowhead whalebone remains and walrus bones:

Atlantic bowheads reached their maximum abundance in the channels of the eastern and central Arctic Archipelago from 11.5 to 8.5 ka [Before Present, BP], but were excluded from areas along northeastern Baffin Island. Pacific bowhead reached their maximum abundance in the western Arctic channels connecting to the Beaufort Sea at the same time. During that interval, whales extended into areas well beyond their present ranges, and then retreated abruptly at about 8.5 ka [BP]. The bowhead range may have expanded as sea-ice export from the Archipelago was enhanced by abundant of meltwater during the interval of rapid glacial recession. Alternatively, greater summer warmth may alone account for reduced summer sea-ice cover. Sea-salt sodium concentrations in Penny Ice Cap and the Greenland Ice Sheet are at highest levels in early Holocene ice (11.5 to 9.0 ka [BP]), consistent with minimal sea-ice cover. … Bowhead whale ranges re-expanded in the middle Holocene (6-3 ka) [BP] … [but] the range did not attain early Holocene extent.

Arctic Map

Figure 2. Map of the Western Arctic. Numbered points represent the 140 sites examined by Kaufman et al. (2004). Kaufman found evidence suggesting “warmer-than-present conditions” at 120 of these sites during the Holocene. [Adapted from Kaufman et al., 2004]

These three examples point clearly to harsher, more extreme Arctic conditions in the past, highlighting how various Arctic habitats responded to those rapid climatic events and gradual environmental changes.

What about the serious ACIA claims on the projected melting of half of the summer sea ice? Again a broader perspective from the current paleoclimatic data and understanding can help.

Sea Ice Extent

Figure 3. 10-year mean anomalies, relative to the 1750-2000 means, of the April (left panel) and August (right panel) sea-ice extents for the Nordic Seas. [Adapted from a conference presentation paper by Torgny Vinje of the Norwegian Polar Institute]

Figure 3 shows the 10-year mean anomalies of both the April and August (summer) extent of sea ice around the Nordic Seas region from 1750 to 2000. The author of this sea-ice extent time series, Dr. Torgny Vinje of the Norwegian Polar Institute, recently remarked that:

The current ice extent reduction in the European Sector of the Artic is in continuation from processes that commenced in the 18th century, prior to the main industrial epoch. It is noted that the minimum August ice extent in the 1930s [right panel of Figure 3] compares with the previous minimum observed in the 1780s. …The maximum temperature during the 1930s corresponds to the minimum ice extent observed in the Nordic Sea at that time. However, while extreme maximum temperature is observed during the 1990s the ice extent has not regained its 1930 extreme minimum [extension] yet.

The April record (Figure 3, left panel) from 1864 to 1998 (first published by Vinje in 2001) shows that sea ice around this region of the North Atlantic has decreased by 33% over the past 135 years. Again Vinje explained that the most likely explanation for the April sea-ice retreat is that the earth is recovering from the cold period of Little Ice Age from 1300-1900 or so. According to Vinje (2001):

Nearly half of this [33%] reduction took place before 1900, that is, before the warming of the Arctic, which took place during the first three decades of the twentieth century … The time series indicates that we are in a state of continued recovery from the cooling effects of the Little Ice Age during which a maximum [April] sea-ice expansion was observed around 1800, both in the Iceland Sea … and in the Barents Sea. [T]he mean annual reduction of the April ice extent is decelerating by a factor of 3 between 1880 and 1980.

Nor did the 700-to-1000-year-long high-resolution, geochemical data collected from the Penny Ice Cap (PIC) on Baffin Island (e.g., see Figure 2) by Grumet et al. (2001) reveal any exceptional sea-ice extents around this critical part of the western Arctic during the last 50 years:

The PIC record of springtime sea-ice coverage illustrates that despite warmer temperatures during the turn of the century, sea ice conditions in the Baffin Bay/Labrador Sea region, as least during the last 50 years, are within ‘Little Ice Age’ variability. Our observations from the PIC record are consistent with an increase in sea-ice extent in the Baffin Bay/Labrador Sea region of the past 30 years (Chapman and Walsh 1993) and cooler surface air temperatures in this region (Hansen et al. 1996).

Heat Transfer

Figure 4. Two slightly different schemes for the parameterization of its heat transfer efficiency within the atmospheric boundary layer. Note the drastic differences between them: For Greenland, a temperature discrepancy of 2ºC to 10ºC appears depending on weak vs. stronger heat mixing parameterization scheme. [Adapted from Viterbo et al., 1999]Clearly, it is impossible, at this point, to make objective scientific claims for the very alarming scenario concerning the large melting of the Greenland Ice Sheet.

Figure 4 shows one of the main reasons why the projected Arctic warming of 4ºC to 7ºC at 2100 by the ACIA can and should be seriously challenged and questioned. The figure shows the extreme sensitivity of surface winter temperature change on local and regional scales to how a model represents the physics of the boundary layer—the layer of turbulent air between the surface and free troposphere with a few hundred meters in thickness. This boundary layer is critical to how both heat and moisture are exchanged between the surface and the free atmosphere, especially during winters and springs over Greenland.

Again, Figure 4 clearly shows that with only a very slight change or tuning of how heat is being exchanged or mixed, the model predictions of the winter surface temperature can vary from as much as 2ºC to 10ºC. That large uncertainty raises a serious question about the ability of any current generation of the state-of-the-art climate model to cast any realistic warming prediction—let alone forecast a warming of 3ºC to 7ºC for the Arctic with the dire consequence of melting the Greenland Ice Sheet.

It has also long been known that the Greenland Ice Sheet probably originated some 2.4 million years ago. Geological records indicate that the Greenland Ice Sheet is most likely the only Northern Hemisphere ice sheet to have survived the last Interglacial warm period around 130,000 to 115,000 years ago (roughly known as the Eemian warm period, identified through terrestrial records from Europe).

The observed climatic and environmental changes during the last Interglacial around the North Atlantic region are indeed dramatic. For example, tall mixed hardwood forests with a closed canopy covered much of Europe during the peak warm period. But after about 115,000 years ago, open vegetation replaced the mixed forests in northwestern Europe.

Yet despite the rather extreme climatic conditions and swings during the last Interglacial period, the Greenland Ice Sheet did not simply melt away. During the persistent warmth of the Eemian Interglacial, climatic records for the North Atlantic-Greenland region indicate no signs of a weakening or a total shutdown of the North Atlantic thermohaline circulation. That fact is remarkable given the distinct possibility of excessive freshening of the North Atlantic Ocean from enhanced rainfall and the melting of Greenland coastal ice during the last Interglacial warm period around 130,000 to 115,000 years ago.

The ACIA’s gloom and doom perspective on current Arctic climate and the coming century’s Arctic climate change is simply unjustifiable. The best available scientific evidence fails to support the report’s claims.

Perhaps worse is the ACIA’s proposed prescription of “fixing” all the ills of the purported Arctic climate trend by taking steps toward the reduction of anthropogenic greenhouse gases. In the end, that premise is obviously more of a political posture than a legitimate scientific conclusion.

As Professor Marcel Leroux of the University of Jean Moulin at Lyon, France, recently commented:

The global warming scenario as asserted today is not proven. But, by reason of its “moral” content one must be either for or against it, a choice that is indeed a nonsense from a scientific point of view. In what domain does conviction take the place of knowledge? A reformulation of the [global warming] issue is therefore urgent, and needs to be made carefully and without complacency, strictly devoted to climatology.

References:

Arctic Climate Impact Assessment (ACIA) Overview Report, 2004. Impacts of a Warming Arctic: Arctic Climate Impact Assessment (Cambridge University Press).

Grumet et al., 2001. Variability of sea-ice extent in Baffin Bay over the last millennium. Climatic Change, 49, 129-145.

Hu et al., 2000. Pronounced climatic variations in Alaska during the last two millennia. Proceedings of the National Academy of Sciences, 98, 10552-10556.

Kaufman et al., 2004. Holocene thermal maximum in the western Arctic (0-180ºW). Quaternary Science Reviews, 23, 529-560.

Shiyatov, 2003. Rates of change in the upper treeline ecotone in the Polar Ural Mountains. PAGES News, 11 (no.1), 8-10.

Vinje, 2001. Anomalies and trends of sea-ice extent and atmospheric circulation in the Nordic Seas during the period 1864-1998. Journal of Climate, 14, 255-267.




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