Glaciers (North America) -- Summary
Does the history of North American glacial activity support the climate-alarmist claim that anthropo-genic CO2 emissions drove temperatures to new and unprecedented heights near the end of the 20th century? We here review some studies of North American glaciers that speak to this issue.
Dowdeswell et al. (1997) analyzed the mass balance histories of the 18 Arctic glaciers that have the longest observational records, finding that just over 80% of them displayed negative mass balances over the last half of the 20th century. However, they note that "ice-core records from the Canadian High Arctic islands indicate that the generally negative glacier mass balances observed over the past 50 years have probably been typical of Arctic glaciers since the end of the Little Ice Age." Also, they emphatically state "there is no compelling indication of increasingly negative balance conditions which might, a priori, be expected from anthropogenically induced global warming." Quite to the contrary, they report that "almost 80% of the mass balance time series also have a positive trend, toward a less negative mass balance." Hence, although most of these High Arctic Canadian glaciers continue to lose mass, as they have probably done since the end of the Little Ice Age, they are losing smaller amounts each year, in the mean, which is not what one would expect in the face of rapidly rising atmospheric CO2 concentrations if they truly drive global warming as dramatically as climate-alarmists say they do.
Also reporting from Canada, Clague et al. (2004) documented glacier and vegetation changes at high elevations in the upper Bowser River basin in the northern Coast Mountains of British Columbia, based on studies of the distributions of glacial moraines and trimlines, tree-ring data, cores from two small lakes that were sampled for a variety of analyses (magnetic susceptibility, pollen, diatoms, chirono-mids, carbon and nitrogen content, 210Pb, 137Cs, 14C), similar analyses of materials obtained from pits and cores from a nearby fen, and by accelerator mass spectrometry radiocarbon dating of plant fossils, including wood fragments, tree bark, twigs and conifer needles and cones. All this evidence suggested a glacial advance that began about 3000 years ago and may have lasted for hundreds of years, which would have placed it within the unnamed cold period that preceded the Roman Warm Period. There was also evidence for a second minor phase of activity that began about 1300 years ago but was of short duration, which would have placed it within the Dark Ages Cold Period. Finally, the third and most extensive Neoglacial interval began shortly after AD 1200, following the Medieval Warm Period, and ended in the late 1800s, which was, of course, the Little Ice Age, during which time Clague et al. say that "glaciers achieved their greatest extent of the past 3000 years and probably the last 10,000 years."
These data clearly depict the regular alternation between non-CO2-forcecd multi-century cold and warm periods that is the trademark of the millennial-scale oscillation of climate that reverberates throughout glacial and interglacial periods alike. That a significant, but by no means unprecedented, warming followed the most recent cold phase of this cycle is in no way unusual, particularly since the Little Ice Age was likely the coldest period of the last 10,000 years. The significant warming of the 20th century would have occurred within the same timeframe and been just as strong even if the atmosphere's CO2 content had remained constant at pre-industrial levels; it was simply the next scheduled phase of this ever-recurring natural climatic oscillation.
In a study based in Alaska, Calkin et al. (2001) reviewed the most current and comprehensive rese-arch of Holocene glaciation along the northernmost portion of the Gulf of Alaska between the Kenai Peninsula and Yakutat Bay, where several periods of glacial advance and retreat were noted during the past 7000 years. Over the latter part of this record, there was a general glacial retreat during the Medieval Warm Period that lasted for a few centuries prior to A.D. 1200, after which there were three major intervals of Little Ice Age glacial advance: the early 15th century, the middle 17th cen-tury, and the last half of the 19th century. During these latter time periods, glacier equilibrium line altitudes were depressed from 150 to 200 m below present values as Alaskan glaciers also "reached their Holocene maximum extensions." Hence, it is only to be expected that Alaska's temperatures would rise significantly and its glaciers would lose mass at significant rates during the planet's natural recovery from the coldest period of the current interglacial.
In another study from Alaska, Wiles et al. (2004) derived a composite Glacier Expansion Index (GEI) for the state based on "dendrochronologically-derived calendar dates from forests overrun by advancing ice and age estimates of moraines using tree-rings and lichens" for three climatically-distinct regions -- the Arctic Brooks Range, the southern transitional interior straddled by the Wrangell and St. Elias mountain ranges, and the Kenai, Chugach and St. Elias coastal ranges -- after which they compared this history of glacial activity with "the 14C record preserved in tree rings corrected for marine and terrestrial reservoir effects as a proxy for solar variability" and with the history of the Pacific Decadal Oscillation (PDO) derived by Cook (2002).
As a result of their efforts, Wiles et al. discovered that "Alaska shows ice expansions approximately every 200 years, compatible with a solar mode of variability," specifically, the de Vries 208-year solar cycle; and by merging this cycle with the cyclical behavior of the PDO, they obtained a dual-parameter forcing function that was even better correlated with the Alaskan composite GEI, with major gla-cial advances clearly associated with the Sporer, Maunder and Dalton solar minima.
In describing the rational for their study, Wiles et al. said that "increased understanding of solar vari-ability and its climatic impacts is critical for separating anthropogenic from natural forcing and for predicting anticipated temperature change for future centuries." In this regard, it is most interesting that they made no mention of possible CO2-induced global warming in discussing their results, presu-mably because there was no need to do so. Alaskan glacial activity, which in their words "has been shown to be primarily a record of summer temperature change (Barclay et al., 1999)," appears to be sufficiently well described within the context of centennial (solar) and decadal (PDO) variability superimposed upon the millennial-scale (non-CO2-forced) variability that produces longer-lasting Medieval Warm Period and Little Ice Age conditions.
Dropping down into the conterminous United States, Pederson et al. (2004) used tree-ring recons-tructions of North Pacific surface temperature anomalies and summer drought as proxies for winter glacial accumulation and summer ablation, respectively, to create a 300-year history of regional glacial Mass Balance Potential (MBP), which they compared with historic retreats and advances of Glacier Park's extensively-studied Jackson and Agassiz glaciers. What they found was most interes-ting. As they describe it, "the maximum glacial advance of the Little Ice Age coincides with a sus-tained period of positive MBP that began in the mid-1770s and was interrupted by only one brief ablation phase (~1790s) prior to the 1830s," after which they report that "the mid-19th century retreat of the Jackson and Agassiz glaciers then coincides with a period marked by strong negative MBP." From about 1850 onward, for example, they note that "Carrara and McGimsey (1981) indicate a modest retreat (~3-14 m/yr) for both glaciers until approximately 1917." At that point, they report that "the MBP shifts to an extreme negative phase that persists for ~25 yr," during which period the glaciers retreated "at rates of greater than 100 m/yr."
Continuing with their history, Pederson et al. report that "from the mid-1940s through the 1970s retreat rates slowed substantially, and several modest advances were documented as the North Pacific transitioned to a cool phase [and] relatively mild summer conditions also prevailed." There-after, however, from the late 1970s through the 1990s, they say that "instrumental records indicate a shift in the PDO back to warmer conditions resulting in continuous, moderate retreat of the Jackson and Agassiz glaciers."
The first illuminating aspect of this glacial history is that the post-Little Ice Age retreat of the Jack-son and Agassiz glaciers began just after 1830, in harmony with the findings of a number of other studies from various parts of the world (Vincent and Vallon, 1997; Vincent, 2001, 2002; Moore et al., 2002; Yoo and D'Odorico, 2002; Gonzalez-Rouco et al. 2003; Jomelli and Pech, 2004), including the entire Northern Hemisphere (Briffa and Osborn, 2002; Esper et al., 2002), which finding stands in stark contrast to what is suggested by the IPCC-endorsed "hockeystick" temperature history of Mann et al. (1998, 1999), which does not portray any Northern Hemispheric warming until around 1910. The second illuminating aspect of the glacial record is that the vast bulk of the glacial retreat in Glacier National Park occurred between 1830 and 1942, over which time the air's CO2 concentration rose by only 27 ppm, which is less than a third of the total CO2 increase experienced since the start of glacial recession. Then, from the mid-1940s through the 1970s, when the air's CO2 concentration rose by another 27 ppm, Pederson et al. report that "retreat rates slowed substantially, and several modest advances were documented."
It is illuminating to note, in this regard, that the first 27 ppm increase in atmospheric CO2 concentra-tion coincided with the great preponderance of glacial retreat experienced since the start of the war-ming that marked the "beginning of the end" of the Little Ice Age, but that the next 27 ppm increase in the air's CO2 concentration was accompanied by little if any additional glacial retreat, when, of course, there was little if any additional warming.
Clearly, and contrary to the strident claims of climate alarmists, something other than the historic rise in the air's CO2 content has been responsible for the disappearing ice fields of Glacier National Park. It should also be clear to all that the historical behavior of North America's glaciers provides no evi-dence whatsoever for unprecedented or unnatural CO2-induced global warming over any part of the 20th century.
Last updated 18 May 2005
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