Effects of climate change at the Poles
Past 1,000 Years
For full article see Past 1,000 years at the Poles
As best we can determine from the graphical representation of William et al.'s data, the peak CH3Cl concentration measured during the MCA is approximately 533 ppt, which is within 3 percent of its current mean value of 550 ppt and well within the range of 520 to 580 ppt that characterizes methyl chloride’s current variability. Hence, we may validly conclude that the mean peak temperature of the MCA (which we refer to as the Medieval Warm Period) over the latitude range 30°N to 30°S—and possibly over the entire globe—may not have been materially different from the mean peak temperature so far attained during the Current Warm Period. This conclusion suggests there is nothing unusual, unnatural, or unprecedented about the current level of earth’s warmth.
Dahl-Jensen et al. (1998) used data from two ice sheet boreholes to reconstruct the temperature history of Greenland over the past 50,000 years. Their analysis indicated that temperatures on the Greenland Ice Sheet during the Last Glacial Maximum (about 25,000 years ago) were 23 ± 2 °C colder than at present. After the termination of the glacial period, however, temperatures increased steadily to a value that was 2.5°C warmer than at present, during the Climatic Optimum of 4,000 to 7,000 years ago. The Medieval Warm Period and Little Ice Age were also evident in the borehole data, with temperatures 1°C warmer and 0.5-0.7°C cooler than at present, respectively. Then, after the Little Ice Age, the scientists report “temperatures reached a maximum around 1930 AD” and that “temperatures have decreased during the last decades.”
The results of this study stand in stark contrast to the predictions of general circulation models of the atmosphere, which consistently suggest there should have been a significant CO2-induced warming in high northern latitudes over the past several decades. They also depict large temperature excursions over the past 10,000 years, when the air’s CO2 content was relatively stable. Each of these observations raises serious doubts about the models’ ability to correctly forecast earth’s climatic response to the ongoing rise in the air’s CO2 content.
Similar trends are present in the Antarctic, where the study of temperatures has provided valuable insight and spurred contentious debate on issues pertaining to global climate change. Key among the pertinent findings has been the observation of a large-scale correlation between proxy air temperature and atmospheric CO2 measurements obtained from ice cores drilled in the interior of the continent. In the mid- to late-1980s, this broad correlation dominated much of the climate change debate. Many jumped on the global warming bandwagon, saying the correlation proved that changes in atmospheric CO2 concentration caused changes in air temperature, and that future increases in the air’s CO2 content due to anthropogenic CO2 emissions would therefore intensify global warming.
By the late 1990s and early 2000s, however, ice-coring instrumentation and techniques had improved considerably and newer studies with finer temporal resolution began to reveal that increases (decreases) in air temperature precede increases (decreases) in atmospheric CO2 content, not vice versa (see Indermuhle et al. (2000), Monnin et al. (2001)). A recent study by Caillon et al. (2003), for example, demonstrated that during Glacial Termination III, “the CO2 increase lagged Antarctic deglacial warming by 800 ± 200 years.” This finding, in the authors’ words, “confirms that CO2 is not the forcing that initially drives the climatic system during a deglaciation.”
A second major blow to the CO2-induced global warming hypothesis comes from the contradiction between observed and model-predicted Antarctic temperature trends of the past several decades. According to nearly all climate models, CO2-induced global warming should be most evident in earth’s polar regions, but analyses of Antarctic near-surface and tropospheric air temperatures contradict this prediction.
Computer simulations of global climate change have long indicated the world’s polar regions should show the first and severest signs of CO2-induced global warming. If the models are correct, these signs should be especially evident in the second half of the twentieth century, when approximately two-thirds of the modern-era rise in atmospheric CO2 occurred and earth’s temperature supposedly rose to a level unprecedented in the past millennium. In this subsection, we examine historic trends in Arctic glacier behavior to determine the credibility of current climate models with respect to their polar predictions.
In a review of “the most current and comprehensive research of Holocene glaciation,” along the northernmost Gulf of Alaska between the Kenai Peninsula and Yakutat Bay, Calkin et al. (2001) report there were several periods of glacial advance and retreat over the past 7,000 years. Over the most recent of those seven millennia, there was a general retreat during the Medieval Warm Period that lasted for “at least a few centuries prior to A.D. 1200.” Then came three major intervals of Little Ice Age glacial advance: the early fifteenth century, the middle seventeenth century, and the last half of the nineteenth century. During these very cold periods, glacier equilibrium-line altitudes were depressed from 150 to 200 m below present values, as Alaskan glaciers “reached their Holocene maximum extensions.”
In summarizing the results of their work in Antarctica, Hall and Denton say “the Wilson Piedmont Glacier appears to have undergone advance at approximately the same time as the main phase of the ‘Little Ice Age’, followed by twentieth-century retreat at some localities along the Scott Coast.” This result and the others they cite make it clear that glacial activity on Antarctica has followed the pattern of millennial-scale variability that is evident elsewhere in the world: recession to positions during the Medieval Warm Period that have not yet been reached in our day, followed by significant advances during the intervening Little Ice Age.
Arctic climate is incredibly complex, varying simultaneously on a number of different timescales for a number of different reasons (Venegas and Mysak, 2000). Against this backdrop of multiple causation and timeframe variability, it is difficult to identify a change in either the extent or thickness of Arctic sea ice that could be attributed to the increase in temperature that has been predicted to result from the burning of fossil fuels. The task is further complicated because many of the records that do exist contain only a few years to a few decades of data, and they yield different trends depending on the period of time studied.
In Antartica, sea ice is actually expanding in some areas. Watkins and Simmonds’ findings, i.e., that Southern Ocean sea ice has increased in area, extent, and season length since at least 1978, are supported by other studies. Hanna (2001) published an updated analysis of Antarctic sea ice cover based on SSM/I data for the period October 1987-September 1999, finding the serial sea ice data depict “an ongoing slight but significant hemispheric increase of 3.7(±0.3)% in extent and 6.6(±1.5)% in area.” Parkinson (2002) utilized satellite passive-microwave data to calculate and map the length of the sea-ice season throughout the Southern Ocean for each year of the period 1979-1999, finding that although there are opposing regional trends, a “much larger area of the Southern Ocean experienced an overall lengthening of the sea-ice season … than experienced a shortening.” Updating the analysis two years later for the period November 1978 through December 2002, Parkinson (2004) reported a linear increase in 12-month running means of Southern Ocean sea ice extent of 12,380 ± 1,730 km2 per year.
Caillon, N., Severinghaus, J.P., Jouzel, J., Barnola, J.-M., Kang, J. and Lipenkov, V.Y. 2003. Timing of atmospheric CO2 and Antarctic temperature changes across Termination III. Science 299: 1728-1731.
Calkin, P.E., Wiles, G.C. and Barclay, D.J. 2001. Holocene coastal glaciation of Alaska. Quaternary Science Reviews 20: 449-461.
Dahl-Jensen, D., Mosegaard, K., Gundestrup, N., Clow, G.D., Johnsen, S.J., Hansen, A.W. and Balling, N. 1998. Past temperatures directly from the Greenland Ice Sheet. Science 282: 268-271.
Hanna, E. 2001. Anomalous peak in Antarctic sea-ice area, winter 1998, coincident with ENSO. Geophysical Research Letters 28: 1595-1598.
Indermuhle, A., Monnin, E., Stauffer, B. and Stocker, T.F. 2000. Atmospheric CO2 concentration from 60 to 20 kyr BP from the Taylor Dome ice core, Antarctica. Geophysical Research Letters 27: 735-738.
Monnin, E., Indermühle, A., Dällenbach, A., Flückiger, J., Stauffer, B., Stocker, T.F., Raynaud, D. and Barnola, J.-M. 2001. Atmospheric CO2 concentrations over the last glacial termination. Nature 291: 112-114.
Parkinson, C.L. 2002. Trends in the length of the Southern Ocean sea-ice season, 1979-99. Annals of Glaciology 34: 435-440.
Parkinson, C.L. 2004. Southern Ocean sea ice and its wider linkages: insights revealed from models and observations. Antarctic Science 16: 387-400.
Watkins, A.B. and Simmonds, I. 2000. Current trends in Antarctic sea ice: The 1990s impact on a short climatology. Journal of Climate 13: 4441-4451.
Williams, M.B., Aydin, M., Tatum, C. and Saltzman, E.S. 2007. A 2000 year atmospheric history of methyl chloride from a South Pole ice core: Evidence for climate-controlled variability. Geophysical Research Letters 34: 10.1029/2006GL029142.