From Climate Change Reconsidered, a work of the Nongovernmental International Panel on Climate Change
One more measure of climatic change is temperature variability. Has the earth experienced more record highs or lows of temperature during the Current Warm Period?
Oppo et al. (1998) studied sediments from Ocean Drilling Project site 980 on the Feni Drift (55.5°N, 14.7°W) in the North Atlantic. Working with a core pertaining to the period from 500,000 to 340,000 years ago, they analyzed δ18O and δ13C obtained from benthic foraminifera and δ18O obtained from planktonic foraminifera to develop histories of deep water circulation and sea surface temperature (SST), respectively. In doing so, they discovered a number of persistent climatic oscillations with periods of 6,000, 2,600, 1,800 and 1,400 years that traversed the entire length of the sediment core record, extending through glacial and interglacial epochs alike. These SST variations, which were found to be in phase with deep-ocean circulation changes, were on the order of 3°C during cold glacial maxima but only 0.5 to 1°C during warm interglacials.
Similar results were obtained by McManus et al. (1999), who also examined a half-million-year-old deep-sea sediment core from the eastern North Atlantic. Significant SST oscillations were again noted throughout the record, and they too were of much greater amplitude during glacial periods (4 to 6°C) than during interglacials (1 to 2°C). Likewise, in another study of a half-million-year-long sediment core from the same region, Helmke et al. (2002) found that the most stable of all climates held sway during what they called “peak interglaciations” or periods of greatest warmth. In this regard, we note that the temperatures of all four of the interglacials that preceded the one in which we currently live were warmer than the present one, and by an average temperature in excess of 2°C, as determined by Petit et al. (1999). Hence, even if the earth were to continue its recent (and possibly ongoing) recovery from the global chill of the Little Ice Age, that warming likely would lead to a state of reduced temperature variability, as evidenced by real-world data pertaining to the past half-million years.
Shifting our focus to the past millennium, Cook et al. (2002) report the results of a tree-ring study of long-lived silver pines on the West Coast of New Zealand’s South Island. The chronology they derived provides a reliable history of Austral summer temperatures from AD 1200 to 1957, after which measured temperatures were used to extend the history to 1999. Cook et al. say their reconstruction indicates “there have been several periods of above and below average temperature that have not been experienced in the 20th century.” This finding indicates that New Zealand temperatures grew less variable over the twentieth century.
Manrique and Fernandez-Cancio (2000) employed a network of approximately 1,000 samples of tree-ring series representative of a significant part of Spain to reconstruct thousand-year chronologies of temperature and precipitation, after which they used this database to identify anomalies in these parameters that varied from their means by more than four standard deviations. In doing so, they found that the greatest concentration of extreme climatic excursions, which they describe as “the outstanding oscillations of the Little Ice Age,” occurred between AD 1400 and 1600, during a period when extreme low temperatures reached their maximum frequency.
In yet another part of the world, many long tree-ring series obtained from widely spaced Himalayan cedar trees were used by Yadav et al. (2004) to develop a temperature history of the western Himalayas for the period AD 1226-2000. “Since the 16th century,” to use their words, “the reconstructed temperature shows higher variability as compared to the earlier part of the series (AD 1226-1500), reflecting unstable climate during the Little Ice Age (LIA).” With respect to this greater variability of climate during colder conditions, they note that similar results have been obtained from juniper tree-ring chronologies from central Tibet (Braeuning, 2001), and that “historical records on the frequency of droughts, dust storms and floods in China also show that the climate during the LIA was highly unstable (Zhang and Crowley, 1989).” Likewise, in a study of the winter half-year temperatures of a large part of China, Ge et al. (2003) identified greater temperature anomalies during the 1600s than in the 1980s and 1990s.
Focusing on just the past century, Rebetez (2001) analyzed day-to-day variability in two temperature series from Switzerland over the period 1901-1999, during which time the two sites experienced temperatures increases of 1.2 and 1.5°C. Their work revealed that warmer temperatures led to a reduction in temperature variability at both locations. As they describe it, “warmer temperatures are accompanied by a general reduction of variability, both in daily temperature range and in the monthly day-to-day variability.” We see that even on this much finer time scale, it is cooling, not warming, that brings an increase in temperature variability.
In a study based on daily maximum (max), minimum (min), and mean air temperatures (T) from 1,062 stations of the U.S. Historical Climatology Network, Robeson (2002) computed the slopes of the relationships defined by plots of daily air temperature standard deviation vs. daily mean air temperature for each month of the year for the period 1948-1997. This protocol revealed, in Robeson’s words, that “for most of the contiguous USA, the slope of the relationship between the monthly mean and monthly standard deviation of daily Tmax and Tmin—the variance response—is either negative or near-zero,” which means, he describes it, that “for most of the contiguous USA, a warming climate should produce either reduced air-temperature variability or no change in air-temperature variability.” He also reports that the negative relationships are “fairly strong, with typical reductions in standard deviation ranging from 0.2 to 0.5°C for every 1°C increase in mean temperature.”
In Canada, according to Shabbar and Bonsal (2003), “extreme temperature events, especially those during winter, can have many adverse environmental and economic impacts.” They chose to examine trends and variability in the frequency, duration, and intensity of winter (January-March) cold and warm spells during the second half of the twentieth century. From 1950-1998, they found that western Canada experienced decreases in the frequency, duration, and intensity of winter cold spells. In the east, however, distinct increases in the frequency and duration of winter cold spells occurred. With respect to winter warm spells, significant increases in both their frequency and duration were observed across most of Canada, with the exception of the extreme northeastern part of the country, where warm spells appear to be becoming shorter and less frequent. In the mean, therefore, there appear to be close-to-compensating trends in the frequency and intensity of winter cold spells in different parts of Canada, while winter warm spells appear to be increasing somewhat. As a result, Canada appears to have experienced a slight amelioration of extreme winter weather over the past half-century.
In another study that suffers from the difficulty of having but a few short decades of data to analyze, Iskenderian and Rosen (2000) studied two mid-tropospheric temperature datasets spanning the past 40 years, calculating day-to-day variability within each month, season, and year. Averaged over the entire Northern Hemisphere, they found that mid-tropospheric temperature variability exhibited a slight upward trend since the late 1950s in one of the datasets; but, as they note, “this trend is significant in the spring season only.” They also admit that “the robustness of this springtime trend is in doubt,” because the trend obtained from the other dataset was negative. For the conterminous United States, however, the two datasets both showed “mostly small positive trends in most seasons.” But, again, none of these trends was statistically significant. Therefore, Iskenderian and Rosen acknowledge they “cannot state with confidence that there has been a change in synoptic-scale temperature variance in the mid-troposphere over the United States since 1958.”
In an attempt to determine the role that might have been played by the planet’s mean temperature in influencing temperature variability over the latter half of the twentieth century, Higgins et al. (2002) examined the influence of two important sources of Northern Hemispheric climate variability—the El Niño/Southern Oscillation (ENSO) and the Arctic Oscillation—on winter (Jan-Mar) daily temperature extremes over the conterminous United States from 1950 to 1999. With respect to the Arctic Oscillation, there was basically no difference in the number of extreme temperature days between its positive and negative phases. With respect to the ENSO phenomenon, however, Higgins et al. found that during El Niño years, the total number of extreme temperature days was found to decrease by around 10 percent, while during La Niña years they increased by around 5 percent. With respect to winter temperatures across the conterminous United States, therefore, the contention that warmer global temperatures—such as are typically experienced during El Niño years—would produce more extreme weather conditions is found to be false.
Over the same time period, Zhai and Pan (2003) derived trends in the frequencies of warm days and nights, cool days and nights, and hot days and frost days for the whole of China, based on daily surface air temperature data obtained from approximately 200 weather observation stations scattered across the country. Over the period of record, and especially throughout the 1980s and 1990s, there were increases in the numbers of warm days and nights, while there were decreases in the numbers of cool days and nights, consistent with an overall increase in mean daily temperature. At the extreme hot end of the temperature spectrum, however, the authors report that “the number of days with daily maximum temperature above 35°C showed a slightly decreasing trend for China as a whole,” while at the extreme cold end of the spectrum, the number of frost days with daily minimum temperature below 0°C declined at the remarkable rate of 2.4 days per decade.
The two papers of Wang et al. (2010) and Cattiaux et al. (2010) discuss the extreme cold winter of 2009/10 in the Northern Hemisphere, when extreme low temperatures were recorded in Europe and North America. This record-breaking cold winter over the Northern Hemisphere has been linked to the extreme (negative) phase of the NAO (North Atlantic Oscillation, a large-scale index based on surface pressure difference between Reykjavik, Iceland, and Lisbon, Portugal) during the winter months. According to Wang et al., the NAO has been trending downward since the early 1990s and the extreme low value of the NAO during December 2009-January 2010 produced a severe cold-air outbreak over North America, Europe, and parts of Asia. The authors further add that if "this downward trend of NAO continues, more frequent cold outbreak and heavy snows are likely in the coming years.” However, Cattiaux et al. suggest that future winters over Europe may not be as cold as experienced in 2009/10. In view of these contrasting conclusions, continued research and improved climate modeling is necessary.
In considering this entire body of research, it is evident that air temperature variability almost always decreases when mean air temperature rises.
Braeuning, A. 2001. Climate history of Tibetan Plateau during the last 1000 years derived from a network of juniper chronologies. Dendrochronologia 19: 127-137.
Cattiaux, J., Vautard, R., Cassou, C., Yiou, P., Masson-Delmotte, V., and Codron, F. 2010. Winter 2010 in Europe; A cold extreme in a warming climate. Geophysical Research Letters 37: L20704 doi:10.1029/2010GL044613.
Climate Change Reconsidered: Website of the Nongovernmental International Panel on Climate Change. http://www.nipccreport.org/archive/archive.html
Cook, E.R., Palmer, J.G., Cook, B.I., Hogg, A. and D’Arrigo, R.D. 2002. A multi-millennial palaeoclimatic resource from Lagarostrobos colensoi tree-rings at Oroko Swamp, New Zealand. Global and Planetary Change 33: 209-220.
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Helmke, J.P., Schulz, M. and Bauch, H.A. 2002. Sediment-color record from the northeast Atlantic reveals patterns of millennial-scale climate variability during the past 500,000 years. Quaternary Research 57: 49-57.
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