September 10, 2007

Increasing Variability in a Warmer World?

Filed under: Temperature History

One of the pillars of the global warming scare is that as temperatures rise, the variability of climate will increase and we will see an increase in temperature extremes. More heat waves will result, more lives will be lost, and the increase in greenhouse gases will be to blame. The concept is illustrated in the Figure 1 – should the temperature distribution simply shift up a few degrees (as seen in a), fewer low temperatures would be experienced while the incidence of high temperatures would increase. However, should the variance of the temperature distribution increase (as seen in b), then the corresponding increase in extreme high temperatures would be even greater. Even the occurrence of record-breaking low temperatures can be blamed on global warming if one accepts the hypothesis that temperature variance is related to any change in mean temperature.


Figure 1. Schematic diagram illustrating the shifts in extremes that can occur under changing mean conditions, through either a symmetrical change in the probability density function of temperature (a), or through a shift in probability density function where the variance and the kurtosis of the distribution changes (b) (from Beniston and Goyette, 2007).

An interesting paper has appeared on this subject in Global and Planetary Change by Martin Beniston and Stéphane Goyette of Switzerland’s University of Geneva. They begin their article noting “It has been assumed in numerous investigations related to climatic change that a warmer climate may also be a more variable climate; such statements are often supported by climate model results.” We learn that “In terms of the extremes of temperature, for example, many of these studies have suggested that shifts in the intensity and/or the frequency of the hot and cold tails of the probability density function (PDF) of temperature are likely to be associated more with changes in the variance than changes in the mean temperature.” Of course, “If a warming climate is indeed accompanied by more variability, then it would follow that severe heat waves are likely to become commonplace, as reported by several studies following the 2003 heat wave that affected much of Europe from May to August of that year based on regional climate modeling studies.”

As a goal of their paper, they tell us “In view of the generally-accepted notion that a warmer climate may also be a more variable climate, it is of interest to investigate whether, based on long time-series of observational data, this hypothesis is indeed verified in a climate that has experienced a warming of 2°C or more at certain locations in Switzerland since the beginning of the 20th century.” Their final paragraph in the introduction strongly hints that this could be a World Climate Report feature as they write “This paper investigates the trends in means, variability and persistence of climate at both low and high elevation sites in Switzerland in order to provide some answers to the questions posed and show that commonly-accepted assumptions are, somewhat surprisingly, not necessarily fulfilled, at least in the Alpine region.” Sounds interesting to us!

The team collected daily maximum and minimum temperature records from a low elevation station (Basel, 369 meters above sea level) and a high elevation station (Saentis, 2500 meters above sea level) from 1901 to 2004. These stations were selected based on their quality, representativeness, and completeness. For each year, they calculated the mean and variance of the daily maximum and minimum temperatures, and they converted the mean and variance values into anomalies based on a 1961-1990 “normal” 30-year period.

The two figures below tell the story loudly and clearly. Figure 1 shows that minimum temperatures at both sites have increased by approximately 2°C over the most recent four decades, but during the same period, variance decreased. Similarly, as shown in Figure 3, maximum temperatures have increased by 1°C over the most recent several decades, and variance levels decreased during that same time period. They authors note “Whatever the period considered (e.g., the first 40 years of the century through to the mid-century maximum, the last 20 years when temperature anomalies are systematically positive, or the entire 104-year record), the variance decreases with increasing temperature.”


Figure 2. Time series of minimum temperature anomalies (a) at Basel (solid line) and Saentis (dashed line) and minimum temperature variance anomalies (b) from 1901–2004. A 5-point filter has been applied to remove the noisiness of interannual variability (from Beniston and Goyette, 2007).


Figure 3. Time series of maximum temperature anomalies (a) at Basel (solid line) and Saentis (dashed line) and maximum temperature variance anomalies (b) from 1901–2004. A 5-point filter has been applied to remove the noisiness of interannual variability (from Beniston and Goyette, 2007).

Their conclusions are priceless as they write “This investigation, carried out for a low (Basel) and a high (Saentis) elevation site in Switzerland, has shown that contrary to what is commonly hypothesized climate variability does not necessarily increase as climate warms. Indeed, it has been shown that the variance of temperature has actually decreased in Switzerland since the 1960s and 1970s at a time when mean temperatures have risen considerably. Nevertheless, these findings are consistent with the temperature analysis carried out by Michaels et al. (1998) [that would be usn’s -eds.] where their results also do not support the hypothesis that temperatures have become more variable as global temperatures have increased during the 20th century.”

We’re not going to say we told you so, but … we told you so a decade ago!

References:

Beniston, M., Goyette, S., 2007. Changes in variability and persistence of climate in Switzerland: Exploring 20th century observations and 21st century simulations. Global and Planetary Change, 57, 1-15.

Michaels, P.J., Balling Jr., R.C., Vose, R.S., Knappenberger, P.C., 1998. Analysis of trends in the variability of daily and monthly historical temperature measurements. Climate Research, 10, 27–33.




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