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Natural Climate Variability on Decade-to-Century Time Scales
Response to Pinatubo for various scenarios calculated by Hansen et al. (1992). See text for details. (From Hansen et al., 1992; reprinted with permission of the American Geophysical Union.)
global temperature cold compared to the expected warming for a long time; however, the lower panel fails to show this persistence in a model where expected greenhouse warming is reduced. A reading of the text of Hansen et al. (1992)
TABLE 1 Ersatz Distribution of Volcanic Eruptions
Year of Eruption
Bandai San, Ritter Island
Bogoslav (1890), Awu
Una Una (1898), Mt. Pelée, Soufrière
Santa Maria, Minami Iwoshima (1904)
Taal (1911), Katmai, Sakurashima (1914)
Calculated response to all volcanos between Krakatoa and Katmai for various choices of model gain (expressed in terms of characteristic ocean delay).
shows that this scenario also included an additional volcanic eruption in 1995!
There would appear to be some possibility of distinguishing different t's by considering sequences of volcanos, since larger t's imply cumulative effects for a sequence of volcanos while smaller t's imply largely independent responses to each individual volcano within a sequence.
These considerations should be relevant to the period 1883-1912 (Krakatoa to Katmai), when there were a number of major eruptions whose total dust production was about three times that of Krakatoa alone (Oliver, 1976). This period was followed by a relative absence of eruptions until about 1950. We have modeled the volcanism of the period 1883-1912 by an ersatz distribution in which various closely spaced eruptions are combined. This is described in Table 1. Figure 9 shows the response to this distribution of volcanism for various choices of t. Clearly, for larger t's (greater than 16 years) each eruption adds cooling to the response of successive volcanos, leading to cumulative, long-lasting cooling following the period 1883-1912, while for smaller t's (less than 16 years) the response has little cumulative character, although the large number of eruptions leaves the temperature depressed for the period 1883-1912.
On the whole, the temperature record shows no evidence of a cumulative effect of volcanism (viz., Figure 5),4 suggesting that appropriate t's are less than 16 years, while Figure
Granted, the warming trend seen after 1920 might be an extension of a similar trend that would have occurred during 1883-1912, had it not been for the cooling due to the volcanos. However, there is no evidence of such a trend before 1883. In addition, the warming ceased after 1940. Finally, had the volcanos been masking a strong trend before 1912, the net warming that would have occurred would have been unprecedented and in excess of anything we could account for by greenhouse considerations—especially given the long ocean delay that would pertain to large greenhouse sensitivity.