Climate Science

Beyond Natural Variation

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Science  08 Nov 2002:
Vol. 298, Issue 5596, pp. 1139
DOI: 10.1126/science.298.5596.1139b

In their efforts to determine whether physical processes provide any evidence that anthropogenic activity has begun to affect Earth's climate, researchers have long pointed out that the historical and geologic records of mountain glaciers show that they have receded substantially in the past few centuries. In order to ascertain whether this retreat lies within the range of natural variation, Reichert et al. have conducted calculations of mountain glacier (from Norway and Switzerland) fluctuations, using a mass balance model of intermediate complexity and a dynamic ice flow model. Fluctuations were produced exclusively by internal variations in the climate system. Their simulations indicate that the preindustrial fluctuations of these glaciers can be explained by internal climate variability, independently of external forcing factors such as solar irradiation changes or volcanic or anthropogenic effects. In contrast, the observed present-day retreat is too large to be due to natural causes alone and must result from external events, with anthropogenic climate forcing being a likely candidate. — HJS

J. Clim.15, 3069 (2002).

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