Man-made and Natural Variation

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Science  14 Jul 2000:
Vol. 289, Issue 5477, pp. 217-219
DOI: 10.1126/science.289.5477.217e

Average global surface atmospheric temperatures have risen about 0.6°C over the past 150 years, although not in an uninterrupted fashion. How much of this is due to external factors (anthropogenic fossil fuel burning, solar variability, volcanic eruptions) versus an internal and natural variability of the climate system has been debated for years.

Andronova and Schlesinger attempt to quantify these contributions by using 135 years of data on atmospheric greenhouse gas concentrations, volcanic activity, and solar irradiation as boundary conditions for a simple climate/ocean model, and then by calculating the effects on Earth's radiation balance. They find that anthropogenic greenhouse forcing has become the dominant external factor, but a residual factor, presumably associated with the internal dynamics of the climate system, has influenced the global climate, too (see also Crowley et al., Research Article, this issue, p. 270). The rapid rate of temperature increase that occurred between 1904 and 1944, and the cooling that took place from 1944 to 1976, are the results of a powerful internal climate variation of unknown origin. This variation, with a period of 65–70 years, and which may be related to the North Atlantic Oscillation, augmented or retarded the steady trend to warmer conditions that greenhouse gases have caused. Therefore, caution should be exercised when looking at temperature trends, and a pause or even reversal of current warming should not be interpreted as evidence against global warming. — HJS

Geophys. Res. Lett. 27, 2137 (2000).

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