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ScienceDaily (Feb. 17, 2005) — A new study of climate in the Northern Hemisphere for the past 2000 years shows that natural climate change may be larger than generally thought. This is displayed in results from scientists at the Stockholm University, made in cooperation with Russian scientists, which are published in Nature on 10 Feb 2005.
This study builds on an analysis of indirect climate data, such as information from ocean and lake bottoms, ice sheets, caves and annual tree rings. The use of this kind of material to reconstruct climate far back in the past is nothing new in itself. The difference between the new study from previous ones, is the selection of data series and the method used to estimate temperatures from them.
A 1000-year long climate simulation, undertaken (by another research group) with a computer model for the physics of the atmosphere and the oceans, show large similarities with the new reconstruction. The climate in this model is governed by reconstructed variations of solar radiation and the amount of volcanic dust in the atmosphere (which reflects sun-light back into space). The fact that these two climate evolutions, which have been obtained completely independently of each other, are very similar supports the case that climate shows an appreciable natural variability - and that changes in the sun's output and volcanic eruptions on the earth may be the cause.
This means that it is difficult to distinguish the human influence on climate from natural variability, even though the past 15 warm years are best explained if one includes human influence in the simulations. The new study underscores the importance of including natural climate variability in future scenarios. It is not only the humans that can cause appreciable climate changes - nature does it all the time by itself.
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