Azara Blog: Fluctuations in global temperature over time

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Date published: 2005/02/10

Nature says (subscription service):

Fluctuations in global temperature during the past millennium may have been larger and more frequent than previously thought, says a fresh analysis of the climate record.

The analysis is likely to reignite a long-standing controversy over the cause and extent of natural climate variability, scientists say, although the unprecedented nature of global warming since the mid-1980s remains unquestioned. The study was conducted by Anders Moberg of Stockholm University, Sweden, and his team.

According to an earlier study, which produced the widely cited 'hockey stick' graph, average Northern Hemisphere temperatures during the past millennium were relatively stable until the late nineteenth century, when they began to increase sharply. In 2001, this assessment was used to underpin the most recent report of the Intergovernmental Panel on Climate Change (IPCC) -- the scientific branch of the United Nations Framework Convention on Climate Change.

But the Moberg study, which is published just as the Kyoto Protocol comes into effect, suggests that notable climate changes have occurred throughout the recent past. If such natural fluctuations continue in the future, they may "amplify or attenuate anthropogenic climate change significantly", the authors conclude.

Moberg's group used a combination of different 'proxies' to reconstruct decadal and centennial temperature changes. Proxies are climate indicators such as tree rings, pollens and boreholes, and the researchers used each one at the timescale that it records most accurately: tree rings are used for reflecting annual variations, for example, and sediments for longer-term changes. The researchers then used 'wavelet analysis' to combine the timescales in the optimum manner.

Time will tell whether this model is more or less accurate than previous ones. As with all such studies the political repercussions are almost more important than the scientific ones, although they should not be since the study has no direct bearing on the current debate about "global warming".

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