BioEd Online

The Earth's temperature may have fluctuated more wildly during the past 1000 years than previously thought, according to a new study that challenges how researchers use tree rings and corals to give us a picture of the Earth's past.

If true, the study suggests that recent warming might not be as unique as was thought previously, and might partly be due to natural temperature cycles, rather than humans spewing carbon dioxide into the atmosphere.

To work out the planet's temperature during the past few hundred years, researchers often look at the width and density of annual rings in trees or the growth of corals. Such temperature indicators, known as proxies, are then used to construct average global temperatures.

But this method could be tainted by a systematic error, according to Hans von Storch, a climate modeller at the GKSS Institute for Coastal Research in Geesthacht, Germany, and his colleagues. Consequently researchers might have underestimated the size of temperature fluctuations from medieval times until the nineteenth century, by a factor of two or more.

Recalculating climate

Storch and his colleagues started with a computer model of climate history, and extracted the temperature for specific points in the Northern Hemisphere at particular times. They used these temperatures to figure out what the corresponding proxies should look like at these times and places. This picture was fed back into a model to recalculate the past global climate and the team's results are published in Science¹.

Surprisingly, temperature fluctuations in the reconstruction were smaller than in the original climate model. The researchers suspect that any attempts to reconstruct the global climate from real proxies, such as tree rings, corals, ice cores and historical weather records might similarly underestimate temperature changes. This calls into question the results of many such climate models.

Scientists currently use a combination of proxies and written records from the twentieth century to estimate past climate change and the probable future course of the current warming trend.

The mean global temperature has increased by 0.6ºC during the past century, with particularly pronounced warming in the last 20 years. At present researchers think that this warming is different from anything that has happened on our planet in the past 10,000 years.

Proxy or impostor?

Chris Forest, a climate researcher at the Massachusetts Institute of Technology in Boston, says there is no question that the Earth is warming up at the moment, and that greenhouse gases have contributed to this change. But the study does challenge how much of the warming, particularly prior to 1980, is a result of natural fluctuations in temperature.

"This is the first critical look I have seen into possible limitations of this widely-used methodology," says Forest. "If the analysis stands up, we will have to re-examine what proxy data are really telling us."

But Michael Mann, a climate researcher at the University of Virginia in Charlottesville, who has carried out the most widely cited proxy-based reconstructions of historic climate variability², says that he already knows about the problems raised by Storch and has tried to take them into account in his calculations.

4 October 2004: This article has been corrected. The study covered the past 1000 years, not the past 2000 years as previously stated.