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Twentieth century 'warmest in 500 years'

Source: BBC

17th Feb 2000

Drilling down into subsurface rocks shows
temperature changes over centuries

Studies of temperature records preserved deep in underground rocks show that the Earth has been gradually warming over at least the last 500 years. And the studies, by scientists in the US and Canada, show that the trend accelerated markedly during the 20th Century, which was the warmest of the past five centuries.

Since 1500, the Earth's temperature has increased by about one degree Celsius, with half of that increase occurring in the last century.

Trend picks up

The warming trend is speeding up. Almost 80% of the net temperature increase observed occurred in the 19th and 20th centuries. In the Northern Hemisphere, the five-century change has been 1.1 degrees, with 0.6 recorded since 1900. The studies, reported in the science journal Nature, are based on analysis of borehole temperatures from 616 sites on every continent except Antarctica. The scientists lowered sensitive thermometers into holes drilled down from ground level to discover how surface temperature altered in the past. A typical borehole was measured at 10-metre depth intervals down to as far as 600 m.

Records preserved

The technique is possible because of heat conduction, which means that temperature changes at the surface generate "signals" that penetrate subterranean rocks.

The signals from short-term daily or seasonal variations penetrate only a few metres and are rapidly lost. But changes over centuries are preserved in deeper rock, although the signals travel very slowly, penetrating only about 500 metres in 1,000 years.

One of the team, Professor Henry Pollack of the University of Michigan, said: "The upper 500 metres is an archive. Like any historical archive, there are of course missing pages, and the ink has run in a few places. "But in principle, if you drilled a borehole anywhere on a continent, you could observe a temperature profile and be able to reconstruct what had happened at that location."

The team's work involved calculating averages from all the boreholes investigated, and built on a previous analysis of borehole temperature data from 358 sites.

The scientists also compared their results with those obtained from other methods of estimating past temperature change, including studies of tree rings, ice cores, lake sediments and coral growth.

"All the methods generally show a very unusual 20th Century, and ours does too," said Professor Pollack. "It is the warmest century of the last five, and the one which is most rapidly changing."

Forecast re-inforced

"What we show that is somewhat different is that the total temperature change over the past five centuries has been greater than some of the other methods are showing."

In an accompanying article in Nature, Jonathan Overpeck, of the University of Arizona, Tucson, says the team's results re-inforce the forecast for this century: continued warming ahead.

"But they also provide unsettling indications that human alteration of the climate system over the past century will make the reliable prediction of climate change an even tougher business than expected.

"Their analysis is the latest of several to indicate that late 20th Century warming is without precedent in the past 400 to 1,000 years.

"We do not know of any combination of natural mechanisms that can explain this phenomenon. So we are left with the likelihood that human-induced global warming is under way."

And he adds a warning. "The results show yet again that the 20th Century record of climate variability is too short and cloaked with human-induced influences to provide a clear indication of natural climate variability. "Earlier studies may have underestimated the full amplitude of natural decade-to century-scale climate variability."

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