The Heat Is Online

New Borehole Study Confirms Warming

Twentieth century hottest in a long time

Reuters News Service, Feb. 18, 2000
LONDON - The 20th century was the hottest for more than 500 years, scientists said.

The earth's temperature has increased by about one degree Centigrade (1.8 degrees Fahrenheit) since the 1500s, they said. In the Northern Hemisphere it was even faster: 1.1C (2F) in the last 500 years and 0.6 C (1.1F) in the 20th century alone.

"The 20th century was the warmest for the last five, and the one which was most rapidly changing," Henry Pollack, of the University of Michigan, said in a statement.

Pollack and his colleague Shaopeng Huang and Po-Yu Shen of the University of Western Ontario reconstructed past trends in climate change by using data on sub-surface temperature gathered from holes drilled into the ground at 616 sites on every continent except Antarctica.

Their research is published in the latest edition of the science journal Nature.

Highly sensitive thermometers in the holes show just how much temperature has changed over the years because signals from surface temperature travel below the earth and are preserved in rock and soil. Temperatures of the past 1,000 years are recorded to a depth of 500 metres (yards) down.

"So the upper 500 metres is an archive - a historical record of temperature changes that have occurred in the last thousand years," said Pollack

"Like any historical archive, there are of course missing pages, and the ink has run in a few places. But in principle, if you drill a borehole anywhere on a continent, you can observe a temperature profile and reconstruct what has happened at that location," he added.

By averaging the temperatures taken from the boreholes, the researchers reconstructed a picture of past climates. Pollack and his team had previously examined data from 358 borehole sites around the globe.

Their findings are consistent with other ways of estimating past temperature such as studying ice cores, lake sediment and coral growth.

"All the methods generally show a very unusual 20th century, and ours does, too," Pollack added. REUTERS NEWS SERVICE

Letters to Nature: Feb. 17, 2000

Temperature trends over the past five centuries reconstructed from borehole temperatures


For an accurate assessment of the relative roles of natural variability and anthropogenic influence in the Earth's climate, reconstructions of past temperatures from the pre-industrial as well as the industrial period are essential. But instrumental records are typically available for no more than the past 150 years. Therefore reconstructions of pre-industrial climate rely principally on traditional climate proxy records, each with particular strengths and limitations in representing climatic variability. Subsurface temperatures comprise an independent archive of past surface temperature changes that is complementary to both the instrumental record and the climate proxies. Here we use present-day temperatures in 616 boreholes from all continents except Antarctica to reconstruct century-long trends in temperatures over the past 500 years at global, hemispheric and continental scales. The results confirm the unusual warming of the twentieth century revealed by the instrumental record, but suggest that the cumulative change over the past five centuries amounts to about 1 K, exceeding recent estimates from conventional climate proxies. The strength of temperature reconstructions from boreholes lies in the detection of long-term trends, complementary to conventional climate proxies, but to obtain a complete picture of past warming, the differences between the approaches need to be investigated in detail.