ANN ARBOR --- Earth's 500-year warming trend accelerated considerably in the 20th century, the warmest of the past five centuries, a new study of borehole temperatures from more than 600 sites around the world confirms. What's more, the results suggest that at least in the Northern Hemisphere, the 500-year warm-up has been even greater than previously estimated with other techniques.

The study, co-authored by Shaopeng Huang and Henry N. Pollack of the University of Michigan and Po-Yu Shen of the University of Western Ontario, appears in the Feb. 17 issue of Nature.

Since 1500, Earth's temperature has increased about 1 degree Celsius (1.8 degrees Fahrenheit), with half of that increase taking place in the 20th century alone, says Pollack, a professor of geological sciences. In the Northern Hemisphere, the temperature change has been 1.1 C (2 F) over the past five centuries and 0.6 C (1.1 F) in the 20th century.

The scientists based their analysis on temperature readings taken by lowering sensitive thermometers into holes drilled from Earth's surface. These readings reveal how surface temperature changed in the past. That's possible because, thanks to heat conduction, temperature changes at the surface generate "signals" that travel downward into subsurface rocks, says Huang, a research scientist at the U-M. Signals from short-term daily or seasonal variations penetrate only a few meters, and Earth quickly "forgets" them, but temperature changes that take place over hundreds of years are preserved in deeper rock.

Temperature signals travel slowly through the rocks, penetrating only about 500 meters in 1,000 years, "so the upper 500 meters is an archive---a historical record of temperature changes that have occurred in the last thousand years," says 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 would drill a borehole anywhere on a continent, you could observe a temperature profile and be able to reconstruct what had happened at that location."

But a single borehole can't tell a global story. For that, the scientists calculate averages from hundreds of borehole sites around the world. Their current work builds on a previous analysis of borehole temperature data from 358 sites in eastern North America, central Europe, southern Africa and Australia, which showed a similar worldwide warming over the past 500 years. By adding more sites to the analysis and greatly expanding the geographical coverage, the scientists increased their confidence in the results and were able to look at regional, as well as global, trends. In the new study, they also compared their results with those obtained with other methods of estimating past temperature change, such as 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. The 20th century is the warmest century of the last five, and the one which is most rapidly changing," says Pollack. "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."

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• Climate
• Global Warming
• Environmental Issues

• Instrumental temperature record
• Temperature record of the past 1000 years
• Temperature record
• Geologic temperature record
• Greenland ice sheet
• Sequence stratigraphy