Showing that the continents have warmed along with the other principal components of Earth's climate system indicates that the warming of our planet has been truly global, the researchers say.

"Our findings remove any last doubt that this is anything other than a global phenomenon," says Henry Pollack, U-M professor of geological sciences, who collaborated on the work with U-M assistant research scientist Shaopeng Huang, U-M graduate student Jason Smerdon, and Hugo Beltrami of St. Francis Xavier University in Nova Scotia. The researchers report their work in the April 15 issue of Geophysical Research Letters, a leading geology journal.

"Until recently, the story of global warming has been built up primarily on the basis of temperature measurements at the surface of the land and oceans," says Pollack.

"These measurements have been painstakingly acquired and put together, and there has been enough information to reconstruct a temperature history for the Earth's surface for the past 140 years. But it's all based on surface measurements."

That approach was augmented about a year ago when another group of researchers determined how much heat had been gained during the last half of the 20th century throughout the atmosphere, the depths of the oceans, and the cryosphere (the portion of Earth's surface where water is in solid form such as sea ice, snow cover, glaciers, ice caps and permafrost). However, their analysis left out one major component of the climate system: continental rock, which covers almost 30 percent of the planet's surface.

Now, Pollack, Beltrami and colleagues have completed the picture by determining how much the continental rock has warmed in recent centuries. The scientists based their analysis on temperature readings taken by lowering sensitive thermometers into holes drilled from Earth's surface into rock formations on six continents (Africa, Asia, Europe, North America, South America, and Australia).

These readings can reveal how temperatures have changed in the past, because the heat that surface rocks absorb from the atmosphere travels slowly downward into subsurface rocks, leaving a distinct signature in the rocks.

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.

The researchers' calculations, based on data from 616 bore holes, found evidence of an increase in the heat content of the continents over the past 500 years, with more than half of that heat gain occurring during the 20th century and nearly one-third of it since 1950.

"The magnitude of the warming we estimate is very similar to that which has come from the studies of the ocean, atmosphere and ice," says Pollack. "We believe it makes a persuasive case that the warming has been truly global."

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International Heat Flow Commission
The University of Michigan
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