Borehole temperatures confirm global warming

Researchers use a drilling rig to bore into the earth in central Brazil, one of 616 borehole sites across the world.

By Environmental News Network staff

February 17, 2000
Web posted at: 2:54 p.m. EST (1954 GMT)

Temperature readings taken from more than 600 holes drilled into Earth's surface confirm that a 500-year warming trend accelerated in the latter half of the 20th century.

"Some 80 percent of that warming corresponds with the growth of industrialization," said Henry Pollack, a geology professor at the University of Michigan in Ann Arbor and co-author of the study in today's issue of Nature.

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The study is the latest piece of evidence that indicates the warming trend in the latter half of the 20th century 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," writes Jonathan Overpeck, a geoscientist at the University of Arizona in Tucson, in an accompanying Nature article.

"So we are left with the likelihood that human-induced global warming is under way, caused by emissions of 'greenhouse gases' such as carbon dioxide, and that the next century is going to see even greater warming," he added.

Pollack and colleagues carried out their analysis — the results of which do not address the causes of the warming, they caution — by lowering thermometers into holes drilled into Earth's surface, primarily at mining sites.

The borehole study confirms that a 500-year warming trend accelerated in the latter half of the 20th century.

The readings reveal how Earth's surface temperature has changed in the past. Temperatures at the surface penetrate Earth's crust; the longer a temperature exists at the surface, the deeper it penetrates.

Daily, weekly and even monthly temperatures do not penetrate more than a few meters, but century-long temperatures propagate slowly down, reaching a depth of about 492 feet (150 meters) in 100 years and 1,640 feet (500 meters) in 1,000 years.

"So the upper 500 meters of the crust is an archive of what has taken place over the past 1,000 years," said Pollack. "We send a thermometer down and take the temperature at a number of depths along the way, establishing a profile of temperature down the borehole."

The borehole method complements other systems of temperature analysis such as tree-ring data and ice cores. While other proxies do a good job of showing year-to-year variation, the borehole method shows a robust long-term trend, said Pollack.

Borehole temperatures in Pollack's study also show greater warming over the past 500 years than temperatures calculated by other means. "My hunch is boreholes do a better job of establishing long-term trends," he said.

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