SAN FRANCISCO—A new 300-site survey of borehole temperatures spanning four continents and five centuries has confirmed what most scientists already believe—the Earth is getting warmer and the rate of warming has been accelerating rapidly since 1900.

“In terms of climate change, the 20th century has not been just another century,” said Henry N. Pollack, University of Michigan professor of geological sciences. “Subsurface rock temperatures confirm that the average global surface temperature has increased about 1 degree C. (1.8 degrees F.) over the last five centuries with one-half of that warming taking place in the last 100 years. The 20th century is the warmest and has experienced the fastest rate of warming of any of the five centuries in our study.”

Pollack presented temperature readings from 300 underground boreholes in Europe, North America, Australia and South Africa at the American Geophysical Union meeting held here this week.

Pollack is one of several geologists who take the Earth’s temperature by lowering sensitive thermometers into boreholes drilled from the surface. Because subsurface rocks preserve a record of actual surface temperature changes over time, boreholes are an important data source for scientists studying global climate change. Short-term changes, such as seasonal variations, penetrate only a few meters underground. Long-term changes on scales of hundreds of years are preserved at greater depths. Since meteorological data has been recorded globally only for the last 100 years or so, borehole temperatures are especially important in determining surface temperature for previous centuries.

Individual borehole temperatures can be skewed by local topography or climate conditions, so Pollack and Shaopeng Huang, U-M assistant research scientist, merged the readings into continental data ensembles to balance out local effects and let regional trends come through. They then combined all four regions to get a global average. Because meteorologists track long-term climate changes in 100-year intervals, Pollack and Huang also looked for century-long trends in borehole data.

When they compared the average worldwide borehole temperature change with global meteorological records over the last century, they found both recorded a 0.5 degree C. average global temperature increase since 1900. “The ground says the same thing the air says,” Pollack explained.

According to Pollack, 80 percent of the total 1 degree C. warming recorded in borehole readings from 1500 to the present occurred after 1750 when people began large-scale burning of coal, wood and other fossil fuels during the Industrial Revolution. Since most warming has taken place after 1750, Pollack believes it is likely a direct result of human activity, rather than a natural climate fluctuation.

“If the upward trend of greenhouse gas emissions continues, we can expect another 1 degree C. increase in average global temperature by 2050,” Pollack said. “This estimate is not based on model computations, but a projection of actual data. Our results agree with the estimates of global climate warming issued by the United Nations’ Intergovernmental Panel on Climate Change (IPCC) and are fully consistent with the conclusion of the IPCC’s scientific panel that human activity is a significant driving force behind global warming.”

Pollack’s study has been funded by the National Science Foundation and the Czech-USA Cooperative Science Program.

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