What boreholes tell us about the Earth's climate
A new study of temperatures in 300 boreholes, spanning four continents and five centuries, has confirmed what most scientists already believed - the Earth is getting warmer and the rate of warming has been accelerating rapidly since the 1900s. "In terms of climate change, the 20th century was not like any other century," said Henry N. Pollack, professor of geology at the University of Michigan. "Rock temperatures below the Earth's surface confirm that the average global surface temperature has increased by about one degree Celsius over the past five centuries. Half of the increase took place in the past 100 years. The 20th century is the warmest and has seen the greatest rate of warming of any of the five centuries we examined in our study.” (see Spectrum of Science 8/93, page 68).
During the American Geophysical Union conference held this week in San Francisco, Pollack presented temperature measurements from 300 underground boreholes in Europe, North America, Australia and South Africa.
Pollack is one of several geologists who measure the Earth's temperature by lowering sensitive thermometers into boreholes. Because subterranean rocks record temperature changes on the surface, boreholes are an important data source for scientists studying global climate change. Short-term variations, such as seasonal fluctuations, only penetrate a few meters into the earth. Long-term changes over several centuries are preserved at greater depths. Because meteorological data has only been recorded worldwide for about 100 years, borehole temperatures are particularly important in determining surface temperatures over the past few centuries.
Temperatures of individual boreholes can be distorted by local topography or climatic conditions. That is why Pollack and his assistant Shaopeng Huang combined the measurements from a continent into one data set in order to compensate for local effects and take account of regional trends. Then you combined all four blocks of data to get a global average. Because meteorologists study long-term climate changes at 100-year intervals, Pollack and Huang also analyzed trends in borehole data at 100-year increments.
When they compared average worldwide changes in borehole temperatures to the global meteorological record over the last century, they discovered that since 1900 there had been a 0.5 degree Celsius increase in temperature in both. "The ground tells the same story as the air," Pollack explained.
According to Pollack, 80 percent of all 1 degree warming occurred after 1750, when people began burning wood, coal, and other fossil fuels on a large scale during the Industrial Revolution. He therefore suspects that the increase in temperature is due to human activities rather than natural fluctuations.
"If the upward trend in greenhouse gas emissions continues, we can expect a further 1 degree Celsius increase in global temperature by 2050," warned Pollack. “This estimate is not based on model calculations but on a projection of the actual data. Our results are consistent with estimates of global warming released by the United Nations Intergovernmental Panel on Climate Change (IPCC). They are also consistent with the conclusion of the IPCC scientific panel that human activity is a significant driver of global warming.”
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