CORVALLIS, Ore. - A temperature analysis of more than 600 boreholes from throughout the Northern Hemisphere suggests that the Earth's climate may be warming at a higher rate than tree-ring analysis and other methods had led scientists to believe.
"If we're right, these boreholes are showing that the Earth is more sensitive to whatever is forcing the climatic change," said Robert N. Harris, an associate professor in the College of Oceanic and Atmospheric Sciences at Oregon State University and a principal investigator in the study.
Results of the research by Harris and colleague David S. Chapman of the University of Utah were just published in the Journal of Geophysical Research. The researchers also will present their data in December at the annual meeting of the American Geophysical Union.
Borehole temperatures have been measured since the 1920s, but only recently has this temperature analysis been applied to global warming studies. Unlike most "proxy" methods to reconstruct climate models, which depend entirely on statistical analysis, borehole temperature research is based on the physics of heat diffusion.
Harris offers an analogy to describe how it works.
"On a smaller scale, it's similar to underground pipes freezing in the spring instead of during the coldest part of winter," he said. "It takes time for the cold winter temperatures to propagate through the ground. Similarly, if you put one end of a steel poker into a fire, and hold the other end, the heat propagates toward your hand.
"If at some later time you take a series of temperature measurements along the length of the rod, you would be able to estimate the temperature of the fire and how long the poker had been in the fire. The distance the poker had warmed is related to time, and the amount of warming is related to the temperature of the fire."
In the ground, rocks are such poor conductors of heat that the effect of a changing surface temperature 500 years ago is felt at a depth of about 200 meters, Harris says. The scientists make careful temperature measurements in boreholes that are as deep as 500 meters. These temperatures reflect the adjacent rock and tell the researchers how temperatures have changed over long periods of time.
What the research cannot tell scientists is what the temperature may have been for a particular year, Harris said.
"Heat diffusion causes the signal to get smeared out, so the deeper you look, the smaller the signal," he pointed out. "Eventually, the signal is lost in background noise. This process also means that you only get multi-year averages."
Harris and Chapman examined temperature data from boreholes throughout the Northern Hemisphere, which helps eliminate regional anomalies in their findings. They estimate that the Earth has warmed 1.1 degrees C. over the past 500 years - more than double the 0.4- to 0.5-degree estimates suggested by most tree-ring analysis.
In their article, they say the difference may be that tree-ring analysis primarily reflects temperatures when trees are actively growing during the warm season, but doesn't reflect changes in winter temperatures. Much of the annual warming recorded by instruments over the past 100 years has occurred during the winter season, they add.
The boreholes used in the research were generated from a variety of sources, including mineral exploration, dry water wells and those done specifically for the temperature research. The best environment for drilling, Harris says, is where the rock is solid and impermeable, limiting advection.
A typical borehole may be six inches in diameter and 200 meters deep. Much deeper and the temperature differences become too minute to pick up, Harris said. However, that depth allows them to take measurements that go back about 500 years - or roughly the time Columbus was first approaching the New World.
"We know by comparative data that borehole analysis, as remarkable as it may seem, really works," Harris said. "For the periods of overlap when we can compare with recorded temperature data, the correlation is excellent. Beyond that, it is simply a matter of applying the physics of heat diffusion. "And those measurements tell us the Earth is warming faster than we previously thought."