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Natural Thermometer Put to Use

An MIT scientist has found that a natural phenomenon that can be measured from anywhere in the world is a sensitive thermometer of surface air temperature in the tropics and could be used to detect global warming.

The work was reported in Science magazine earlier this year.

Currently scientists studying global warming base their work on surface air temperatures collected from around the globe. But the data are incomplete. "Only a fraction of the measurements made [at field stations] are actually reported because of inefficiency and financial constraints," said Earle R. Williams, an associate professor in the Department of Earth, Atmospheric and Planetary Sciences.

In addition, those data that are reported may be contaminated by "heat-island" effects, Professor Williams said. Parking lots and other structures that absorb heat are often built near instrumentation shelters, which can affect readings.

Professor Williams has found that the Schumann resonance (SR), a global electromagnetic phenomenon, can be used to measure temperature without these drawbacks. "Because it's global, the SR shouldn't have the bias of the heat-island effect, plus it will get around the poor measurements currently made, particularly in the tropics," he said.

Professor Williams discovered the correlation between the SR and temperature through his work with lightning. The SR is a product of lightning-at any one moment it integrates the electrical noise from every lightning bolt on earth.

And lightning, he found, is dependent on temperature. Although "everyone's noticed that a hot summer day has more lightning," he said, no one had ever done a scientific study to quantify it.

Professor Williams did just that by comparing the amount of lighting with temperature in three locations around the world. He found that in all three-Darwin, Australia; Kourou, French Guiana; and Orlando, Florida-lightning, as expected, increased with temperature.

From there, he concluded that "if the same sensitive dependence shows up in three different places, that relationship has to be global." Hence his interest in the SR. He suspected that it, too, would be sensitive to temperature, but didn't have the SR measurements to prove it.

Until last fall, when he found that Charles Polk, a colleague at the University of Rhode Island, had measured the SR continuously for about six years with equipment hidden in a forest near the university (since the SR is global, it can be measured anywhere). With Polk's data, Professor Williams compared the average value of the SR every month over five and a half years with the average temperature change for the entire tropics every month over the same period. The SR closely followed temperature change.

Professor Williams also found that the SR was extremely sensitive. "When it was only a little hotter, the SR was greatly enhanced, approximately doubling for every 1'C change in temperature," he said.

Such sensitivity is important in tracking global warming. "When you look at the historical record," Professor Williams explained, "the natural variations in temperature are huge compared to the very small trend over the last several decades [claimed to result from global warming] we're looking for."

Professor Williams notes that the SR is most effective as a thermometer of temperature in the tropics, because conditions there are most conducive to creating lightning (warm, moist air that rises quickly up into the upper atmosphere). However, he said, global temperatures have been found to follow fluctuations in the tropics. "So if the tropics are warming," he concluded, "odds are the whole earth is warming, too."

In addition to being a sensitive tool for studying global warming, the SR will also help scientists learn more about lightning itself. At present, Professor Williams said, "there's a lot of evidence that ice particles at the tops of thunderstorms are responsible for the charge separation that causes lightning, but we don't understand all the details of this phenomenon." Temperature measurements of the surface air in the tropics, where most lightning occurs, are critical to this research. Unfortunately, these measurements are also among the most poorly reported.

"It's not exactly high-tech in the middle of the Amazon [or some other lands around the equator], but those are precisely the areas we want temperature data from," Professor Williams said. The SR will give scientists a new way to make those measurements.

Currently Professor Williams is working to organize "a vastly improved temperature data set for the tropics" based on conventional measurements. That way, he said, "we can see how good the SR really is as a thermometer."

In addition, he is measuring the SR from the top of the Green Building on campus. (He notes, however, that he is thinking of measuring the signal from Rhode Island with Polk's equipment, which is currently not being used, because in Cambridge the SR is often hidden by electrical noise from motors, arc welders and other equipment.)

Professor Williams' work was primarily supported by the National Science Foundation.

A version of this article appeared in the November 12, 1992 issue of MIT Tech Talk (Volume 37, Number 13).

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