February 22, 2001 – Measuring temperatures inside holes in the ground is an accurate way of showing that Earth's Northern Hemisphere has warmed about 2 degrees Fahrenheit (1.1 degrees Celsius) since the Industrial Revolution began, University of Utah scientists found.

"This is another piece of independent evidence that says global warming is real, and that it is proceeding at a rate faster than we have observed in recent geologic history," said David S. Chapman, graduate school dean and professor of geology and geophysics at the University of Utah.

"The warming we found implies a link between global warming and greenhouse gas emissions from industrialization" that began in about the 1750s, said Robert N. "Rob" Harris, an assistant professor of geology and geophysics. "The warming is real and significant."

Harris and Chapman conducted the new study, which is featured on the cover of the March 1 issue of Geophysical Research Letters, a journal published by the American Geophysical Union.

The new study found the same 2-degree-Fahrenheit warming of the Northern Hemisphere as another study published a year ago in the journal Nature by scientists at the University of Michigan and University of Western Ontario. That study was based on measurements made when thermometers were lowered into more than 600 boreholes throughout the Northern Hemisphere.

Harris said the new study confirms and strengthens the earlier findings by showing the borehole temperature method accurately reflects real changes in air temperatures measured by weather stations during the past 100 years.

Warm and cold weather at Earth's surface sends "thermal waves" underground. The waves warm or cool subterranean rock. In recent years, geophysicists have measured temperatures in abandoned mineral- or water-exploration holes or other boreholes, and then subtracted the effect of heat rising upward from within the Earth. That leaves a pattern of underground temperatures from which scientists can calculate the extent to which Earth's climate has warmed or cooled during the past 500 years or so.

Most weather stations have existed to measure surface air temperatures only during the last century or less, with a few such records going back to 1860. Harris and Chapman showed borehole temperature measurements for the past century or so correlated accurately with actual weather station temperature records. That gives researchers confidence that borehole temperature measurements accurately reflect real air-temperature changes for the past 500 years, Harris said.

"The difference between their study and our study is that they looked at the borehole data alone, whereas we combined an analysis of the borehole data with the past 100 years of weather records," Harris said. "We confirmed that ground and air temperatures do track each other. We are adding credibility and confidence to the [borehole] method."

Harris and Chapman analyzed weather-station temperature records since the 1860s and data collected by several groups of scientists who measured temperatures in 439 boreholes throughout the Northern Hemisphere, including Utah, the U.S. Great Plains, eastern Canada, central Europe, Russia and China. The holes range from 650 to 2,000 feet deep.

They found that average Northern Hemisphere temperatures have increased 1.25 degrees Fahrenheit (0.7 degrees Celsius) between the advent of the Industrial Revolution in the last half of the 1700s and the average temperature recorded by weather stations during the 1961-1990 period. Weather stations show average temperatures rose another 0.72 degrees Fahrenheit (0.4 degrees Celsius) during the 1990s.

So warming since pre-industrial times totals almost 2 degrees Fahrenheit (1.1 degrees Celsius), Harris and Chapman concluded.

The Industrial Revolution saw a major increase in combustion of oil, coal and other fossil fuels, leading to increased emissions of carbon dioxide, the main "greenhouse gas" that is emitted by human activities and traps heat in Earth's atmosphere to warm climate.

Harris and Chapman said their study's primary new contribution was in determining more accurately the average "baseline" temperatures before the 20th century. Estimates of climate change based on tree growth rings, growth patterns in corals, pollens in lake sediments and other such "proxy" indicators suggested there was little change in global temperatures from 1500 to 1900, Chapman said.

"Our findings suggest the warming started about 1750 to 1800," he added.

Some critics have argued that global warming is not real, and that the warming trend during the 1900s simply represents a return to normal conditions after a cool period during the 1800s. Harris said the new study undercuts that argument because it found temperatures were cooler before industrialization than they are now.

Chapman said that while few borehole temperature measurements have been made in the Southern Hemisphere, those measurements are consistent with the warming measured in the Northern Hemisphere, indicating warming has occurred worldwide. Harris said research now in progress appears to confirm that opinion.

Like earlier studies, the new one found less warming near equatorial latitudes and more warming closer to the Arctic.

Polar regions are ecologically sensitive, so greater warming means "there could be greater ecological effects in high-latitude regions, such as melting of permafrost, release of methane [a greenhouse gas] from tundra regions [which could aggravate global warming] and potential melting of polar ice sheets," Chapman said.