Posted on 04/17/2002 9:15:35 AM PDT by cogitator
Rock Measurements Suggest Warming is Global
ANN ARBOR, Michigan, April 16, 2002 (ENS) - Over the past 50 years, the rocks of Earth's continental crust have warmed in concert with the warming of the oceans, atmosphere and ice, says a team of Michigan and Canadian researchers.
"Our findings remove any last doubt that this is anything other than a global phenomenon," said Henry Pollack, University of Michigan (UM) professor of geological sciences.
"Until recently, the story of global warming has been built up primarily on the basis of temperature measurements at the surface of the land and oceans," said Pollack. "These measurements have been painstakingly acquired and put together, and there has been enough information to reconstruct a temperature history for the Earth's surface for the past 140 years. But it's all based on surface measurements."
About a year ago, a group of researchers determined how much heat had been gained during the last half of the 20th century throughout the atmosphere, the depths of the oceans, and the cryosphere - the portion of Earth's surface where water is in solid form such as sea ice, snow cover, glaciers, ice caps and permafrost.
However, their analysis left out one major component of the climate system: continental rock, which covers almost 30 percent of the planet's surface.
Now, Pollack and colleagues have completed the picture by determining how much the continental rock has warmed in recent centuries.
The scientists based their analysis on temperature readings taken by lowering sensitive thermometers into holes drilled from Earth's surface into rock formations on six continents, including Africa, Asia, Europe, North America, South America and Australia. These readings can reveal how temperatures have changed in the past, because the heat that surface rocks absorb from the atmosphere travels downward into subsurface rocks, leaving a distinct signature in the rocks.
Signals from short term daily or seasonal variations penetrate just a few meters, but temperature changes that take place over hundreds of years are preserved in deeper rock.
The researchers' calculations, based on data from 616 bore holes, found evidence of an increase in the heat content of the continents over the past 500 years, with more than half of that heat gain occurring during the 20th century, and almost one-third of it since 1950.
"The magnitude of the warming we estimate is very similar to that which has come from the studies of the ocean, atmosphere and ice," said Pollack. "We believe it makes a persuasive case that the warming has been truly global."
The research is reported in the April 15 issue of the geology journal "Geophysical Research Letters."
The subsurface heat profile is taken into account. I would guess that these are mostly water wells or geophysical exploration (i.e. core profile), so they aren't that deep.
It seems to me that in this thread (which dates back to last April) that I posted links to the best explanations I could find of the method on-line. I'll check and refer you to the numbers in a subsequent reply.
There is no doubt about it. They have probably developed something like a tree-ring history with at least a time scale and a temperature function so they can estimate what the temperature in the atmosphere was, say, 5000 years ago.
The link above has a temperature profile derived from borehole temperatures. The plot goes back 500 years; the article says they can go back about 1,000 years.
The article also addresses some of the other questions you raised. The data does complicate the question of natural vs. anthropogenic warming, because the borehole data shows a 500-year warming trend that accelerated in the 20th century. But if the world was in a Little Ice Age period during that time, with surface ups and downs that are seen in the tree-ring and sediment core record, it makes sense that there has been warming since 1500 when the surface variability is smooothed out.
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