Both questions got closer to definitive answers last week. These holes, typically 2 to 4 inches wide and a quarter of a mile deep, originally were drilled for geological research, oil exploration or other reasons. Now they provide a handy means of taking the Earth's past temperature.
Heat moves two ways through the ground.
The other is from the bottom up, as heat from the Earth's molten innards travels upward. By lowering thermometers into these holes, and subtracting the bottom-up signal from the readings, scientists can find evidence of past temperature shifts as they propagate down through the rock. Any warming detected during the 16th, 17th and 18th centuries was almost certainly not caused by human society. Atmospheric carbon dioxide levels did not rise appreciably until the early 19th century. In the Feb. 18 issue of Science, several of the NRC panel members confirmed and expanded that idea.