Here is our final question-and-answer session with Dr. Richard Alley, Evan Pugh Professor of Geosciences at Penn State. To review, Dr. Alley studies paleoclimates from ice cores and is one of the authors of the IPCC's 4th Assessment.

I'd like to say thank to you to Dr. Alley for sharing so much information with the blog over the last couple of weeks.

Question: Dr. Alley, how can you tell what forcings were involved with past climate changes?

Dr. Alley's Answer: One can use strength of magnetization of lava flows and sediments to learn about the strength of the magnetic field, and then the changes in cosmic-ray-produced things not explained by the magnetic field are probably changes in the sun (we don't think the total cosmic ray flux changes much). There are small changes in the sun recorded, and small changes in climate going along, with a behavior that matches what we expect. The large changes in magnetic field that have happened don't seem to have had an effect on climate, so that doesn't seem to be an important forcing. There is little evidence of changes in micrometeorites/space dust (except for the occasional giant meteorite such as the one way back that killed the dinosaurs), so space dust doesn't seem important. Volcanoes block the sun and bring cold--a degree or two for a year or two--this is well-recorded, but isn't "organized"--one eruption doesn't trigger another, so the volcanoes mostly make "noise". The very skinny version is that climate has been mostly controlled by sun (with small changes in total brightness giving small climate changes), and by greenhouse gases (mostly CO2). In addition, features of Earth's orbit (which move the sunshine around over the planet over tens of thousands of years--more in the north or the south, or more at the equator or the poles) have paced ice ages, in part by affecting CO2. Over long times, drifting continents affect where ocean currents go and other such things, which matter to climate.

Question: What degree of confidence do paleoclimatologists have in the climate reconstructions? Does it diminish farther back in time?

Dr. Alley's Answer: Given the huge range of indicators, the degree of confidence goes from pound-on-the-table/this-is-almost-surely-right to this-is-more-likely-than-not-to-be-right/can't-say-much-more. In general, confidence is better more recently.

Question: Can you compare the current warming in the Arctic with the warming of the 1930s and 1940s? What were the climate forcings that brought on that warming?

Dr. Alley's answer: This is still a topic of research, but the recent paper by Johannessen et al., and other work, give good pointers. The most obvious difference between the earlier warming in the Arctic, and the more recent one, is that the earlier one was mostly restricted to the Arctic, and the more-recent one is almost everywhere. The geographic pattern of the earlier one did not look like that expected for greenhouse gases (which had not risen much yet), and the more recent one does look like the greenhouse-gas pattern. Some worker points to a role for sun and volcanoes in the earlier one (occasionally, by accident, brighter sun and fewer volcanoes happen at the same time), together with a "dynamic" component--perhaps a change in the ocean overturning linked to the Atlantic Meridional Oscillation. The climate is a complex-enough beast that glib answers should be viewed with caution--the IPCC or National Academy statements on global warming are NOT glib, but are cautious and carefully reasoned, and must be.