As I mentioned last week, Dr. Richard Alley, Evan Pugh Professor of Geosciences at Penn State, has taken the time to answer some of my questions about paleoclimatology. In today's post, he talks more specifically about the information that can be gleaned from an ice core.

Question: Specifically related to ice cores, can you explain what information is captured in glacial ice and how you "read" it? How can you tell the difference between a cold year and a year with higher than average precipitation, for instance?

Dr. Alley's Answer: Temperature, I explained earlier (Trees and Ice are Links to Past Climate).

Another we use (a "fuzzy" one--doesn't reveal short-lived temperature changes a long time ago, but is reliable) is the physical temperature of the ice. If you throw a frozen turkey in the oven, turn on the heat and then leave, and your significant other comes home and wants to know how long the turkey has been in but doesn't have your cell phone number, why, drill a hole in the turkey, measure the temperature, and the colder the inside, the shorter time the turkey has been cooking. The Greenland ice sheet a mile down is colder than the surface is, and is colder than the bedrock two miles down, because the ice is still warming from the cold of the ice age, and we can estimate how cold the ice age was in central Greenland from this.

Another is related to the isotopic composition of the water in the ice. If you have a bunch of water molecules, a few of them will have an extra neutron or two ("heavy water"). The heavier water is still water, but just a tad heavy. The heavier stuff rains or snows out of a cloud first. As an air mass moves over an ice sheet and cools, the heavy falls out, so the remaining vapor comes closer and closer to being all "light", so the next rain or snow becomes more and more "all light". The colder the air mass, the lighter the isotopic makeup of the rain or snow. So, the isotopic history in an ice core is a temperature history. If you look at isotopes, and isotopes of gases, and borehole temperatures, you can start to get a reliable history of temperature at the site.

In favorable ice cores, we date by counting annual layers (summer snow and winter snow look different, are isotopically different, chemically different, electrically different, etc.). We check as far back as we can using known time markers (find the fallout from historically dated volcanic eruptions, or from atomic-bomb explosions), and it works really well. We have several people count several things several times, try really hard not to "cheat" by checking on the answers of others, and then compare our ages to those for correlative climate events in other records (tree rings, etc. from elsewhere). The thickness of an annual layer (after a correction for thinning from compaction under weight of snow above and from ice flow) gives the snow accumulation rate. Changes in "dirtiness" of ice (how much dust) either represent changes in the dust delivery, or in the delivery of water to dilute the dust; know the accumulation rate, and you have the dilution.

A refrozen-meltwater layer shows it was warm. Fallout of odd nuclides made by cosmic rays in the atmosphere tell how many cosmic rays were getting in (and because the magnetic field and the sun's activity help protect us from cosmic rays, tell about the magnetic field and the sun's activity). One can find micrometeorites, pollen, sea salt, etc. and learn about those. And, the trapped bubbles are our only reliable samples of old air and the greenhouse gases in that old air. So, there is plenty to learn.