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Category Archives: glaciation

Ice age chicken and egg: Did warming or carbon dioxide rise come first?

Crystals in a thin slice of Antarctic ice from Talos Dome seen through a polarized microscope.
Frédéric Parrenin

Ice cores have certainly done their share for climate science, but nobody’s perfect. While they contain wildly useful samples of ancient air and proxy records of temperature, linking the two together is harder than you might expect. Once you get down to fine-scale questions of timing, that difficulty becomes important.

Several studies that have looked at the start of the warming out of the last ice age have concluded that temperature began to rise several centuries before carbon dioxide increased. This week, a new study in Science shows that may not be the case.

The complications arise from the way that air is trapped in ice. The ice, of course, forms from snow compressed by the weight of the snow on top of it. Above the ice, then, it’s possible for air to move between the bits of snow (or partially compacted “firn”). So while this year’s snow (from which temperature is determined) is found at the surface, it won't be encapsulated as much as 100 meters below. That means that when you analyze a layer from an ice core, the age of the ice and the age of the air bubbles are offset. If we want to know whether temperature rose before CO2, we have to know which layer of ice corresponds to which layer of air bubbles.

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Bermuda’s golf courses sit on land that’s (partly) an African import

Enlarge / Bermuda's red soils sit atop limestone bedrock.
Photo by Diana Johnson, University of Illinois

The characteristics of a region’s soils are largely controlled by two factors: climate and geology. Soils are mostly composed of broken-down minerals from the bedrock (along with, critically, organic matter), and the weathering process is driven by climatic conditions. Soils can also form within the sediments deposited in an area, however, as is the case in the flood plains of river valleys. And there are other things that can transport sediment to a new home—like wind.

This is what makes the soil of the island of Bermuda so interesting. It seems like they don’t belong. The island’s shallow bedrock is composed of gray limestone, mainly in the form of broken-up bits of skeletons from marine organisms—the same stuff that makes up the beach sand. The soil, however, is rusty-red and clay-rich.

The reason for this sharp contrast has long been debated, with two hypotheses gaining the most support. One possibility is that the limestone contains a fair amount of clay, which is left behind as the limestone dissolves. (Limestone weathers quickly in rainwater, as evidenced by hard-to-read gravestones.) In this view, it’s not that the clay soil “doesn’t belong”, it’s just that it’s concentrated by the removal of the other minerals.

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Last big chill suggests lower climate impact of carbon

One of the key measures of the impact of atmospheric carbon dioxide is called the climate sensitivity, which provides an estimate of how much the planet will warm in response to a doubling of the CO2 concentration. This figure has been estimated using a variety of methods, producing a range of values; the Intergovernmental Panel on Climate Change estimates that the most likely value is 3 Kelvin, but recognizes there's a reasonable chance it could range anywhere from 2.4-4.5K. A new study that uses a climate model to evaluate the peak of the last glacial period, however, suggests that the IPCC's figure might be a bit high, and that very high values are overwhelmingly unlikely.

Glacial periods are triggered by small changes in the Earth's orbit. These aren't enough by themselves to alter the global climate, but they set off a drop in atmospheric CO2 and an expansion of ice, which reflects sunlight back to space. These feedbacks help the Earth enter a deep chill during glacial periods.

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