Climate clues in ice

Antarctic core reveals lowest levels of atmospheric carbon dioxide on record

A kilometers-long ice core from Antarctica has recorded climate information for the past 800,000 years and has revealed a three millennia–long period when carbon dioxide levels in the air were lower than any previously measured.

IT’S IN THERE The concentrations of carbon dioxide and methane found in bubbles of ancient air (dark spots) trapped in Antarctic ice provide clues to ancient climate. AWI Bremerhaven, University of Bern

GOING WAY BACK Researchers drilled a 3.2 kilometer–long, 98 millimeter–diameter ice core from East Antarctica, one portion shown here, that includes precipitation that fell during the past 800,000 years. L. Augustin/LGGE

The longest detailed records of atmospheric gases previously reported, from the uppermost sections of a 3.2 kilometer–long ice core drilled in eastern Antarctica, go back 650,000 years, says Thomas Stocker, a climate physicist at the University of Bern in Switzerland. Isotopic analyses of the ice in the deepest portions of that sample — at depths between 3,060 meters and 3,190 meters — have revealed how temperature in the region varied between 650,000 and 800,000 years ago. But researchers previously hadn’t assayed the gases trapped in bubbles in that portion of the core, Stocker notes. He and his colleagues have now performed those analyses and report their findings in the May 15 Nature.

Once snow piles up more than 80 meters or so deep, the pressure at the bottom of the heap converts the densely packed, somewhat porous snow into impermeable ice, thereby locking bubbles of air in place. As snow continues to accumulate, the mass of ice — whether a mountain glacier or a continent-wide ice sheet — becomes a chronicle of long-term variations in the atmospheric concentrations of various gases, including those such as carbon dioxide and methane that are linked to climate change.

In many aspects, the new results provide no surprises, Stocker says. Earth still plunged into an ice age every 100,000 years or so, punctuated with warm spells, or interglacials, that lasted about 10,000 years. And, as found in previous studies of this core’s shallower ice, the rises and falls of the region’s temperature are closely linked to increases and decreases in levels of carbon dioxide and methane trapped in the ice’s bubbles. In other aspects, however, the samples provide new clues about ancient climate.

Previously, ice core studies have found that natural levels of atmospheric carbon dioxide varied between 180 and 300 parts per million. However, during a 3,000-year period that began about 670,000 years ago, CO2 levels dropped to a minimum of 172 ppm, a low level unseen in other, more recent samples. Since the Industrial Revolution began, CO2 concentrations have been on the rise because of the burning of fossil fuels; today, levels of that greenhouse gas exceed 380 ppm and are increasing, on average, about 2 ppm each year (SN: 5/10/08, p. 18).

The geologic record, including seafloor sediments, suggests that the long-term average amount of CO2 in the air has been declining for at least 50 million years. The new findings, however, hint that atmospheric levels of CO2 in general rose from 800,000 to 400,000 years ago and then began to decline again. Also, the researchers report, interglacial periods between 800,000 and 400,000 years ago weren’t as warm as those that have occurred more recently. These variations, although small, may reveal previously unrecognized cycles in climate that scientists don’t yet understand, Stocker says.

Determining the duration and magnitude of these cycles, if indeed they are real, may require scientists to discover Antarctic locales that harbor ice older than 800,000 years, says Ed Brook, a paleoclimatologist at Oregon State University in Corvallis. The coldest and thickest parts of the ice sheet in East Antarctica, for example, may retain deep ice that fell as precipitation more than 1.5 million years ago, he notes. Samples that old, he adds, could help solve a long-standing mystery posed by the geologic record: Why does a 100,000 year–long climate cycle recorded in recently deposited ocean sediments disappear in rocks laid down as sediments more than 900,000 years ago?

Even if scientists never find ice more than 800,000 years old, the new findings confirm that Earth’s atmosphere today is unusual, Brook says. “Modern levels of greenhouse gases have no natural analogue in the ice record,” he notes.

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