Ice Core Record Extended

Climate Change

Ice cores drilled at vostok Station in East Antarctica provide evidence of Earth's temperatures and greenhouse gases for the past 440,000 years. Now, data from a new ice core called EPICA Dome C, drilled roughly 600 miles from Vostok, extends that record back another 210,000 years.

A study by Thomas F. Stocker of the Physics Institute at the University of Bern, in Switzerland, and colleagues describes Dome C core data that reveal the relationship between global temperatures and atmospheric carbon dioxide concentrations for the period 390,000 to 650,000 years before present (Science 2005, 310, 1313). The data indicate that the current concentration of CO₂, at 380 ppm, is 27% higher than the preindustrial level and higher than any level attained during the past 650,000 years.

A second study based on Dome C data, also by Stocker and colleagues, lays out the relationship between global temperatures and the greenhouse gases methane and nitrous oxide (Science 2005, 310, 1317). It shows that methane levels over the past 650,000 years were never higher than they are today.

We have added another piece of information showing that the timescales on which humans have changed the composition of the atmosphere are extremely short compared to the natural time cycles of the climate system, Stocker says.

The record also indicates that the oldest two periods between frigid glacial eras were cooler, and lasted longer, than the more recent interglacials. The older interglacial periods persisted for 20,000 to 30,000 years, in contrast to more recent periods, which lasted about 10,000 years. The research shows that nature can generate warm [interglacial] phases that are long, with corresponding greenhouse gases that are stable, Stocker says, contrary to what some other studies had previously concluded.

As snow falls in Antarctica, it is compressed by overlying precipitation. Compressed snow transforms into ice at a depth of about 80 meters and traps air bubbles. Estimates of global temperatures are obtained by analyzing the ice for the isotopic ratio of deuterium to hydrogen. In warm periods, the ratio is higher because more heavy water (D₂O) molecules evaporate from the ocean, enriching the deuterium content of snowfall. The temperature record from ice layers is corroborated by data on foraminifera (single-celled organisms) from sediment cores drilled in the ocean floor. These well-established methods have been used since the late 1960s.

The levels of greenhouse gases are determined by analyzing CO₂, CH₄, and N₂O in the tiny air bubbles trapped in ice core layers. In general, as global temperatures rise, the concentrations of these gases also rise, Stocker explains.

The Dome C ice core was drilled and analyzed by the European Project for Ice Coring in Antarctica (EPICA), an international collaboration funded by the European Science Foundation and the European Commission scientific program. Stocker and colleagues created a climate record for 650,000 years by combining data from the Vostok, Dome C, and other ice cores.

In a Science commentary, Edward J. Brook, a paleoclimatologist at Oregon State University, Corvallis, writes that the primary message of Stocker's research is that the modern atmosphere is highly anomalous. The second message, he says, is that there is an intimate connection in these records between temperature change and greenhouse gas change.