Carbon dioxide and the end of the last ice age

Antarctic ice-core records covering the past million years reveal a sawtooth pattern of temperature rise and fall roughly every 100 000 years, corresponding globally to major advances and retreats of year-round ice cover at high latitudes and altitudes. That temperature oscillation is closely tracked by a corresponding rise and fall of the atmosphere’s carbon dioxide concentration, as one might expect from the importance of CO₂ as a greenhouse gas. But the Antarctic data, in isolation, seem to show CO₂ increase generally lagging temperature rise by a few centuries—a puzzle often cited by climate-change skeptics. Now a team led by Jeremy Shakun has compiled temperature-proxy data from ice and sediment cores at 80 locations worldwide to create a thermometric record of unprecedented global scope and temporal resolution for the most recent deglaciation, which began about 20 000 years ago and leveled off 10 000 years later to initiate the present ‘interglacial’ epoch. The new record shows global mean temperature rise, unlike Antarctic temperature, clearly trailing CO₂ increase during most of that period. (Global temperature is plotted here as differences from the early interglacial mean.) The team attributes the misleading early rise of Antarctic temperature to interruption by meltwater of a northward Atlantic current that ordinarily exports heat from the Southern Hemisphere to the Northern Hemisphere. That conjecture is supported by data from ocean-floor sediments and climate-model simulations. The team’s simulations strongly suggest that the rising CO₂ concentrationwas the last deglaciation’s principal driver. (J. D. Shakun et al., Nature 484, 49, 2012 .)—Bertram Schwarzschild