BioEd Online

Current global average temperatures exceed those experienced for about 75 percent of the past 11,300 years, a new study suggests. And if climate models are any clue, by the end of this century they’ll be the highest they’ve been since the last ice age ended.

Instrumental records of climate only extend back to the late 19^th century. Beyond that, scientists depend on analyses of natural chronicles such as tree rings and cave formations. But even these archives have their limits: Many detailed reconstructions of climate, particularly of temperature, apply only to limited regions or extend back at most a couple of millennia, says Shaun Marcott, a climate scientist at Oregon State University in Corvallis.

So Marcott and his colleagues set about reconstructing global climate trends all the way back to 11,300 years ago, a time when the Northern Hemisphere was emerging from the latest ice age. To do so, they collected and analysed data gathered by other teams. The 73 climate records they considered included sediment cores drilled from lake bottoms and seafloors worldwide, along with a handful of ice cores collected in Antarctica and Greenland, says Marcott.

Each of these chronicles spanned at least 6,500 years, including a baseline period that stretched from 3550 to 2550 BC, a millennium-long interval that began near the centre of the post-ice-age period. For some records, the researchers inferred past temperatures from the magnesium/calcium ratio in shells of tiny creatures that had died and dropped to the ocean floor; for others they did so by measuring the lengths of long-chain organic molecules, called alkenones, trapped in the sediments.

Coming out of the ice age, global average temperatures warmed until reaching a plateau between 7550 and 3550 BC. Then a long-term cooling trend set in, with the coldest stretch of the post-ice-age era falling between AD 1450 and 1850. Since then, temperatures have been on the rise at a dramatic clip: From the first decade of the 20^th century to now, global average temperatures rose from near their coldest point since the last ice age to nearly the warmest, Marcott and his team report today in Science [1].

The temperature trends the team identified for the past 2,000 years are statistically indistinguishable from results obtained by other researchers in a previous study, says Marcott. “That gives us confidence that the rest of our record is right too,” he adds.

Marcott and his colleagues “have put together a pretty impressive set of climate proxies,” says Gavin Schmidt, a climate scientist at the NASA Goddard Institute for Space Studies in New York City. “The overall climate picture has been clear for a long time, mostly from the Northern Hemisphere, but this compilation really puts the rest of the world in context,” he adds.

“Prior to this study, researchers could only guess whether global temperatures had exceeded the warmest part of the present interglacial period,” says Darrell Kaufman, a geologist at Northern Arizona University in Flagstaff. The new findings show that the recent high temperatures are not necessarily the warmest, but they are unusually high, he notes.

The temperature trends seen during most of the post-ice-age period match those expected from natural factors such as the long-term variation in the tilt of Earth’s axis, says Marcott. But in the last century and a half, industrial emissions of carbon dioxide have increased—a factor that helps explain why global temperatures have increased so quickly in recent decades, he suggests.

Climate models suggest that by the end of this century, regardless of the carbon dioxide emissions scenario presumed for coming decades, temperatures will be the highest seen since the last interglacial period, the researchers contend.