Global average temperatures are now higher than they have been for about 75% of the past 11,300 years, a study suggests. By the end of this century they will be the highest ever since the end of the most recent ice age according to climate models.
Instrumental records of climate extend back to only the late nineteenth century. Beyond that, scientists depend on analyses of natural chronicles such as tree rings and isotope ratios in cave formations.
Climate scientist, Shaun Marcott and his team at Oregon State University have set about reconstructing global climate trends all the way back to 11,300 years ago, when the Northern Hemisphere was emerging from the most recent ice age. To do this, they collected and analysed data gathered by other teams. The 73 overlapping climate records that they considered included sediment cores drilled from lake bottoms and sea floors around the world, along with a handful of ice cores collected in Antarctica and Greenland.
Each of these chronicles spanned at least 6,500 years, and each included a millennium-long baseline period beginning in the middle of the post-ice-age period at 3550 BC.
After the ice age, they found, global average temperatures rose until they reached a plateau between 7550 and 3550 BC. Then a long-term cooling trend set in, reaching its lowest temperature extreme between ad 1450 and 1850. Since then, temperatures have been increasing at a dramatic rate: from the first decade of the twentieth century to now, global average temperatures rose from near their coldest point since the ice age to nearly their warmest.
The temperature results indicated by separate methods are statistically indistinguishable from results obtained by other researchers which confirms that the methods are valid.
The temperature trends 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 past century and a half, industrial emissions of the greenhouse gas carbon dioxide have increased — which helps to explain why global temperatures have risen so quickly in recent decades, he suggests.
Climate models from the Intergovernmental Panel on Climate Change suggest that by the end of this century, regardless of future carbon dioxide emissions, temperatures will be at their highest since the end of the most recent ice age, the researchers say.
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