ananyo writes "Global average temperatures are now higher than they have been for about 75% of the past 11,300 years, a study published in Science suggests. Researchers have reconstructed global climate trends all the way back to when the Northern Hemisphere was emerging from the most recent ice age. They looked at 73 overlapping temperature records including sediment cores drilled from lake bottoms and sea floors around the world, and ice cores collected in Antarctica and Greenland. For some records, the researchers inferred past temperatures from the ratio of magnesium and calcium ions in the shells of microscopic creatures that had died and dropped to the ocean floor; for others, they measured the lengths of long-chain organic molecules called alkenones that were trapped in the sediments. 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, they report (abstract)."