This is Scientific American's 60-second Science, I'm Julia Rosen.How much colder was it at the peak of the last ice age? That's a question scientists have been trying to answer for decades.And now they have a new best guess: 11 degrees Fahrenheit.That's a lot, especially considering it's a global average. Parts of North America were much colder."First of all, large areas of the northeast were completely under ice.So that would have been pretty chilly; you wouldn't be living there.But even here in the west, right, where we weren't covered by an ice sheet, it would have been something like 20 degrees Fahrenheit lower."Jessica Tierney, a paleoclimatologist at the University of Arizona.Tierney and her colleagues spent years compiling information about Earth's climate at the height of the last glacial period, about 20,000 years ago."We obviously don't have thermometers in the glacial period, so we have to instead look for these kinds of stand-in indicators."One kind of stand-in is plankton that lived in the ocean and got preserved in marine sediments.Scientists use these fossils to infer past ocean temperatures by studying changes in the chemistry of their shells and in the kinds of fats and other compounds they produced.Tierney and her team then combined these data with a climate model to give a full picture of glacial conditions."It's actually a technique used every day in weather forecasting. What's new is we're using it for the past, not the future.So we are actually, you know, hindcasting, if you will, rather than forecasting."