SPACE: These days the climate news is all about global warming, but global freezing was the biggest climate worry in Earth’s distant past. Long periods of severe cold – like Ice Ages on steroids – brought glaciers down to the equator and froze much if not all of the oceans. Scientists still debate what triggered these so-called Snowball Earths, but equally uncertain is how the Earth unfroze itself. One research group is studying the hyper-greenhouse warming that would be needed to end a million-year-long…