There isn't really a zero. In its billions of years of history, Earth's temperature has fluctuated widely between large extremes, as far as we can tell extrapo

There isn't really a zero. In its billions of years of history, Earth's temperature has fluctuated widely between large extremes, as far as we can tell extrapolating from traces we've found. Within "recent" history, its temperature has fluctuated between ice ages and warm periods, each of which last some hundreds of thousands of years, and the few thousand years of human existence seems to coincide with the tail end of an ice age (though keep in mind we're talking about very large time frames).