The last ice age: how the northern hemisphere froze over

When half of Europe lived under three kilometres of ice

Why was half of Europe buried under three kilometres of ice while the southern hemisphere barely noticed? The question is not rhetorical: between 20,000 and 22,000 years ago, during the Last Glacial Maximum, the northern hemisphere's ice sheet reached its maximum extent, and today's political geography would have stood no chance.

The first temptation is to blame the Gulf Stream. Analyses of microfossils show that the thermohaline circulation changed in intensity, but a circular dilemma remains: did the ice age slow the currents, or did the slowdown trigger the ice age? There are studies on the matter, but the ultimate cause remains unresolved. Meanwhile, the famous contrast between Madrid and New York, both at 40ºN, becomes a laboratory: if the ocean current weakens, Iberian winters start to resemble those of the Atlantic east coast.

The southern hemisphere didn't freeze because there was nowhere for it to

Simulations of the Last Glacial Maximum, such as those shown by Scotese's project, explain the asymmetry. The north has far more landmass; the ice sheet reached down to 50ºN, with thicknesses of up to three kilometres over solid ground, and icebergs drifted as far as the Gulf of Cádiz. In the south, at 50ºS only the tip of Patagonia and Antarctica emerge, and the ice expands over ocean, which limits its growth. That is why the map of the southern hemisphere doesn't look so covered: it wasn't that it wasn't cold, it's that there was little continental surface.

The irony is that the ice, as it retreats, is still leaving its mark: the isostatic rebound of the crust is still measured today at latitudes once buried. And more than one person, with summer upon us, would suggest that another ice age would freeze a few problems along the way. It's not a good idea, but the remark says a lot about how the climate debate works: somewhere between the data and the wishful thinking.