Greta Thunberg thinks warm weather is a crisis. And since she’s easily upset it’s a good thing she doesn't realize her own country of Sweden used to be much hotter than today. For most of the past 10,000 years, in fact. A 2006 study based on lake bed sediments found that today's July temperatures in Northern Sweden average about 11 degrees C, but from 8,000 years ago to about 200 years ago, averaged 13 to 14 degrees C. Another study in 2008 found the same thing for Southern Sweden: Summers today are two to three degrees cooler than they were for thousands of years. How did the Swedes survive?
Studies of past temperatures are notoriously controversial, as the battle over the hockey stick illustrates. The best ones don't try to reconstruct the entire planetary climate. Instead they focus on one region and use a single type of temperature proxy that offers a reasonably consistent record over time. And these two Swedish studies fall into that category.
They use the chemical composition of mud layers at the bottom of Swedish lakes to infer what the pollen and diatom populations were, which in turn vary with temperature in predictable ways. The picture that they provide is that, after the glaciers retreated 10,000 years ago, temperatures began rising pretty quickly, reaching a maximum some time in the interval from 6,000 to 8,000 years ago during the revealingly named “Holocene Climatic Optimum”, then started drifting down while staying high compared to the present, until sharply dropping in the past 200 years to today's relatively cold conditions.
That pattern seems to fit the picture for a lot of Northern Europe. Since the end of the last ice age, the warmest temperatures were around 6,000 years ago with long warming and cooling intervals both before and after, and the Little Ice Age that ended in the mid-1800s was the coldest period we've seen since the glaciers retreated.
We are bouncing back from an exceptionally cold period that, had it not started warming, might have marked the onset of another ice age. Which, had it happened, would have made it very hard for Greta to sail to New York to protest global warming. Or perhaps skate.