From Climate Central's Michael D. Lemonick:

The so-called Medieval Warm Period (MWP), a 400-year span from about 950 to 1220 A.D. when the Vikings colonized Greenland, was relatively balmy by the standards of the past 2,000 years, leading some to argue that the global warming we’re now experiencing isn’t that big a deal. But a new report in the journal Geology argues that the MWP wasn’t all that warm after all — and certainly not as warm as the climate is today.

According to William D’Andrea of Columbia University’s Lamont-Doherty Earth Observatory and his co-authors, summer temperatures in the Svalbard Archipelago, a group of islands in the Arctic Ocean about 400 miles north of Norway, have been between 3.6°F and 4.5°F higher over the past 25 years, on average, than the summers the Vikings enjoyed.

“As more research has come in over the past decade,” D’Andrea said, “it’s becoming clear that this medieval period was not uniformly warm, and we can see that for sure in this one location.”

The question isn’t just academic. Climate scientists are convinced that rising temperatures during the 20th century, and especially over the past 50 years, are largely due to heat-trapping properties of greenhouse gases generated by the burning of coal, oil and other carbon-based fuels. If they’re going to project how fast and how high temperatures will rise in the future — to know where we’re going — it’s important to know where we’ve been.

What complicates the matter is that global temperatures can also change for purely natural reasons — and if they spiked dramatically during the MWP, it might suggest that nature has a significant role in today’s warming.

“We need to disentangle natural variability from the changes we humans are provoking,” D’Andrea said, and the best way to do that is to look to a time before humans began burning fossil-fuels in earnest.

The problem, of course, is that the Vikings didn’t have thermometers. We know from their records and those of others that the MWP was warm, but not precisely how warm. So like all scientists who want to understand ancient temperatures, D’Andrea and his colleagues relied on proxies — natural processes that change with temperature.

In this case, they looked at lipids, or fat molecules created as a biological by-product by algae in Kongressvatnet, a lake on the island of Spitzbergen. When the lake water is colder, the algae tend to churn out unsaturated fats; when it’s warmer, the fats tend to be saturated. Then, when the algae die, their fat-containing corpses drift to the lake bottom, where they’re buried deeper and deeper each year by new layers of algae and other debris.

D’Andrea and his co-workers extracted about 1,800 years’ worth of layers from the lake bottom, measured the relative amounts of saturated and unsaturated fats in each one, and came up with a detailed temperature profile of the lake water going back 18 centuries (these were summer temperatures only: in winter, the lake is frozen).

In order to keep their lake-bottom “thermometer” honest, the scientists compared the most recent hundred years’ worth of fat levels with records from actual thermometers located on Spitzbergen. “It turns out,” D’Andrea said, “that the lipid levels really do a very accurate job of recording temperature.”

That, D’Andrea said, gave them the confidence that they really could say something meaningful about temperatures back to the Medieval Warm Period and beyond. “We can say that summer temperatures at this location have been warmer in the recent past than they have over the past 1,800 years.”

One possible criticism of the study is that it’s based on just one location. Maybe this lake, or this region, was significantly cooler during the MPW for some reason than the rest of the world. For that reason D’Andrea and his scientific collaborators are doing the same sort of research in other lakes — in Greenland, Alaska, and the Ural Mountains of Russia; and on Ellesmere Island, in the Canadian High Arctic.

Those results haven’t been published yet, but, D’Andrea said, “the more work we do, the more this finding seems to hold up.”

The research is important, not just because it reinforces the conclusion that humans are now putting their own imprint on the climate, but also, D’Andrea said, because it can help scientists predict where the climate is going from here.

“If we can understand how the climate system reacted to natural forces in the past,” he said, “we’ll have a better understanding of how it will respond to the changes we’re imposing on it.”

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