The cold logic of hockey sticks

It's fitting that so much of the "debate" over global warming concerns a sport played on

ice: hockey sticks, to be precise. For the non-initiated, "hockey stick" in climate-speak means any temperature reconstruction that shows relatively steady temperatures for 1000 years, followed by rapid warming in the last 100 (the "blade" of the hockey stick). We've been over this before, but global warming deniers just can't criticize hockey sticks enough.

The most recent spat comes from TV weatherman Anthony Watts, who claims that a recent study by some Canadian scientists, Rolland et al, proves all hockey sticks wrong. Watts asserts that according to the study, which uses insects called "midges" as proxies for temperature in a small region of Canada, we had more global warming between about 1100 and 1400 AD than we do today. Here's the temperature graph he shows:

If you just glance at the graph, it appears that the hockey stick is broken, and temperatures have been higher in the geologically recent past than today. Look a little closer though, and you realize the study says no such thing, as it refers only to summer temperatures in a single island in Canada, not global climate change. The study's conclusion states:

The paleolimnological study of this northern Southampton Island lake provides information and extends the spatial understanding of Northern Hemisphere climatic events(Medieval Warm Period and Little Ice Age) in the Foxe Basin region. Both chironomid-based August air temperature inferences and sedimentological assemblages suggest that Southampton Island was affected by a regional warming between cal yr AD 1160–1360 and a regional cooling between cal yr AD 1360–1700. These results compare well with both archaeological studies made on Southampton Island and paleoclimatic studies. (More about the medieval warm period can be found here.)

So the paper makes no claims about *global* climate. Rather, it was testing how accurately a certain method of analysis could estimate past temperatures. It was already well-understood that there was *regional* warming in parts of the northern hemisphere during the time period in question, and the authors' midge analysis confirmed previous findings. For Watts to wave it around triumphantly as if it's the "latest" crack in global warming is misleading.

In fact, if Watts had read the study, he would have seen this in the very first paragraph:

Evidence of rapid climate change at northern latitudes has focussed research efforts on arctic environments. Due to possible feedback mechanisms, such as snowand sea ice extent (albedo), these regions are believed to be particularly sensitive to global warming... Many studies have already shown that some arctic areas have undergone major modifications of their annual thermal budget during the second half of the last century. They specifically showed an increase of surface air temperatures during summer, and a drastic reduction of winter sea ice cover thickness and summer extent (Johannessen et al.,1995,1999; Dickson, 1999; Rothrock et al., 1999; Comiso, 2002). On the other hand, regions surrounding the Foxe Basin, the Hudson Bay, and the Hudson Strait are so far only slightly affected by such global warming effects.

In other words, the authors acknowledge the reality of "rapid" global warming, with the potential for large feedbacks, and were trying to understand why one region in the arctic might be anomalous to this trend. To infer from the study that the medieval warm period was a global warming trend equivalent to today's is to commit several errors of logic.

But for the moment, let's assume that Watts' assertion was true, and temperatures today are similar to the "medieval warm period." Would it matter?

The reason the "medieval warm period" is important to Watts and his ilk is that in their interpretation, it shows that today’s temperatures are not out of the ordinary. If temperatures were just as high in the past as they are today, what do we need to worry about?

But there's a fallacy in that reasoning – what matters is not how high today's temperatures are compared with the past, but rather how high they’ll be 50-100 years from now. It's not where we are; it's where we're going.

We know that increasing carbon dioxide in the atmosphere warms the climate (check here for the best explanation out there). We also know that CO2 is already at a higher concentration than during the medieval warm period - indeed, than at any point in the last 400,000 years, and likely the last 15 million; since it takes a few decades for temperatures to catch up with CO2 levels, we've still got a few degrees of warming in the pipeline even if we stopped emitting CO2 today. And if we keep going like we are, it won't stop there: we'll double the preindustrial level of CO2 by 2050, and triple it by 2090. In fact, the last time CO2 levels were as high as they were today, the earth was 5 to 10 degrees Fahrenheit hotter, sea level was 75 to 120 feet higher, and there was no permanent ice cap in the Arctic and very little ice on Antarctica and Greenland. And we're poised to triple that level.

So even if Watts proved that the earth was as warm 700 years ago as it is today, it is completely irrelevant, because we're about to get a whole lot hotter if we don't act now. Any time you hear someone "debunk" the hockey stick, just remember, it doesn't matter.

Medieval warm period debunking global warming? The only thing that's medieval is Anthony Watts's grasp of logic and science.

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