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Followup: Is the late arrival of summer in Chicago proof of global warming?
July 9, 2009

Dear Cecil:

Regarding your column about the late arrival of summer:

As I recall, around the year 2000 there was a conference on climate in Canada which was the first to declare that there was a preponderance of evidence supporting global warming.
Someone with a computer model kept finding a "precession" of the seasons: the beginning of winter was delayed, and likewise peaked later than the historical average. Similarly spring was pushed back, and the summer heat lingered into when fall weather should have predominated. (Nobody complains about Indian summer except the PC people who insist on calling it something else.) The modeler then looked for this pattern in climate records and found it. Confronted with a model which yielded a prediction resulting in a testable hypothesis, and confirmation of the hypothesis in data, the scientists applied the scientific method (they love that) and issued a declaration. Sorry if the details are vague, I leave it to you to supply them.

— Hwrd, via the Straight Dope Chicago Message Board

Cecil replies:

I'll be damned.

Normally I pride myself on being at the cutting edge of science — I mean, seriously, how many people do you know who've explained quantum mechanics in dactylic tetrameter? In this case, however, I find myself on the trailing edge — meaning I'm providing corroborating evidence for an effect first noticed by someone else. I comfort myself with the thought that the effect I'm corroborating is considered one of the signature proofs of global warming. So fine, I don't get the Nobel. I'm happy to do my bit.

Let's take it from the top. In my original column, I analyzed 60 years of weather data (no kidding — 21,000 days! Is your columnist working for you or what?) to prove what many suspected — early summer in Chicago has gotten noticeably cooler in recent decades, or to put it another way, the onset of summer has been delayed.

Turns out this may be a global phenomenon. Your recollection of that Canadian scientific conference is perhaps a bit scrambled — not to be critical, but the Teeming Millions' ability to accurately recall essential details has, historically, pretty much sucked. However, my guess is you heard something about the work of David Thomson, currently a mathematician at Queen's University, Kingston, Ontario.

In 1995 Thomson published a paper in Science entitled "The Seasons, Global Temperature, and Precession." It's heavy going even for me, with much opaque discussion of arctangents, Gibb's ripples, and the Cramιr–Rao bound — I'm telling you, you don't ever want to sit next to a statistician at a football game. However, the gist is that, for astronomical reasons having to do with orbital precession, the timing of the seasons over the centuries has drifted progressively later in the year. In other words, the onset of summer heat (and winter's chill, and everything else having to do with the seasons) has been gradually delayed.

For 300 years the drift was slow but steady. After 1940, though — Professor Thomson provides a little chart that makes the point vividly — the drift rate fell off the table. The seasons have shifted more in the past 50 years than they did in the previous three centuries. The dramatic change closely corresponds to the buildup of carbon dioxide in the atmosphere during the same period. Ergo — this is definitely a time when you want to say ergo — human-caused global warming is real. The chilly Junes we've been having in Chicago are further proof.

As usual, however, there's more to it than that. Last January a team of researchers led by A. R. Stine published a paper in Nature entitled "Changes in the phase of the annual cycle of surface temperature." Stine and company confirmed Thomson's finding that greenhouse gases were affecting the timing of the seasons. Thomson himself  wrote an appreciative note in the same issue of Nature: "This shift [is] highly anomalous when compared with [earlier] data … implicating human agency as the cause."

Just one problem. Thomson had noticed a trend toward later seasons. Stine and associates found a trend toward earlier seasons — on average, 1.7 days earlier on land outside the tropics between 1954 and 2007. It's all very well to have signature studies confirming global warming. However, it's confusing to have the signature studies show opposite things.

I've been attempting to reach Professor Thomson to talk this over, so far without success. However, I've studied the papers, and I suspect his answer is going to be: it's complicated. (This is the scientific response to a lot of things.) Thomson based his conclusion on weather data for central England, where they've been keeping track of the temperature for a long time. Stine and friends based their findings on twentieth-century weather data from around the world. Different things happen in different places. While the seasons over land seem to be earlier, seasons over the oceans apparently are later. What's more — here's why I say your recollection is suspect, Hwrd — none of the mathematical climate models predicts shifting seasons.

Your typical media type waits till the experts figure it out. However, the Straight Dope being the proactive enterprise it is, we felt we should help get the project off the dime. Our thought was, never mind what's happening with the seasons in central England, over the oceans, or in other distant locations. Let's worry about the seasons right here. We remembered a friend of mature years saying once that while Chicago springs had gotten worse, the weather had gotten nicer in the fall. If in fact the seasons were shifting and the springs were staying chillier later, the warm temps ought to linger longer in autumn.

So we went back to the spreadsheet and the 21,000 lines of data. This time we computed the average high temperature in early fall, September 15 to October 15, for the period from 1950 to 2008. Result:

Your first thought is: This blows. The overall temperature trend is down, not up. However, the question bears closer examination. First let's look at average temperatures in January:

Here we begin to see a pattern. The long-term winter temperature trend is up, though not dramatically. Clearly the 1950s were an unusually warm time. Considered over a shorter period, however — from the bitterly cold winters of 1977-79 to the present — winter temperatures in Chicago have risen sharply. The past decade has been the warmest stretch in the past 60 years. That's in line with the common observation among climate-change specialists that winters have warmed up more than summers.

Now look again at the early fall chart. We see the same rising trend, although it started later. The average high temperature declined until 1993, but has risen markedly since then. Is this evidence of the seasonal shift some experts claim to have detected — an unmistakable sign of global warming? Eh, 15 years is too short a time to judge. But could be.

— Cecil Adams

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