The last time I wrote about Chicago's warm winter (and what it might have to do with SPACE WEATHER), someone asked me on Facebook whether it had anything to do with global warming. The best I could do was basically "that's not exactly how to think about it":

I think the best way to think of it is a baseball metaphor (sorry, that's how my mind works): month-to-month, year-to-year weather is like how many runs are scored from game to game; climate change is like the steroid era or lowering the mound, which adjust things across the board. Like, if you lower the mound, you're going to get *more* 20-run games, but you're still going to get shutouts, just not as many.

So I was kind of pleased with myself to see one of my favorite weather bloggers, Dr. Jeff Masters, use a nearly identical metaphor the other day:

The natural weather rhythms I've grown to used to during my 30 years as a meteorologist have become significantly disrupted over the past few years. Many of Earth's major atmospheric circulation patterns have seen significant shifts and unprecedented behavior; new patterns that were unknown have emerged, and extreme weather events were incredibly intense and numerous during 2010 – 2011…. I appeared on PBS News Hour on December 28 (video here) to argue that watching the weather over the past two years has been like watching a famous baseball hitter on steroids–an analogy used in the past by climate scientists Tony Broccoli and Jerry Meehl. We're used to seeing the slugger hit the ball out of the park, but not with the frequency he's hitting them now that he's on steroids. Moreover, some of the home runs now land way back in the seats where no one has ever been able to hit a home run before. We can't say that any particular home run would not have occurred without the steroids, but the increase in home runs and the unprecedented ultra-long balls are highly suspicious. Similarly, Earth's 0.6°C (1°F) warming and 4% increase in global water vapor since 1970 have created an atmosphere on steroids.

But even that's simplifying things a lot. Other things have paralelled the steroid era in baseball, like the expansion of the major leagues, meaning that pitchers who never would stay in the majors have been around for hitters to tee off on. Licit medical technology has extended players' careers; baseball has expanded overseas recruiting; computer technology has vastly increased our understanding of the sport. It's a lot of different moving parts.

The climate is generated by a machine as big as the planet; the effects are difficult to predict because of all the moving parts. As Masters notes:

Extreme weather years like 2010 and 2011 are very likely to increase in frequency, since there is a delay of several decades between when we put heat-trapping gases into the atmosphere and when the climate fully responds. This is because Earth's oceans take so long to heat up when extra heat is added to the atmosphere (think about how long it takes it takes for a lake to heat up during summer.) Due to this lag, we are just now experiencing the full effect of CO2 emitted in the late 1980s; since CO2 has been increasing by 1 – 3% per year since then, there is a lot more climate change "in the pipeline" we cannot avoid.

So what is going on? It's the warmest La Niña on record. That brings the global temperature down, but causes different effects in different places. Chicago is going through a near-record warm spell—strong La Niñas correlate with above average temperatures, like the 65 degrees we hit in 1989 when the mean January max was 52 degrees, three degrees higher than this month's mean. Meanwhile, Alaska and northern Europe are suffering through deadly cold snaps. Here's how that works:

"First of all," he explains, "we are experiencing a La Niña pattern of sea surface temperatures in the Pacific Ocean. This pushes the jet stream and the cold arctic air northward."

"On top of that, this year's Arctic Oscillation has been stronger."

The Arctic Oscillation is a see-sawing pressure difference between the Arctic and lower latitudes.  When the pressure difference is high, a whirlpool of air forms around the North Pole. Last year, the whirlpool motion was weaker, allowing cold air to escape from the polar regions and head southward to the US.

"This year the whirlpool has been more forceful, corralling the cold air and keeping it nearer the pole. That has reinforced the La Niña impact."

That's what I wrote about last time—the Arctic Oscilliation and its possible connection to solar activity. Basically the top of the climate machine is spinning really fast right now, keeping all the cold air away from us.

So what actually causes the Arctic Oscilliation to change? No one really knows. As Greg Postel wrote last year at the Washington Post's outstanding weather blog, making an important distinction about the AO:

[T]he AO really isn't a mechanism that brings us these weird weather patterns. It's a symptom of them. Common wisdom has latched on to the idea that the AO is a dynamical structure—one that moves around the world and occasionally creates meteorological havoc by producing lengthy spells of uncommon weather. It is not.

Rather, the AO is a response to, or a byproduct of, the interaction among a group of fundamental atmospheric mechanisms. On more technical terms, it can be understood without appealing to fluid dynamics, and it is not a solution to any governing mathematical equation.

The real question is what causes the atmosphere to slosh back and forth in this way that removes cold air from the poles. No one knows for sure. But when weather systems capable of long-range air displacements eject tropical warmth into polar territory and dislodge cold air from it, the effects tend to last more than a few days.

One theory, as noted, is solar activity. Another is Siberian snow cover:

Cohen claims that rapidly advancing snow cover in Siberia can set off a chain of events from Earth’s surface to the stratosphere. The quick expansion can lead to a large dome of cold high pressure over Siberia. That dome, in turn, perturbs the jet stream so it flows more north to south in addition to west to east, resulting in more intense cold-air outbreaks in eastern North America and western Europe, which often breed snowstorms.

But it's just one piece, and not an isolated predictor:

Second, while I have no doubt that enhanced snow pack in Eurasia is amplifying these waves that warm the Polar stratosphere and creating our mammoth blocking patterns (translation: cold U.S., Europe, and Asia winter), something else is initiating these waves. The reason I believe this is that in October 1998, we had well above normal Eurasian snowfall, but we DID NOT have a cold Eastern U.S. winter. Instead, tropical forcing from a powerful La Niña dominated our warm winter weather. The Siberian snow was there, but the waves were not.

1999 also saw a pretty long warm spell in January and February in Chicago.

The most important thing to remember about global warming is that it's global. The entire globe heating up by a degree isn't going to directly affect what boots you wear in the morning. But applied over the many, many other powerful inputs, it can work its way into your wardrobe in various ways. Whether that means warmer or cooler weather can depend on a lot of other things, from Siberia to the sun.

Update: In related news, the USDA updated its plant hardiness zone map for the first time in 12 years, using a much longer timeframe.

Related: U. of C. prof David Archer's class lectures on climate change are all online.

Photograph: Juan Carlos Martin-Loeches (CC by 2.0)