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Commentary

Cold Winters And Global Warming

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Our recent cold winters don’t seem to fit with the story about global warming. Commentator and University of Dayton professor Bob Brecha explains how to put these pieces together.

It can be a tough sell to talk about global warming after the winter we just went through.  Even my dog didn’t want to go out for a walk when it was -10°F outside.  How can we reconcile the last couple of freezing-cold winters with global warming?

Of course, the “global” in global warming means that we have to look beyond our own front yard to get a sense of what’s going on.  We also have to think about the longer-term patterns that are taking place, not just what we happen to observe this year. It’s not just global warming, but truly climate change.  Remember late winter in 2012?  I was able to ride my bike to work in the morning in shorts – in March – since temperatures were in the 80s!  This year it was Alaska’s turn to celebrate – or not.  Above average temperatures there forced the start of the Iditarod dog-sled race to move farther north.

This brings me to the sometimes surprising consequences of a changing climate.  I’ll mention just two that have to do with our North American winters. For over a century, scientists have predicted that adding carbon dioxide to the atmosphere would warm the planet.  It’s also been known that far-northern latitudes would warm more quickly than where we are in Ohio – exactly as we’ve been observing. 

The surprising bit is that the decreasing difference in temperature between the Arctic and lower latitudes can lead to some colder weather patterns down here.  The jet stream, a wind pattern that exists in the upper atmosphere, tends to drive winds from west-to-east in North America.  As the temperature difference decreases, the jet stream winds can weaken and cold regions can begin to bulge down further into the lower 48 states.   There is also some growing evidence that a complicated set of circumstances can lead to the troughs in the jet stream not only dropping down further, but also getting stuck in place instead of traveling quickly toward the east.

For an atmospheric physicist this is all fascinating stuff and it’s led to a flurry of research trying to better understand the details of the science behind these phenomena.  But if you were in Boston this winter, it might be more annoying than interesting.   In fact, the massive snowfall they received is also consistent with a warmer climate.  The Atlantic Ocean was unusually warm, and therefore more moisture evaporates.  A warmer atmosphere also  holds more of that moisture, about 4% more for every degree of temperature increase. When the Arctic air comes barreling down toward Massachusetts, the moister air dumps its increased precipitation in the form of snow, leaving those huge piles we saw on the news.

Climate scientists are not just trying to play both sides of the game, simply claiming that both cold and warm weather are the result of climate change.  Even though the climate is a very complex system, much of the behavior we see goes back to basic physical principles that don’t depend at all on political-party affiliation. It may make for an amusing news sound-bite when a Senator brings a snowball into the chamber in March and tries to use that prop as an argument against climate change, but those who depend on weather patterns to be somewhat predictable are not amused by a changing reality.

Single events like a cold-snap can rarely be attributed directly to climate change.  However, our human-induced climate change is really loading the dice toward more extreme events that are exactly consistent with what scientists expect to happen.  It may be satisfying as a scientist to understand the earth system, but it would be more satisfying if we actually tried to do something to prevent future unpleasant consequences of climate change.

Bob Brecha is a professor of the renewable and clean energy program at the University of Dayton.