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What effect will global warming have on the next ice age?

June 12, 2015

Dear Cecil:

I recently reread Bill Bryson's Short History of Nearly Everything. In his chapter on ice ages, he says geologists believe the earth has had numerous glaciation events, we're currently in an interglacial period, and we’re likely due for another round of ice. Bryson also writes that global warming could paradoxically accelerate the next glaciation, although no one really knows. I was wondering: what impact would global warming have on an impending ice age?

Cecil replies:

This one’s easy. As a result of global warming, the next ice age in all likelihood has been postponed until further notice. Bask in that thought for a moment. OK, time’s up. What we may get instead could be worse — not just droughts and hurricanes, but winters from hell.

In the 1970s scientists thought the next ice was going to arrive, if not imminently, at least disconcertingly soon — possibly within 1,500 years. Abundant geological and archeological evidence showed the earth had experienced many ice ages, the most recent of which concluded about 10,000 years ago. Warm periods, or interglacials, typically lasted about 10,000 years (I’m giving the simplified version of this). You see the nub of the problem right there.

Modern humans managed to survive the last ice age, but the experience was brutal. The epicanthic eye folds, flatter facial features, and compact bodies typical of today’s east Asians are thought to be the result of having been trapped behind the glaciers. Unprotected eyes, prominent noses, and long limbs were an invitation to frostbite and death.

The entirety of what we now think of as civilization was created during the current interglacial — in the grand scheme, an astonishingly short period of time. The thought that this hospitable era was drawing to a close gave scientists of the time the willies. Yeah, we’d gotten through it before, and we’d get through it again. But cheezit, at what cost — back to the caves?

Thanks to global warming (yay!), we can now put this grim prospect behind us. In a 2013 report, the National Climate Assessment and Development Advisory Committee, convened by the U.S. Department of Commerce, declared that “humans have so altered the composition of the atmosphere that the next glaciation has now been delayed indefinitely.”

So fine. We, and not untrammeled nature, now control our destiny. That’s not necessarily good.

I pause to acknowledge here that, like everyone else who isn’t determined to ignore the evidence, I buy the overall contention that human activity affects climate — not just now, but throughout history.

Mostly we’ve warmed things up. Cutting down forests and draining wetlands for agriculture may have forestalled an ice age about 5,000 years ago. (To simplify again, deforestation added carbon dioxide to the atmosphere.) Soot produced by burning wood, coal, and other fuels coated ice and snow, causing them to soak up more solar heat and melt faster. This phenomenon is thought to be responsible for ending the Little Ice Age, a colder-than-average period from 1350 to 1800, which in turn had been triggered by the reforestation of Europe after farms were abandoned in the wake of bubonic plague.

So global warming in principle isn’t new. What’s different is that we’re pumping CO2 into the atmosphere at a greater rate than before, with unpredictable consequences. In his book, Bryson speculates that greater warming would increase cloud cover, cooling the planet. Others posit that as the ice sheets melt and temperatures rise in the northern latitudes, the amount of vegetation will increase, resulting in reduced atmospheric carbon dioxide and again, more cooling.

Still others believe a sudden addition of melted fresh water into the oceans could disrupt critical ocean currents, such as the Gulf Stream, and lead to much colder temperatures in the north and the return of the glaciers. One such event occurred more than 8,000 years ago, when a giant glacier meltwater lake in the middle of North America drained into the ocean and triggered a chilling of the northern hemisphere by more than 5 degrees Fahrenheit.

Change may occur even in the absence of catastrophic events. Global-warming deniers (a dwindling breed, from what I can tell) have made much of the harsh winters of the past couple years in the U.S. northeast and midwest. Granted, two cold seasons don’t a long-term trend make. However, a few years ago I analyzed Chicago weather data for late spring (May 15 through June 15) from 1950 to 2009 and found two things. First, year-to-year temperature variation, modest in the 1950s and ’60s, increased after 1969 and since 1977 has been characterized by sharp swings. Second, on average, late springs in Chicago now are about 4 degrees cooler than in 1950.

Does that mean the glaciers are about to return? No, but we’re pumping enormous amounts of energy into a system with few safety valves. Did that cause the distortion of the jet stream that poured polar air into northern cities, causing the recent catastrophic winter in Boston? It’d be foolish to make such a claim now. Only in hindsight will we be able to say: that was the year the future arrived.

 

 

 

 

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