Melting Glaciers: Clues to Climate Change
Rapid Change in a Warming World
Climate change isn’t always slow, small, and imperceptible in a human lifetime. One of the most important lessons from ice core analysis is that Earth’s climate in some places can also change rapidly and dramatically, such as a 15-degree temperature change in a decade. This, you’d notice.
About 12,900 years ago, Earth was warming steadily, in resolute recovery from our most recent glaciation. Then, suddenly, things got a lot colder and drier. The ice sheets reversed direction. Much, if not all, of the North Atlantic Ocean froze over. The landmasses flanking it became colder. Elsewhere in the world, Asia and Africa were beset by dust storms, and tropical wetlands dried up. In some areas, flora and fauna—and likewise, early humans—responded to the climate change in as little as two human generations.
During periods of extensive glaciation, polar vegetation such as Dryas octopetala can grow much further south. The Younger Dryas, a period of abrupt climate change, was named after this species.
After about 1,300 years of odd behavior, this interruption, called the Younger Dryas, ended. Earth resumed its warming course. One important study of Greenland ice cores showed that in a single decade of the Younger Dryas’s departure, temperatures over Greenland shot up about 15 degrees C. That’s as if your local climate changed to one typical of that 1,500 kilometers south of you in the geologic blink of an eye.
Some mechanism, then, was overriding the gentle long-term astronomical rhythms, called Milankovitch cycles, that govern when glaciations come and go. (Click here to learn about these cycles in “The Coming and Going of an Ice Age.”)
Could climate switch this abruptly again? It’s not impossible. But global warming complicates the scenario. Interestingly, human impact on the planet may have the power to both force such a rapid change—or prevent it entirely.
The Heat is On
According to the Met Office, the UK government’s meteorological branch, nine out of the ten warmest years on instrumental record (since 1861) were in the last decade. Today’s temperatures, however, aren’t unprecedented in recent geologic history. About 10,000-6,000 years ago, after the Younger Dryas left and the current interglacial period continued, temperatures in many northern locales were roughly 1 degree C warmer than recently. Epochs previous to the Pleistocene saw warmer periods as well.
Warm periods and high concentrations of the greenhouse gases methane and carbon dioxide usually occur together, although cause and effect are not clear. However, current-day levels of greenhouse gases may force the issue. Carbon dioxide is at its highest concentration in 260,000 years. And no mechanism other than our unprecedented burning of organic material such as fossil fuels and forest biomass can explain carbon dioxide’s increasingly rapid rise of late.
Computer models cited by the International Governmental Panel on Climate Change in 2001 predict that if greenhouse gases double their pre-industrial levels, average global temperatures will rise between 1 and 5 degrees C. A February 2005 modeling study in the journal Nature makes a more aggressive claim. Using nearly a hundred-thousand personal computers, the simulation produced a 2 to 11 degree C rise in global surface temperature in response to doubling of atmospheric carbon dioxide. In most IPCC model predictions, doubling will be reached before 2100.
Conveyed by the Ocean
We can already spot earthly changes from global warming. Arctic and subarctic permafrost is thawing. Winter ice on lakes and rivers breaks up earlier in the season. Trees flower earlier.
