A Current Controversy: Is Europe About To Freeze?

Researchers from Oregon State University explored this potential phenomenon, and the fluctuations in "thermohaline circulation" that could trigger it, in an analysis to be published Thursday in Nature, a professional journal. It's by no means certain that climatic changes of this magnitude and speed will come to pass, the scientists say, but even the reasonable possibility that they might are a cause for serious concern.

"To answer difficult questions such as this we depend a lot on our computer models, and in this area different models reach different conclusions," said Peter Clark, an OSU professor of geosciences and one of the world's leading experts on glaciers and prehistoric climate changes.

"What is fairly clear is that if the ocean circulation patterns which now warm much of the North Atlantic were to slow or stop, the consequences could be quite severe," Clark said. "This might also happen much quicker than many people appreciate. At some point the question becomes how much risk do we want to take?"

The big variable in this particular equation, Clark said, is whether or not changes in global temperature and precipitation patterns might affect a gigantic conveyor belt of warm, less-salty surface water that moves from the tropical Atlantic Ocean until it finally becomes so cold and salty in the far north Atlantic that it sinks, moves south and continues the circulation pattern.

This process, called thermohaline circulation, only happens in two regions of the Earth's polar areas. But it is responsible for much ocean circulation, including the critical currents that help keep parts of North America and Europe far warmer than they would otherwise be, considering the far north latitudes at which they lie – most of Great Britain is at the same latitude as central Canada.

This circulation process, researchers say, is not inevitable. Research suggests it may have fluctuated or even stopped numerous times in Earth's distant past, and that it's especially sensitive to moderate increases in temperature or influxes of fresh water. The same very cold, very salty water that sinks in the far North Atlantic Ocean simply won't sink if it's just a little bit warmer or a little bit less salty. And at various times, it appears these changes have happened not in geologic terms of thousands of years, but rather decades.

"This system does not respond in what we call a linear manner," Clark said. "Once you start putting on the brakes, this circulation pattern could slow down faster and faster and eventually stop altogether."

Research has found that some of the Earth's most rapid climatic shifts – up to 15 degrees in decades or less – have in the past occurred during glacial periods, when large ice sheets advanced from the polar regions as far south as New York City, among other places. Some scientists have even theorized that the wild temperature fluctuations of the last ice age may have retarded the evolution and development of humans as a species, as they struggled to cope with rapidly changing conditions.

We are now in an "interglacial" period that, in theory, may have less volatility, but could also be coming to an end.

Global warming will simply delay the inevitable, Clark said, because it actually should be about time for Earth to enter its next ice age. There's a fairly well defined pattern of about 10,000-year-long interglacial periods followed by 90,000-year-long ice ages, and the current interglacial period is already more than 10,000 years old.

"At this point we just aren't sure what to expect in terms of climatic volatility," Clark said. "But the more we learn about them, it becomes clear that these thermohaline circulation patterns are quite sensitive to temperature and influxes of fresh water, such as you might get with changing precipitation patterns triggered by global warming, not to mention melting ice caps or glaciers."

So the paradox, the scientists say, is that the same greenhouse effect which might make the Earth warmer, overall, could have the opposite effect on much of Europe by slowing or shutting down the warm ocean circulation patterns on which it depends.

"Most, but not all, coupled general circulation model projections of the 21st century climate show a reduction in the strength of the Atlantic overturning circulation with increasing concentrations of greenhouse gases," the researchers said in their report in Nature. "If the warming is strong enough and sustained long enough, a complete collapse cannot be excluded." This prospect – the collapse of the thermohaline circulation patterns that dictate its climate - has raised enough concern, Clark said, that Great Britain recently launched a $40 million research program to analyze this phenomena and its possible implications. And the National Academy of Sciences recently issued a report that made reference to an "inevitable surprise" of "climate changes with startling speed."

At this time, Clark said, some of the best potential to improve the ocean and atmospheric computer models that could help resolve some of these questions about future climate lie in studies of the distant past. Ice cores from Greenland glaciers have been instrumental in this work, he said, providing a look backwards at climatic conditions more than 100,000 years into the past, and work in that area will continue.