In an irony worthy of O. Henry, scientists have recently begun to wonder if global warming caused by fossil fuel consumption may trigger a remarkable side effect--mini ice ages in Europe.
"It's kind of a long shot," says Thomas Haine, associate professor of Earth and planetary sciences in the Krieger School of Arts and Sciences, whose oceanography research is shedding light on the strange new possibility. "I don't think it's a likely possibility, but it is one potential outcome of global warming."
Haine is a physical oceanographer who came to Hopkins in January 2000. He uses classical physics to study the circulation and dynamics of ocean currents and their effects on climate. One of his specialties is using chlorofluorocarbons--the ozone-destroying chemicals that countries around the world resolved to stop using in the 1970s--to track ocean currents.
Concern is growing among oceanographers that one of these ocean currents may change substantially or switch off in the coming decades, and it's this potential transformation that forms the unexpected link between global warming and the threat of cooling in Europe.
"What happens today is that warm water runs north from the subtropical Atlantic Ocean," says Haine, pointing to the Atlantic's Gulf Stream on a map, "and it squirts out [across the Atlantic] toward Europe. As it crosses the Atlantic, the heat is dragged out of the ocean. The water gets colder, the heat comes into the atmosphere, and that's what keeps western Europe mild in winter."
Compare climates of cities at the same latitude in Europe and North America to reveal the warming effect, which can make a city in Europe as much as 36 degrees Fahrenheit warmer than its North American counterpart. Balmy Rome, for example, is at the same latitude as Boston, whose winters can be tough.
"I come from England," Haine says, "and if you travel along a latitude line [to North America], you end up in Labrador. That's partly why civilization flourished in Europe rather than in Atlantic Canada--because of the Atlantic Ocean."
As water from the tropics cools on its journey to the north and east, it becomes denser and sinks, powering deep ocean currents in a process oceanographers call thermohaline circulation.
Scientists believe this system would be dramatically slowed down by new influxes of freshwater, which is less dense than ocean water and tends to sit on top of the ocean. Haine and other oceanographers are concerned that new freshwater may already be flowing into the ocean from two sources linked to global warming and higher temperatures: melting Arctic ice and an increase in the speed of the water cycle, which could cause more evaporation in the subtropical Atlantic and more rain in the subpolar Atlantic.
Haine cites historic and contemporary evidence as supporting the possibility of a change in the thermohaline system.
"There's paleooceanographic evidence that in the past the North Atlantic currents were different," Haine says. "Based on these indirect data, we believe the thermohaline circulation was weaker during the last ice age, which peaked about 18,000 years ago. We think [the thermohaline circulation] switched on as the Earth warmed, but then switched off and then back on again as freshwater from the melting glaciers flooded the North Atlantic."
Contemporary measurements have definitively shown that the North Atlantic is getting fresher. In the cover article of this month's issue of Discover magazine, researchers at the Woods Hole Oceanographic Institute describe "huge rivers of freshwater" that have appeared over the last 30 years in the North Atlantic.
Finally, scientists have found that a key branch of the thermohaline circulation that runs between Iceland and Scotland has weakened by about 20 percent since 1950. The branch is near the Faroe Islands.
Based on the available data, which is still incomplete, oceanographers estimate a major change in the thermohaline system could conceivably come as early as 2050, lowering temperatures in Europe by 5 degrees Celsius (9 degrees Fahrenheit) or more.
"Winters in northern Europe could be very cold, like Labrador or Newfoundland," Haine says. "Which, of course, is cause for concern. A program is emerging to make critical measurements in the Atlantic and to monitor these processes for signs of weakening and to see if the thermohaline circulation is switching off. I'm involved in that program."
Although he's been to sea to gather data, Haine has now moved into a portion of his career where the bulk of his research is focused on taking field data and fitting it to computer models of ocean circulation processes. With the support of Johns Hopkins, the National Science Foundation and other international funding agencies, he's been able to acquire a high-power parallel computing cluster that lets him conduct intricate simulations of key regions of the North Atlantic.
"We're trying to build an accurate high-resolution model of the subpolar Atlantic Ocean that will allow us to estimate quantities that we can't measure directly, like the overall strength of the thermohaline circulation or the strength of the freshwater outflows leaving the Arctic," he says.
Scientists can't always measure such quantities directly either because their instruments can't cover a wide enough area, or the logistics of making detailed measurements among the ice-choked subpolar North Atlantic are too challenging.
Using the computer model to probe key aspects of ocean circulation is akin to the methods meteorologists use to forecast weather, according to Haine. Fit the available data to the theoretical model, see if it works and, if it doesn't, alter the model.
The first results are in from the new computing cluster, which arrived earlier last year, and have been submitted to a journal for review. Haine happily reports that the ocean dynamics model used for their initial simulations, which is now being upgraded, appeared to do quite well in matching data.
In the paper, Haine will also offer new insights based on combining chlorofluorocarbon data with the computer model.
"Once they settle in the ocean, CFCs are pretty much indestructible," he explains. "They're like a dye entering the ocean; they have no natural sources, and we know the release history quite well."
By tracking the sinking of CFCs and other aspects of their circulation, Haine can track ocean currents.
"We've made some discoveries about the way in which the subpolar currents mix this tracer coming from the surface, which is relevant to this thermohaline circulation," he explains. "It's helping us understand some of the basic processes in more detail."