New information is leading to a controversial shift in thinking on the impact of global warming on ocean circulation, partly due to the work of a UA researcher.

The scientific community has long believed that as global warming continues and large amounts of freshwater ice melt into the ocean, the ocean's circulation will slow.

This would have a catastrophic impact on the environment as vividly, if somewhat overdramatically, portrayed in the film "The Day After Tomorrow."

But a paper published last week in Nature magazine, the result of several studies of past and possible future weather, says that in fact the very opposite is true and ocean circulation will become stronger as the icecaps melt.

"We missed what was right in front of our eyes," said Joellen Russell, an assistant professor of geosciences at the University of Arizona and co-author of the paper.

She was at the Geophysical Fluid Dynamics Laboratory at Princeton University before coming to the UA two years ago, and spent several years studying and creating models for what weather will be like as global warming continues.

For this study, she and co-author J.R. Toggweiler "pulled all that research together" to conclude that wind pushes the ocean currents and that warming temperatures will increase the speed of these currents.

The westerlies, also known as the trade winds, are the main wind in the middle latitudes of both the Northern and Southern hemispheres. Observing that the winds have been migrating toward their respective poles for 40 years, Russell and Toggweiler realized this could be the explanation of how the last ice age ended 18,000 years ago.

"The question of how atmospheric CO2 and the carbon cycle varied during the ice ages is the big mystery in the field," Toggweiler said.

As the westerlies move, the ocean's circulation increases, releasing more carbon dioxide from the deep ocean, leading to more warming and even stronger circulation in a feedback loop strong enough to push Earth out of an ice age.
It also has a local effect.

The westerlies "are how we get water in the winter," said Russell, and with their movement, the weather will go from "occasional winter storms to not very many at all. Our rain will end up in Oregon." This will become more noticeable during the next few decades.

Previous models had placed the path of the westerlies in the wrong spot to begin with, making any predictions erroneous from the start.

"The new model gets it just about right," Russell said.

Early weather models were based on the idea that ocean circulation was based on wind only for the surface and on buoyancy for deeper circulation. So adding freshwater to the ocean as the Earth warms would lead to less movement of ocean. Now, however, oceanographers mostly agree that it is only wind that has a major effect on ocean circulation.

Evidence from the most recent ice age, which reached its coldest 21,000 years ago, shows that the ocean had very little movement and exchange of deep water and surface water until the warming of the Earth about 18,000 years ago. "The evidence is piling up," that those models predicting a weakened ocean circulation in the coming decades are wrong, Russell said.

The increasing speed of the westerlies and their movement toward the poles "should stir the ocean's salty and fresh waters around and minimize the effect of the polar freshening," Toggweiler said.

Still, the idea that the ocean's circulation will increase as the Earth warms is not fully accepted by scientists.

"It's controversial, but it explains what happened in the past and what is happening now," Toggweiler said.

The mounting evidence has won the new theory a lot of converts, Toggweiler said.
"We were lucky to publish first," Russell said.

"This is what science is all about," Toggweiler said. "Looking for where the common wisdom is wrong."

โ— Contact NASA Space Grant intern Eric Schwartz at 807-8012 or at eschwartz@azstarnet.com.