I often hear complaints that science, writ large, changes its mind a lot. One study says salt is bad for you, another says it isn’t so harmful after all. What people often don’t understand is that the process of science is a provisional and a cumulative one, which makes it self-correcting over time. A small study of a population indicates that sodium intake may increase blood pressure, which is an indicator for a higher risk of heart disease. A later, larger study may show that other factors, like a high-fat diet, are more important predictors of what makes a ticker go bad. Another study indicates that genetic pre-disposition together with diet and exercise habits may be the best predictor for who will have heart attacks and who probably won’t. All the studies may be valid, but the results are provisional and our understanding may continue to evolve as new research is conducted. Human bodies are complicated, individualistic and variable machines so the answers about health and physiology are rarely completely straight-forward and universally applicable. The more we learn about human health and biology, however, the more we understand these complexities and variations.

And so we come to climate, another complex system with many interlocking, interacting parts. For decades, oceanographers and climate scientists have been studying what’s known as the North Atlantic Current, an extension of the Gulf Stream, which carries warm, equatorial waters to North America and Europe. We did a webcast recently on the basics of this phenomenon, hosted by Exploratorium senior scientist Charles Carlson (scroll down to 4-21-2007). As recently as 12,000 years ago, this conveyor belt of warm salty water shut down when a flood of fresh water from melting glaciers poured into the ocean, plunging Europe and North America into a mini ice age (which was also the premise of a recent disaster film, “Day After Tomorrow.”) Scientists have been worried that global warming and the subsequent melting of ice sheets in Greenland would cause a similar disruption of the North Atlantic Current, but the recent IPCC report backed away from this prediction, reports Walter Gibbs in the New York Times. I asked Charlie to comment on this recent finding, here’s what he had to say:

“We can all remove one possible disaster scenario from our fears about global warming. The latest scientific evidence, coupled with more sophisticated computer climate modeling, doesn’t support a northern hemisphere plunge into a mini-ice age, as the Gulfstream current fails. Since the late ’70’s oceanographers have observed that the Gulfstream has periodically and dramatically varied in flow over the earth’s history, and that these variations are associated with major climatic shifts, like ice-ages. Such current fluctuations certainly could account for climatic shifts, since the North Atlantic current accounts for moving about 30% of the equatorial heat towards the poles significantly warming the northern latitudes, making Europe and North America more habitable. But a closer look has revealed that such a current failure isn’t all that likely. It would take a major catastrophic melting of the Greenland ice sheet, and that’s not in the earth’s current climate cards. So no “Day After” ice-age, we’ll all slowly warm instead.”

I had a fascinating breakfast meeting with Rocky Kolb while he was in Berkeley recently giving a talk. Rocky formerly led the particle astrophysics group at Fermilab and is now chair of astronomy and astrophysics at the University of Chicago. He’s also a great public speaker and a bit of a celebrity: Rocky was Dr. December in the Stud Muffins of Science Calendar, circa 1996. One of the things we talked about over eggs and pancakes was the dual personality of physics. More than any other scientific discipline, physics depends on two varieties of scientist: the theorist and the experimentalist. Rocky is a theorist, but probably the most famous theoretical physicist was Einstein. His relativity theories, written early in the last century, kept the experimentalists who build observatories and particle accelerators busy for decades trying to confirm his theoretical predictions. But experimentalists like nothing better than coming up with observations about the universe, matter, or the inner workings of particles that catch theorists with their mathematical pants down. A sign about the doorway of condensed matter physicist Sid Nagel reads: Here is where theories come to die. Rocky’s take: “We’re smarter, better looking and generally taller.” But the shorter ones do occasionally exact their revenge. It happened in 1999 when Saul Perlmutter from the Lawrence Berkeley Lab announced that, billions of years after the big bang, the universe was actually accelerating rather than slowing down and collapsing under the weight of gravity. No theorist had predicted this finding and collectively they’ve been scratching their heads ever since trying to explain it. “This is the first time in 3000 years of cosmology that theorists are playing catch-up to the experimentalists. In the past there were more theories than observations could confirm or refute, but now there are more observations that the theories can not explain, ” Rocky told me. It’s certainly an interesting time to be a cosmologist and a sideline observer.