RENEE MONTAGNE, host:
Here's an issue that President Bush talked about in his State of the Union address, and that presidential hopefuls will likely weigh in on: Global warming. Scientists and government officials from 154 nations are gathering in Paris to write a major document that will sum up the state of our knowledge about global warming. It's expected to underscore a consensus that already exists: the planet is getting warmer, and human activities are a big reason.
NPR's Richard Harris reports.
RICHARD HARRIS: For many years, the favorite horror story about abrupt climate change was that a shift in ocean currents could radically cool Europe's climate. These currents, called the overturning circulation, bring warm water and warm temperatures north from the equator to Europe.
Susan Lozier, an oceanographer at Duke University, says scientists have long worried that this ocean circulation could be disrupted.
Professor SUSAN LOZIER (Physical Oceanographer, Duke University): If global warming slowed down this overturning circulation, there would be, initially, large changes to Europe's climate. It would get colder, which is a paradox of a result, isn't it?
HARRIS: Scientists have found evidence for just that kind of change in recent earth history. The idea that it could happen again got such legs even Hollywood seized on it as a premise for the movie "The Day After Tomorrow," which featured an instant Ice Age and other havoc.
(Soundbite movie "The Day After Tomorrow")
Unidentified Man: Oh, the water coming towards New York City. Everyone is…
HARRIS: Susan Lozier says scientists will have to laud about that movie since nobody in their wildest dreams expects abrupt change to be that sudden. But scientists were a bit startled in 2005 when researchers in Britain reported that the northward circulation of warm water did appear to be slowing down. Harry Bryden at the National Oceanography Centre in Southampton measured a big change.
Mr. HARRY BRYDEN (National Oceanography Centre): The overturning circulation was down by about 30 percent when we analyzed the recent sections compared to the older sections, which go back to 1957.
HARRIS: That triggered screaming headlines in Europe. But it turns out that study was based on five snapshots, if you will, brief glimpses of the ocean during five different research cruises. More recently, Bryden and his colleagues have put a bunch of instruments out in the ocean to get a moving picture of those currents, and these show the ocean changes so much week-to-week and month-to-month the apparent slowing trend was probably an illusion.
Mr. BRYDEN: There's a lot of variability out there.
HARRIS: That has led many oceanographers to breathe a big sigh of relief, including Susan Lozier from Duke.
Prof. LOZIER: There is no clear evidence now that the overturning circulation has slowed down, or of course there's no evidence at all that it has stopped.
HARRIS: So between those observations and deeper understanding of these currents, many scientists are now not so worried that the Atlantic will change abruptly as a result of global warming. But don't relax just yet. Over the past five years scientists have become increasingly concerned that global warming could abruptly change global rainfall patterns. Julia Cole is at the University of Arizona.
Professor JULIA COLE (Geosciences, University of Arizona): And of course, the atmosphere can respond a lot more quickly than the oceans.
HARRIS: Cole says the historical record shows that gradual changes in climate can often lead to abrupt changes in rainfall patterns.
Ms. COLE: Often, the climate system appears to respond in a step-like way, even when the cause of that change is gradual. And so we're seeing increasing evidence for that in systems like the Asian monsoon, in Western United States drought, and in other precipitation sensitive systems.
HARRIS: For example, she said some studies now project that the Southwest United States could suffer prolonged winter droughts, which is when rainfall there is most needed. If that proves to be the case, abrupt climate change will really strike on this side of the Atlantic. That's by no means certain, Cole says, but this she does say with confidence.
Prof. COLE: As we move into a warming world, I think we need to keep in mind that the way that the world warms will not necessarily be slow or gradual or predictable, as we might like.
HARRIS: And that's one message we are likely to hear from the climate officials and scientists who are meeting in Paris this week.
Richard Harris, NPR News.
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