Al Gore's documentary, "An Inconvenient Truth," continues to trigger heated discussions about global warming, as well as Mr. Gore's political ambitions, real or imagined.
But how close to the mark is his representation of the science tying humanity's industrial activity to changing climate aka anthropogenic global warming?
In short, say several climate researchers, he basically gets it right, although one can question some aspects of the presentation.
"I worry that the movie is a little heavy on disaster scenarios," says David Battisti, professor of atmospheric science at the University of Washington in Seattle and director of the university's Earth Initiative. Over the course of the coming century, he says, average conditions are likely to change in ways that should prompt action now; it doesn't require extreme examples to make the point.
After viewing the film, he says, his impression is that nothing in it "misstated the science in a qualitative way."
Indeed, his colleague Eric Stieg, also with the University of Washington, has noted that the film includes research that is only a few months old and clearly relevant to the discussion - research that won't appear in the next UN summary of climate science, slated for release early next year. The newest studies were too late to be included in the next UN summary, which is published about every six years and widely cited in public debates.
Among the nits and notes from the cognoscenti:
In the film, Gore points to Mt. Kilimanjaro as an example of global warming's impact on alpine glaciers. Robert Balling, a climatologist at Arizona State University writes, however, that the shrinking glaciers atop Africa's famed volcano have been disappearing for more than a century. Two studies published in 2004 suggest the retreat was triggered by declining rainfall since the end of the 1800s.
But other researchers note that the glacier has survived far more severe and long-term droughts during its 11,000-year history. Regardless of what may have triggered the glacier's shrinkage, researchers say global warming is a plausible, if not fully verified, reason for its accelerating disappearance. The film also points to a range of other large alpine ice fields worldwide that have declined dramatically over the past several decades. If Kilimanjaro ultimately proves to be the wrong global-warming poster child, there are plenty of others to choose from.
As for Greenland and Antarctica, the film shows dramatic footage of calving ice where glaciers meet the sea. Animated maps show the effect of a 20-foot rise in sea levels if larger chunks of Greenland and Antarctica's inventory slides into the sea. Viewers might assume that the 20-foot rise in sea level will take place this century. A pair of studies published in the journal Science in March do suggest that if CO2 emissions continue to rise at a fairly moderate pace, temperatures likely would rise high enough by the end of this century to render such a meltdown unavoidable. But, they add, it would take several centuries for that scenario to play out, giving humans more time to adapt.
Glaciologists also caution that calving ice is only half the equation: You must balance what an ice sheet loses at the edges against what it gains in the interior through snowfall. Projections of global warming's effect on these two vast ice sheets point to thickening in the interior from increased precipitation, which researchers say is happening. The sheets were in "near balance" from the 1960s perhaps through the 1980s, says Richard Alley, a glaciologist at Penn State University in College Park. "The Greenland picture is starting to come into focus - very recently, melting has gone up more than snowfall ... and the ice sheet is shrinking. Confidence is not 100 percent, because these are hard measurements to make and it's a short data set." He says that Antarctica appears to be following suit.
Some researchers point to the last UN summary of climate science, published in 2001, which indicated that no significant trends in tropical storm strength or numbers have appeared in data for the 20th century. Within the past year, however, several studies have been published that find a trend toward stronger tropical cyclones worldwide. The results have triggered an intense debate among hurricane specialists - not surprising given the stakes, the poor quality of some historical hurricane data, and the "freshness" of the latest results. If the results hold up, they would bolster conclusions from modeling studies that point to more intense tropical cyclones in a warmer world. Meanwhile, trends in rainfall and drought intensity are appearing that are consistent with those projected by global-warming models, researchers add.
The movie is about climate, not marine life. And in many respects, this phenomenon is less controversial because its chemistry is so straightforward, says Victoria Fabry, a marine science professor at California State University at San Marcos. But it's no less critical, she says, because it has implications for the ocean food chain.
The oceans take up CO2 via biological processes. They also take up CO2 via seawater chemistry, which can lead to a more-acidic ocean. So far, the decline in pH, a measure of acidity and alkalinity, looks tiny. But marine life is sensitive to small changes; the net result is to reduce the amount of calcium carbonate that corals need and critters at the bottom of the food chain use to build their shells. This spring, researchers with the National Oceanic and Atmospheric Administration wrapped up a research trip from Tahiti to Alaska, sampling water chemistry. The team, led by oceanographer Richard Feely, notes that, over large sections of the north Pacific, it found evidence of acidification attributable for the most part to ocean uptake of the CO2 humans have pumped into the atmosphere over the past 15 years - consistent with model projections.