NICHOLAS D. KRISTOF: The Big Melt

If we learned that Al Qaeda was secretly developing a new terrorist technique that could disrupt water supplies around the globe, force tens of millions from their homes and potentially endanger our entire planet, we would be aroused into a frenzy and deploy every possible asset to neutralize the threat.

Yet that is precisely the threat that we’re creating ourselves, with our greenhouse gases. While there is still much uncertainty about the severity of the consequences, a series of new studies indicate that we’re cooking our favorite planet more quickly than experts had expected.

The newly published studies haven’t received much attention, because they’re not in English but in Scientese and hence drier than the Sahara Desert. But they suggest that ice is melting and our seas are rising more quickly than most experts had anticipated.

The latest source of alarm is the news, as reported by my Times colleague Andrew Revkin, that sea ice in the northern polar region just set a new low — and it still has another month of melting ahead of it. At this rate, the “permanent” north polar ice cap may disappear entirely in our lifetimes.

In case you missed the May edition of “Geophysical Research Letters,” an article by five scientists has the backdrop. They analyze the extent of Arctic sea ice each summer since 1953. The computer models anticipated a loss of ice of 2.5 percent per decade, but the actual loss was 7.8 percent per decade — three times greater.

The article notes that the extent of summer ice melting is 30 years ahead of where the models predict.

Three other recent reports underscore that climate change seems to be occurring more quickly than computer models had anticipated:

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Science magazine reported in March that Antarctica and Greenland are both losing ice overall, about 125 billion metric tons a year between the two of them — and the amount has accelerated over the last decade. To put that in context, the West Antarctic Ice Sheet (the most unstable part of the frosty cloak over the southernmost continent) and Greenland together hold enough ice to raise global sea levels by 40 feet or so, although they would take hundreds of years to melt. We hope.

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In January, Science reported that actual rises in sea level in recent years followed the uppermost limit of the range predicted by computer models of climate change — meaning that past studies had understated the rise. As a result, the study found that the sea is likely to rise higher than most previous forecasts — to between 50 centimeters and 1.4 meters by the year 2100 (and then continuing from there).

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Science Express, the online edition of Science, reported last month that the world’s several hundred thousand glaciers and small ice caps are thinning more quickly than people realized. “At the very least, our projections indicate that future sea-level rise maybe larger than anticipated,” the article declared.

What does all this mean?

“Over and over again, we’re finding that models correctly predict the patterns of change but understate their magnitude,” notes Jay Gulledge, a senior scientist at the Pew Center on Global Climate Change.

This may all sound abstract, but climate change apparently is already causing crop failures in Africa. In countries like Burundi, you can hold children who are starving and dying because of weather changes that many experts believe are driven by our carbon emissions.

There are practical steps we can take to curb carbon emissions, and I’ll talk about them in a forthcoming column. But the tragedy is that the U.S. has become a big part of the problem.

“Not only is the U.S. not leading on climate change, we’re holding others back,” said Jessica Bailey, who works on climate issues for the Rockefeller Brothers Fund. “We’re inhibiting progress on climate change globally.”

I ran into Al Gore at a climate/energy conference this month, and he vibrates with passion about this issue — recognizing that we should confront mortal threats even when they don’t emanate from Al Qaeda.

“We are now treating the Earth’s atmosphere as an open sewer,” he said, and (perhaps because my teenage son was beside me) he encouraged young people to engage in peaceful protests to block major new carbon sources.

“I can’t understand why there aren’t rings of young people blocking bulldozers,” Mr. Gore said, “and preventing them from constructing coal-fired power plants.”

Critics scoff that the scientific debate is continuing, that the consequences are uncertain — and they’re right. There is natural variability and lots of uncertainty, especially about the magnitude and timing of climate change.

In the same way, terror experts aren’t sure about the magnitude and timing of Al Qaeda’s next strike. But it would be myopic to shrug that because there’s uncertainty about the risks, we shouldn’t act vigorously to confront them — yet that’s our national policy toward climate change, and it’s a disgrace.