Climate change could be 'irreversible' for 1,000 years? Gulp!

Irreversible is a tough word, especially when applied to the effects of global warming.

But physics alone suggests that even now, the changes that humans are imposing on Earths' climate system will last for at least 1,000 years and likely far longer. That's the gist of a new study by an international team of researchers led by Susan Solomon, an atmospheric chemist at the National Oceanic and Atmospheric Administration's Earth System Research Lab in Boulder, Colo.

Dr. Solomon, who has enjoyed a long, scientifically productive career, isn't known for hyperbole. So when she and colleagues use the word "irreversible" over time spans measured in millenniums, it's clear a great deal of thought and data lay behind it.

In some respects, the conclusions are not new. But the emphasis is. In his new book "The Long Thaw," for instance, University of Chicago climate scientist David Archer lays out a case that at least 10 percent of the CO2 that human industrial activity is pumping into the air will affect climate 100,000 years from now.

Why the lock-in? Unlike other compounds humans have added to their environment, newly added CO2 persists in the atmosphere for centuries. It's a bit like a bank account: The balance grows as long as the owner keeps making deposits and never takes anything out, never mind earning interest. In the atmosphere's case, withdrawals -- at least non-technological ones -- take place over geologic time scales.

"The same thing that is holding back climate change today will keep it going in the very long term, and that is the oceans," she says. The oceans are the ultimate "sink" for carbon and they absorb most of the heat. So even after atmospheric concentrations peak and very slowly start to decline (more slowly than you would think, she says), oceans will sustain the warming as it slowly re-releases heat back into the atmosphere.

One rationale for the study, she says, is to underscore the long-term nature of the changes humans are imposing on the climate system. Too often, research and policy is geared to what can or should be done by the end of this century. Among policymakers and the public, this can often look as though once that date is reached, problem solved.

Rather than a call to throw up one's hands in discouragement, she adds, the results show the importance of acting quickly to reduce emissions and so limit the very long-lived effects.

A focus on the big three

Solomon's group looked at three climate features: warming itself; changes in rain and snowfall patterns; and sea-level rise, absent any contribution from Greenland or Antarctic ice caps. (For a tidy summary, colleague  Eoin O'Carroll has a tight post here.)

These three, the team says, involve aspects of climate where scientists have already identified some level of contribution from the effects of rising greenhouse-gas levels, where an understanding of the physical mechanisms driving the changes are well in hand, and where model projections are consistent despite other differences in the models.

They based their work on a range of possible targets for stabilizing carbon-dioxide concentrations in the atmosphere, but focused on a particular range: 450 molecules for every million in the atmosphere parts per million by volume (ppm) to 600 ppm. Concentrations as of 2007 stand at about 383 ppm, according to the Global Carbon Project, compared with 280 ppm at the start of the Industrial Revolution.

The 450-600 range is of special interest, since 450 by the year 2100 represents a target widely cited in negotiations for a new global climate treaty, which its architects hope will be ready to take over when the 1997 Kyoto Protocol's first five-year "commitment period" expires in 2012. The 600 is a business-as-usual emissions path.

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