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.
In short, if CO2 concentrations peak at 450-600 ppm, declines in rainfall during the dry season in regions such as the US Southwest or the Mediterranean are comparable to the Dust Bowl drought in the US during the 1930s and those seasonal declines persist for millenniums. Sea level from heat expansion alone rises by an average of up to three feet through the year 3000.
For simplicity's sake, they didn't take a large number of secondary effects into account. And projections of melting of Greenland's and Antarctica's ice caps are still too uncertain to include. The levels Solomon's team projected are a minimum, based on better-understood physical processes. The results appear in the issue of the Proceedings of the National Academy of Sciences for the week of Jan. 26, 2009.
Records of past climate regimes over tens of millions of years or more and the geophysical processes involved, such as those Dr. Archer explores in his book, certainly add weight to the way Solomon and her colleagues have framing the global warming issue in terms of irreversibility.
Long-term change v. public perceptions
But as calls to action, some analysts wonder if that's enough to galvanize the public.
When pollsters ask about global warming as a stand-alone issue, in some surveys people tend to say it's serious and requires action. Some have even responded that they'd even pay higher energy costs if that helped underwrite the move to more climate-friendly ways to meet energy needs.
But ask people to stack global warming against other issues, and it falls to the bottom of a long list. It remains a concern. But it does not enjoy a level of urgency that many environmental groups and scientists see.
In a poll published Jan. 22 and conducted by the Pew Research Center for People and the Press, out of 1,503 respondents, 85 percent pointed to the economy as a top priority; 30 percent said global warming was a top priority.
To some extent, this isn't surprising. One can find similar patterns in responses when people are asked about space exploration and the US space program.
Over at Framing Science, American University communications professor Matt Nesbit has this to say about what the Pew survey results imply for using charismatic animals, or geographically and temporally remote effects to build a case for action on global warming:
It's ... time to stop focusing narrowly on remote polar impacts, looming environmental disaster, or symbols such as polar bears. These exemplars are either not personally relevant enough to most audiences, are dismissed as remote and far off in the future, or easily re-framed as "alarmism" sending interpretations back into the mental box of lingering scientific uncertainty.
Stand by for a forthcoming paper that details some of the possible alternatives, he adds.
Whatever its role may or may not be as yet another in a long list of calls to arms on climate change, the paper could have some immediate implications for policy, the team suggests.
Some the economic analysts have posited that it's better to back off of quick action and wait for the technology to come along that will scrub the air of excess CO2 in short order and at a time when economic growth will ensure that more countries can afford the technologies needed. But as the team points out, excess CO2 is not a short-lived air pollutant such as ozone or sulfur-dioxide. Once it's in the air, there's no going back -- short of so-far unproven approaches for scrubbing CO2 from the air, especially at the scale needed.
In addition, the Kyoto Protocol approach of throwing all human-generated greenhouse gases into a pot and allowing countries to pick the easiest ones to deal with to meet their emissions targets may help slow temperature increases in the short to medium term. But controls on these shorter-lived gases do nothing to ease the millennial-scale effects.
To which one might add one more policy implication: A need to put the pedal to the metal on increasing adaptation planning efforts in developed countries as well as increasing adaptation help for developing countries. Adaptation is no substitute for emissions reductions. But if the somber song of Solomon et al is correct, the need for some degree of adaptation measures is as inevitable as the changes she and her colleagues see humans imposing on climate.