What's the take-home message of irreversible climate change?

Can't let this week slip any further past without drawing your attention to a new paper on "Irreversible climate change because of carbon dioxide emissions," which has just been published in the Proceedings of the National Academy of Sciences. (It can be found here), and I have a copy and will share some excerpts. You can also read a press release here.

First, the authors, Susan Solomon of NOAA's Earth System Research Laboratory in Boulder and her European colleagues, deal with the defining the most important term in the paper:

(where irreversible is defined here as a time scale exceeding the end of the millennium in year 3000; note that we do not consider geo-engineering measures that might be able to remove gases already in the atmosphere or to introduce active cooling to counteract warming)

While the paper has already attracted a fair bit of attention, I'm not sure this is the best way to go about it, as it's clear people just don't worry much about what's going to happen a thousand years hence. Case in point: One of the strongest criticisms of Al Gore's climate change presentation is that his most dramatic animation, which lays out what happens to coastal cities, including New York, if you melt half the Greenland and West Antarctic ice sheets, comes with no specified time frame. And then it turns out that even in the worst-case scenarios, our best guess at that time frame involves centuries if not several millennia.

What's going to get politicians interested is what will happen in the next few decades ;;;; and even then you have to bias your presentation toward the nearest of terms.

Still, the Solomon paper does make some extremely interesting points. For starters:

It is not generally appreciated that the atmospheric temperature increases caused by rising carbon dioxide concentrations are not expected to decrease significantly even if carbon emissions were to completely cease...

Global average temperatures increase while CO2 is increasing and then remain approximately constant (within 0.5 °C) until the end of the millennium despite zero further emissions in all of the test cases.

and in the case of precipitation:

Increased drying of respective dry seasons is projected by 90% of the models averaged over the indicated regions of southern Europe, northern Africa, southern Africa, and southwestern North America and by 80% of the models for eastern South America and western Australia

Is there a danger in pointing that we've already done "irreversible" damage to the ecological support system of the planet? Will that meme carry so much weight that people will just give up? Not that we shouldn't be exploring such avenues of research, but it's an important question to ask.

Solomon was on NPR Monday afternoon, insisting that her results do not mean we should give up, even though it looks like whatever we do we're looking at very serious consequences of what we've already done.

"I guess if it's irreversible, to me it seems all the more reason you might want to do something about it," she says. "Because committing to something that you can't back out of seems to me like a step that you'd want to take even more carefully than something you thought you could reverse."

Hmmm. What this means to me is those who are pushing for strong climate change mitigation action are going to have to emphasize that what's already in the pipeline will pale compared with what will come if we don't get our act together.

So what we need are some very easy to understand explanations of the difference between letting business as usual continue and moving aggressively and immediately to a decarbonized energy economy.

Solomon et al. ends with a concise summary of the policy implications, especially for those like Bjorn Lomborg who argue that economics is more powerful that ecology and for those advocating a cap-and-trade approach to reducing emissions:

It is sometimes imagined that slow processes such as climate changes pose small risks, on the basis of the assumption that a choice can always be made to quickly reduce emissions and thereby reverse any harm within a few years or decades. We have shown that this assumption is incorrect for carbon dioxide emissions, because of the longevity of the atmospheric CO2 perturbation and ocean warming. Irreversible climate changes due to carbon dioxide emissions have already taken place, and future carbon dioxide emissions would imply further irreversible effects on the planet, with attendant long legacies for choices made by contemporary society. Discount rates used in some estimates of economic trade-offs assume that more efficient climate mitigation can occur in a future richer world, but neglect the irreversibility shown here. Similarly, understanding of irreversibility reveals limitations in trading of greenhouse gases on the basis of 100-year estimated climate changes (global warming potentials, GWPs), because this metric neglects carbon dioxide's unique long-term effects.

UPDATE: Andy Revkin in his NY Times Dot Earth blog, bounced the Solomon paper off John Sterman of MIT, who replied with a lengthy letter that ends on an optimistic note:

One more critical point: it's important that people not react to Solomon's work with despair. Yes, a certain amount of climate change, due to past emissions, is inevitable, and will not be reversible. But it would be tragic if people concluded that therefore there is nothing we can do, that it is futile to reduce emissions, and that therefore all efforts should shift to adaptation. To the contrary: if nothing is done to cut emissions, and soon, the climate our children and grandchildren will face will almost certainly be far less hospitable, and there will be no turning back. By the time we know for certain how bad it will be it will be too late to take any corrective action. The Solomon paper should finally bury the idea that we can wait and see. It further strengthens the case for immediate, strong mitigation. The good news is that it's getting cheaper every day to cut carbon emissions. Through learning, scale economies, R&D, and other forms of innovation, new technologies for carbon-neutral renewable energy are becoming more available and less expensive. Each megawatt of solar or wind capacity we build lowers the cost of the next and the next -- a positive feedback we

need to strengthen if we are too avoid irreversible harm to the ability of the planet to sustain us.
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Susan Solomon (2009). Irreversible climate change because of carbon
dioxide emissions Proceedings of the National Academy of Sciences