350 vs 450

Finally, the issue of whether the “magic number” for the atmospheric CO2 level is 450 ppm (parts per million) or 350 is getting more attention. In this case, the attention is in the form of a NY Times article, Is 350 the New 450 When It Comes to Capping Carbon Emissions?, which I highly recommend:

Nearly 200 countries have signed a U.N. treaty pledging to avoid “dangerous” climate change. But lately, it seems, “dangerous” is lost in translation. Fifteen years since that agreement took effect, scientists and governments are still grappling with what carrying out its promise means.

For the European Union, it means limiting Earth’s warming to just 2 degrees Celsius hotter by the end of this century than it was before the Industrial Revolution. That’s a goal many experts believe is roughly equivalent to capping atmospheric carbon dioxide at 450 parts per million. But a growing number of countries — mostly vulnerable ones and small island nations like the Maldives — say that won’t prevent rising sea levels from swamping their coasts.

They’re calling for an even stricter standard: 350 parts per million, a number endorsed by NASA climatologist James Hansen.

Some scientists mapping out Earth’s potential futures say both targets are arbitrary. What’s essential, they insist, is that countries start cutting their greenhouse gas emissions soon and stay flexible in case the planet behaves in unexpected ways.

“The best guesses are not carved in stone,” says Gavin Schmidt, a climate modeler at NASA’s Goddard Institute for Space Studies. “There well may be surprises, pleasant or unpleasant.”

I hereby nominate Gavin Schmidt for the 2009 Understatement of the Year Award, Science Division.

The article continues:

Even reaching the 450 ppm target isn’t a guarantee — a sentiment stated bluntly in a letter (pdf) that 20 prominent climate scientists recently sent to members of Congress, urging an even more stringent emissions goal “to hold the risk of ruinous climatic change to an acceptably low level.”

“When you put in all those uncertainties, the idea that you can hopefully say 450 [ppm] will be 2 degrees [Celsius], it’ll be the same thing — that’s rather optimistic,” NASA’s Schmidt said.

“If we’re very lucky, 450 might be another 1 degree. If we’re very unlucky, it might be another 3 degrees.” With that in mind, the scientist said he thinks 350 ppm is a good long-term goal.

“We can see changes in ice sheets happening now at roughly 390,” he said, “and the planet hasn’t even caught up with 390 yet. … The prudent thing is to say, ‘If we can dial it back again, let’s do it.’”

This is a key point, one that I doubt many people understand at an intuitive level: The changes humanity is making to the atmosphere are the proverbial sucker punch, from a geological standpoint. But the environment takes much longer to respond. This is why discussions about climate chaos so often talk about there being X degrees of warming “in the pipeline”, and the awful trick we’re playing on ourselves by obsessing over what happens only between now and the year 2100. As David Archer pointed out in his book The Long Thaw (page 162), by limiting our view to the year 2100 we’re creating a “bonus” of 40% in the amount of CO2 we can emit simply because 40% of the change from our emissions up to 2100 won’t happen until after that date. Any bets we’ll hear a serious discussion of this 40% bonus and its implications for public policy in Copenhagen? No, I don’t think so, either.

And notice also that Schmidt seems to be questioning the mapping of 450 ppm to 2°C, a key assumption that I think deserves far more attention than it’s deserved to date.

And wait, there’s more:

Flexibility is also important, said Schmidt, pointing to lessons gleaned from another U.N. environmental treaty, the 1990 Montreal Protocol. Countries that signed the treaty eventually agreed to phase out the use of ozone-depleting chlorofluorocarbons. But it took years of experience and several rounds of revisions to reach that point.

“It took 4 or 5 goes for [treaty parties] to get to those decisions that had a big impact,” Schmidt said. With climate change, he said, governments should have short-term targets “commensurate with the size of the problem” that they can revisit regularly.

“2050 is long enough away that we are going to get a few bites at this cherry,” he said.

Despite the wrestling over treaty targets, scientists said one point is clear: The world needs to cut its greenhouse gas output.

Richard Betts, head of climate impacts at the Met Office, said that if current CO2 emissions trends continue, the world could warm an average of 4 degrees Celsius by 2100, with devastating effects.

My guess is that the first round of public policies, both here in the US and around the world, will be distinctly uninspiring. So we’ll need those multiple cherry bites to get it right (or even merely closer to the mark); my fear is that we’ll only take bites 2..N when pushed by real world impacts. This is where the buffering/timing effect mentioned above becomes much more than a scientific curiosity; if we wait to act until after we’re already feeling the pain of climate chaos, then a sizable additional increment of pain will be unavoidable thanks to locked-in warming and its ramifications. And if that warming just happens to come at a time and temperature that triggers large-scale methane releases from permafrost and/or hydrates, then our delay will have triggered a climate catastrophe that we’ll have virtually no chance of stopping.

I’ve often made the comparison that we’re playing Russian roulette with the climate. While that’s admittedly a very unpleasant piece of imagery, I thin it’s appropriate in that the costs of losing are catastrophic and we really are dealing with at least some level of uncertainty, which brings probability into the picture. One problem, of course, is that that the lock-in effect means our inaction on climate change amounts to placing a bet on multiple trigger pulls at once, and thanks to our emissions being so far ahead of our understanding of how the climate works and responds to such shocks, we’re just now finding out that the number of trigger pulls bundled into that bet is higher than we thought.

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