What it will take to cut global warming in half

A new paper to be published next week in Geophysical Research Letters (which really needs a better name) lays out what kind of effort would be required to reduce the impacts of climate change by half. Actually, what it does is conclude that if we reign in our fossil-fuel emissions by 70%, temperature rise and a few other consequences will be roughly half of what they'd be in the absence of any mitigating effort. But since both targets are arbitrary, it doesn't really matter which way you approach the subject.

Either way, the news ain't good.

In "How much climate change can be avoided by mitigation?" (subs req'd) Warren Washington of the National Center for Atmospheric Research in Boulder, Colo., and seven of his colleagues in Boulder, the Institute for Atmospheric and Climate Science in Switzerland, and Climate Central of New Jersey and California, run a bunch of computer models to compare a non-mitigation (business-as-usual) scenario with "a low emission mitigation" alternative. That alternative sees civilization managing to cut oil, gas and coal emissions by 70% by 2100, by which time CO2 levels would be stabilized at 450 ppm.

They use those numbers because the IPCC frequently talks about them, even though the standard timeframe being bandied about these days involves even more aggressive cuts by as soon as 2050. There are those who fear that if we don't manage the earlier schedule, we're pretty much SOL. So keep that in mind when evaluating the author's conclusions. Which are that :

Compared to a nonintervention reference scenario, emission reductions of about 70% by 2100 are required to prevent roughly half the change in temperature and precipitation that would otherwise occur. By 2100, the resulting stabilized global climate would ensure preservation of considerable Arctic sea ice and permafrost areas. Future heat waves would be 55% less intense, and sea level rise from thermal expansion would be about 57% lower than if a non-mitigation scenario was followed.

The authors don't pretend to know whether or not these kind of reductions will be sufficient to avoid what most people would consider "catastrophic" consequences. That's a policy question, right? It is quite possible that even this much mitigation won't be enough to stop us triggering a tipping point that makes the rest of their calculations irrelevant. No one knows, for example, just how much permafrost can melt before the methane released by that melt kicks off a runaway greenhouse effect.

Instead, their take-home message is "the impacts of climate change with a mitigation scenario are substantially less than with a non-intervention emission strategy, and the amount of climate change that can be averted with mitigation is considerable." (One could observe that we knew as much already, but it is always helpful to have a group of scientists back up common sense.)

When I look at the results graphically, however, a different message occurs to me. Here are a couple of maps illustrating the difference between a do-nothing approach (top) and the 70% mitigation scenario:

To my eyes, the bottom map doesn't look too inviting. Not when you consider an increase of 2°C is widely regarded as "not good." Perhaps this explains why the most common targets being pushed by the more aggressive climate science advisers involve a cut of between 80 and 90% by 2050. James Hansen would have us reduce CO2 levels to significantly below today's 387 ppm by the end of the century, because he says 450 is too dangerous.

Is it important exactly what targets we choose at this point? Some argue that what we really need are non-numerical policy targets, such as Hansen's immediate moratorium on new coal-fired plants and the subsequent total phaseout by 2030. I tend to agree that setting national or even international emissions targets but leaving the details to the market, whether through a cap-and-trade approach or a flat carbon tax, makes me more than a little nervous.

In any event, we need regular and frequent reminders of the cost of doing nothing. And Washingon et al. 2009 is just such a reminder. I would like to see the 70% mitigation scenario compared with 80, 90 and 100% mitigation scenarios, but that's just a quibble.
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Warren Washington, et al. (209). How much climate change can be avoided by mitigation Geophysical Research Letters