Tuesday, July 29, 2014

Why it's still not "game over" for global warming

Buildings are seen near the ocean as reports indicate that Miami-Dade County in the future could be one of the most susceptible places when it comes to rising water levels due to global warming. Joe Raedle / Getty Images

If you haven't read it yet, I'd recommend my colleague Ezra Klein's piece on "7 reasons America will fail at global warming." It's an insightful look at all the ways the US political system is poorly suited to dealing with a massive, long-range problem like climate change.

But the underlying premise of the article is a little ill-defined. Climate change isn't an issue with a single point of "success" or a single point of "failure."

What we're facing are (literally) degrees of change. The world will get hotter as we load more greenhouse gases into the atmosphere. And the higher the temperatures, the greater the risks for human civilization. A 2°C rise in global average temperatures would be disruptive. A 4°C rise would be much more disruptive. And 6°C rise would be far, far more drastic still.

At no point here does it make sense to say that we've "failed" once and for all, or that it's (to use Ezra's phrase) "game over." Things can always get worse. And it's still very unclear where we'll end up on that spectrum.

We're likely to miss the 2°C goal — but the story doesn't end there

Ezra's jumping-off point was a long piece I wrote about the world's likely failure to stay below the agreed-on 2°C temperature limit.

That's a goal that the international community has set for itself: We shouldn't let global average temperatures rise more than 2°C (or 3.6°F) above pre-industrial levels. Go past that, and we step outside the conditions under which human civilization developed.

It's true that the world is very likely to go past the 2°C mark — for the reasons I reported on here. But the 2°C limit also isn't some magical line between "success" and "failure." It's an artificial boundary that happened to become the focal point for policy discussions. The world isn't entirely safe if we stay below it, nor entirely doomed if we go above it.

Indeed, plenty of people would have considered even 2°C of warming a "failure" — it will likely mean that low-lying island nations like Tuvalu get swallowed up by the rising seas, as well as extensive damage to coral reefs and the communities that depend on them as the oceans warm.

This isn't semantic quibbling. The key point is that the disruptions caused by global warming are likely to increase as temperatures keep rising. And the damage is expected to increase non-linearly. Here are a few mainstream forecasts from economists who have tried to model that damage:

Economic loss from climate change as temperatures increase

Stern2013

Annual Consumption Loss as a Fraction of Global GDP in 2100 due to an Increase in Annual Global Temperature in the DICE, FUND, and PAGE Models, from Stern (2013).

Different models have different estimates for how costly global warming will be. But everyone agrees on the general point — risks and damages keep piling up as the world gets hotter. So if the world can't prevent 2°C of warming, it's still a good idea to try and avoid 3°C of warming. If we can't avoid 3°C of warming, it's still a good idea to avoid 4°C. And so on.

Climate policy experts don't like to put things this way, since it increases the odds that the world might get lulled into complacency and postpone cutting emissions, letting temperatures rise higher and higher. (The World Bank, for one, has argued that "4°C warming simply must not be allowed to occur" — because there's no guarantee that humanity can adapt.)

That's understandable. Setting hard boundaries — and framing things in terms of success and failure — is a much more intuitive way to think about the issue. (I've been guilty of this sort of talk myself.)

But it doesn't really make sense to declare "game over" at any point. One place to see that is with that recent news that six of West Antarctica's key glaciers appear to be in a state of irreversible decline and will eventually collapse into the ocean in the coming centuries.

Yes, it's probably too late to keep that ice sheet intact. In that sense, we've failed. But by increasing or decreasing emissions, humanity can still influence how rapidly that ice melts in the centuries ahead — and hence, how much time coastal cities have to adapt to rising sea levels. And that's a far more relevant question.

So how much warming are we in for?

Clarification: The numbers on the Y-axis refer to billion metric tons per year of C, not CO2

That's still hard to say exactly — there are a lot of variables at play here. But if global greenhouse-gas emissions continue growing on their current trajectory, climate models suggest we could face around 4°C or more of warming by the end of the century (that's 7.2°F).

On the other hand, if every country in the world followed through on the pledges that they've already made to cut greenhouse gases, then we could possibly limit that to somewhere around 3.1°C of warming, according to calculations by the Climate Action Tracker, which keeps tabs on government commitments.

Even those cuts are far from assured — most countries would need to adopt considerably more ambitious policies to limit emissions. What's more, 3.1°C of warming is likely to put a lot of stress on crops and the global food system, lead to significant sea-level rise, and bring increased heat waves, droughts, and so on. But 3.1°C is also still less than 4°C.

No matter what, the world is expected to face a significant rise in global temperatures over the coming century. That will likely require costly and often painful adjustments around the world — particularly for poor countries that have fewer resources to cope. But the scale and pace of that rise matters a great deal, and humanity still has a lot of influence over that final outcome.

Further reading:

  • My longer piece on the 2°C temperature target, and why the world is likely to surpass it. (And yes, the headline of that piece is guilty of the same sloppy "failure" language critiqued in this post.)
  • The most recent report from the IPCC went into extensive detail about what to expect as the planet warms — and delved into the prospects for adaptation.

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