Current CO2 concentration in the atmosphere

Carbon budgets and US emissions

The Carbon Brief has a must-read article up, Carbon briefing: Making sense of the IPCC’s new carbon budget, that points out the precarious nature of our emissions situation:

So how big is the budget? For it to remain likely that we stay below two degrees, the total amount of carbon released through carbon dioxide emissions must be less than 1000 billion tonnes, the IPCC says. ‘Likely’ here means a 66 per cent chance.

It’s possible to calculate a budget like this because carbon dioxide, which is the biggest contributor to global warming, has a predictable relationship with temperature. The warming we get is almost directly proportional to the total amount of carbon dioxide that accumulates in the atmosphere.

While carbon dioxide is the main greenhouse gas resulting from human activity, it’s not the only thing causing the atmosphere to warm. Methane, CFCs, ozone, nitrous oxide and soot all have an overall warming effect.

To stick within the two degree target, this means the budget for carbon dioxide emissions ends up being less than the original 1000 billion tonnes.

In the draft Summary for Policymakers given to governments to review back in June, the IPCC scientists stopped short of including a concrete figure for how much these other gases tighten the budget for carbon dioxide emissions.

That was because their warming effect is less straightforward than for carbon dioxide. But after calls from governments for something more tangible to work with, the final version of the Summary for Policymakers included scientists’ best estimate of what the reduced budget would be.

So, the magic number they came up with is 800 billion tons of carbon, including the 531 billion tons already poured into the atmosphere, leaving us about 270 billion tons. GIven that current worldwide emissions are roughly 10 billion tons per year, this is most definitely not happy news. And, as the article points out, this estimate does not include little things like methane emissions from the Arctic not-so-permafrost, which is currently home to about 1.7 trillion tons of carbon. But let us not muddy the water with alarmist talk of impending gloom; we’ll stick to the numbers, which are plenty depressing all by their sterile selves.

Note, as quoted above, that this carbon budget is only assuming a 66% chance of staying below the magic 2C limit on industrial warming. You do realize, Dear Readers, that setting such a low bar means we have exactly the same probability for disaster as playing Russian roulette with two bullets, not the usual one, in the gun, correct?

Some thoughts on all of this:

  • I’ll spare you my usual rending of clothes over the 2C limit except to say that given what we’re already seeing in terms of melting ice in the Arctic, Antarctic, and continental glaciers, plus locked in sea level rise and other horrors, all at a warming of a mere 0.85C, I’m not at all confident that 2C is the right limit.
  • Who decided that a 66% probability of avoiding 2C was enough? That seems like an absurdly low target, considering what’s at stake. In their Unburnable Carbon study, the Carbon Tracker Initiative details a remaining budget for an 80% chance of avoiding 2C of only about 155 billion tons of carbon (reported as 565 billion tons of CO2). Are you comfortable with even a 20% chance of sending the climate off the rails in horribly painful and expensive ways? I’m not.
  • While I generally like the mental model of a “carbon budget” and its usefulness in communicating with non-experts and non-climate-obsessives, there is a downside to it. Far too many people will instinctively look at the numbers above and say, “If we can emit another 270 billion tons of carbon and right now we’re at 10 billion per year, that gives us a long time to fix the problem — 27 years — so we’ll be OK.” This is dangerously naive. It overlooks the fact that if we run right off the end of the budget, we simply can’t wake up on January 1, 2040 and say, “OK, we’re done with carbon, let’s start building wind turbines and solar panels real fast.” It also ignores the possibility that feedbacks, like those nasty Arctic carbon stores, will kick in make us zoom past 2C even if we barely stay under the budget. Hell, even without the Arctic carbon jumping into the fray, we’re still looking at a one in three chance of passing 2C at the limit.
  • How far back, one might reasonably ask, would we have to go to pinpoint a time when we could have made a comfortable transition away from carbon-intensive energy sources? My guess is that even when James Hansen gave his famous Congressional testimony in 1988 it was already too late. It might even have been too late when Jimmy Carter gave his energy crisis speech on April 18, 1977. But I am damn sure that 2013 is way beyond the comfort zone.

And as for the recent reports of the “good news” about US CO2 emissions dropping, Brad Plumer has a sobering assessment that I highly recommend.

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