The trillionth tonne meme gets some traction

The first three of the "America's Climate Choices" reports from a U.S. National Academy of Sciences committee restate the case that there is "strong, credible body of evidence, based on multiple lines of research, documenting that Earth is warming" and calls for the adoption of "an economy-wide carbon pricing system." Not really Earth-shattering news, just climate-disrupting. What is worth drawing your attention to is the embrace of something akin to the "trillionth tonne" idea.

Until now, most government-associated documents dwell on end-point targets. A given a certain chance of keeping greenhouse gas concentrations below a specific threshold and certain temperature by a specific date, say a 75% chance of no more than 2 °C over preindustrial norms by 2050 by cutting emissions to 80% of 1990 levels in hopes of keeping CO₂ to under 450 parts per million.

The new report from the Panel on Limiting the Magnitude of Future Climate Change, however, is the first time I know of such an esteemed group calling for a cumulative emissions budget. The idea, which is based on the fact that what matters most is how much carbon we spew into the atmosphere, not how fast and when we do it, has been around for a while. But it was given a catchy slogan last year when Nature published a couple of papers (here and here) and lots of supplementary commentary on "the trillionth tonne." (It's a good thing that tonnes and tons aren't that different.)

Essentially, the problem boils down to an estimate of how much anthropogenic carbon the atmosphere can absorb before the Earth warms more than 2 °C, a number that turns out to be close to a trillion tonnes. We've already emitted a little over half that much, so we have to figure out how to budget the remaining 500 gigatonnes.

The NAS report doesn't use the language of a trillion tonnes, which is for global calculation purposes, but it does supply some numbers for the U.S., referring to a

'representative' domestic emissions budget in the range of 170 to 200 gigatons of CO_2-equivalent for the period 2012 through 2050.... At the current rate of U.S. emissions (roughly 7 Gt CO_2-eq per year), the proposed budget would be "spent" well before 2050.

The implication is that if we emit more than the quota that nature (not Nature) has afforded us, we will then have committed the planet to a new climate regime, one that little resembles the condition of the past 10,000 years, during which civilization arose. Of course, that will be no reason to give up trying to avoid even worse consequences.

Just how soon we will exceed that budget? The scientists behind trillionthtonne.org, drawing on those Nature papers, calculate that at current global emission rates, our quota will be reached sometime in the Northern Hemisphere's summer of 2028, assuming that we're content with "a three in four chance of global warming remaining below 2°C."

The intriguing thing about using the cumulative emissions approach is that it actually allows for a wide range of policy options. It also focuses the mind on the reality that the sooner we start cutting emissions, the longer we will have before spending the entire budget. Conversely, the later we wait, the steeper the slope of the decline will have to be. If we wait until the 2020s to start bringing real emissions down, we stand a very slim chance of avoiding catastrophic climate change. But if we start now, cutting at between 2% and 4% a year, things look a bit rosier.

That's not to argue that a 2% annual cut for the next half century will be easy. But it's certainly not in the realm of the impossible. Let's hope this approach attracts yet more attention.

I'll note also that one of the climatologists responsible for pushing the idea, Myles Allen of the University of Oxford, is, according to some calculations, the second-most cited climatologist on the planet. He's behind only John Mitchell, chief scientist at the U.K.'s Met Office, and ahead by a whisker of NASA's James Hansen.