How to define success on climate change

Lots of media coverage today of a special edition of Nature that’s just been published, and in particular on two articles that discuss what it will take to limit global average warming to 2 degrees C.

The headline finding that most of the press coverage runs with is that the total, cumulative carbon budget that the world can emit without hitting catastrophic tipping points is estimated at 1 trillion tonnes of carbon – and that we’ve already used up half of this. What’s more, as Wired notes, at present we’re sending another 9 billion tonnes of carbon up into the air each year – meaning that on present rates, we’re going to hit the buffers within half a century.

So, according to the authors of the studies, we need to reduce global emissions by around 80% by 2050 – quite some distance more demanding a target than the 50% by 2050 target that the G8 has committed to, though in line with Obama’s headline objective. (As I noted here back in 2007, the G8 should have known better than to take 50% as their headline global target – which rested on a rather optimistic interpretation of figures set out in the last IPCC assessment report.)

One thing that confused me in the two Nature articles, though, was this point – summed up on Real Climate (emphasis added):

Both [articles] find that the most directly relevant quantity is the total amount of CO2 ultimately released, rather than a target atmospheric CO2 concentration or emission rate. This is an extremely useful result, giving us a clear statement of how our policy goals should be framed. We have a total emission quota; if we keep going now, we will have to cut back more quickly later.

Needless to say, the question of what metric we use to measure success on climate change is a very big deal, given the extent of policy implications that flow from it.  So are the authors right to suggest that instead of aiming for a target CO2 concentration level, we should be focusing primarily on cumulative emissions?

Well, by way of comparison of the different metrics, think of the atmosphere as a bath-tub and CO2 as water.  Too much water, and the bath will overflow (as we start hitting buffers, tipping points, positive feedbacks, abrupt climate change and other Bad Things). In this metaphor:

  • Emissions = the amount of water flowing into the bath
  • Sinks (the amount of CO2 soaked up by oceans, forests etc.) = the amount of water flowing out of the plughole
  • Concentration levels (how much CO2 or CO2e there is in the air, in parts per million) = the level of water in the bath

Now as Myles Allen, lead author of one of the Nature articles, observes in the Guardian today, it’s clearly true that if cumulative emissions matter more than our current rate of emissions right now. To return to the bath-tub metaphor: if you’re worried about the risk of the bath overflowing, then the question of the rate at which is flowing into the bath is clearly less relevant than the total amount of water that’s flowed into the bath since you turned on the tap.

But what I don’t get is why we should be more interested in cumulative emissions (how much water has flowed into the bath) than in concentration levels (the level of water in the bath).

Wired’s article quotes Myles as saying that it’s a “nightmare” to figure out what concentration level is the right one. Fair enough – the question of how much Co2 is absorbed by sinks is indeed fiendishly complex. But I worry about the risks that might be involved in only looking at cumulative emissions – in effect ignoring sinks / water down plughole.

Today, after all, around 40% of our emissions get absorbed by climate sinks.  However, data suggest that this proportion is falling, particularly because oceans’ capacity to absorb CO2 seems to be degrading.  Now if we imagine that sink degradation gets so bad that none of our emissions get absorbed by the oceans, then it clearly follows that we’d have to reduce emissions much faster – because we’d no longer be getting a 40% free ride from the oceans.

But if we’re watching cumulative emissions – the amount of water that’s flowed into the bath – as the key metric, then this crucial question of sink failure doesn’t figure on our climate dashboard.  If, on the other hand, we’re watching concentrations – the level of water in the bath – then the fact that water’s no longer flowing down the plughole will be clearly apparent.

So nightmarish though it may well be to do the maths on a greenhouse gas concentration target, it looks to me as though there’s still a case for framing the overall challenge in terms of a concentration target – as the IPCC did in its last assessment report, when it argued that to limit warming to between 2.0 and 2.4 degrees C, we’d need to stabilise CO2 at between 350-400ppm and CO2e (with other greenhouse gases added in) at between 445-490ppm.

All that said, it IS helpful to have a clear sense of the total remaining global emissions budget that we have to play with – leading, as it inevitably does, towards the long-delayed (and still not commenced) discussion about exactly how to share it out between the world’s nations…