How important is the two degree target?

In a speech given at Dartmouth College at the beginning of this month, US Lead Climate Negotiator Todd Stern caused some consternation in the media by opening up the subject of the global two degree Celsius target. Bylines such as “US Abandons 2° Target” appeared soon after, to the extent that a further statement was made two days later by Todd Stern to say;

 “The U.S. continues to support this goal. We have not changed our policy.”

Reading the speech more closely, Stern had not dismissed the target at all nor questioned the necessity of making substantial reductions in global emissions. Rather, he had outlined a negotiating strategy which might bring nations to the table and actually get them to agree on something, rather than the status quo situation which has so far resulted in little progress.

 For many countries, the core assumption about how to address climate change is that you negotiate a treaty with binding emission targets stringent enough to meet a stipulated global goal – namely, holding the increase in global average temperature to less than 2° centigrade above pre-industrial levels – and that treaty in turn drives national action. This is a kind of unified field theory of solving climate change – get the treaty right; the treaty dictates national action; and the problem gets solved. This is entirely logical. It makes perfect sense on paper. The trouble is it ignores the classic lesson that politics – including international politics – is the art of the possible. . . .

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This kind of flexible, evolving legal agreement cannot guarantee that we meet a 2 degree goal, but insisting on a structure that would guarantee such a goal will only lead to deadlock. It is more important to start now with a regime that can get us going in the right direction and that is built in a way maximally conducive to raising ambition, spurring innovation, and building political will.

The 2°C target has been around for a while, but has no particular scientific basis. Rather, it represents an integrated assessment based on many inputs. From what I have been able to find, it appears to be the point at which various systems may see a step change in their response to rising temperatures. This includes the collapse of some major ice shelves, changes in major ocean current circulation, the demise of some marine ecosystems, extensive coral bleaching and so on. Much of this is summarized in an output document from a 2005 conference, Avoiding Dangerous Climate Change, convened by the British Government prior to their hosting of the G8. Although the EU had proposed a 2°C target well before 2005, it was at this conference and the following G8 meeting where it really took hold. Finally, at the UNFCCC COP in Cancun it was agreed as a formal goal, given that the objective of the Convention is to “avoid dangerous anthropogenic interference with the climate system”.

But the 2°C objective is just the beginning  of a long chain which must ultimately stretch down to the allowable emissions of a given power plant or the need to store a tonne of CO2 in a subsurface reservoir. This chain is riddled with uncertainty which almost never gets a mention. For example, many now link the 2°C objective with atmospheric stabilization of CO2 at a level of 450 ppm and in previous postings I have talked about 2°C equating to an atmospheric stock of one trillion tonnes of carbon. But these are levels that are associated with something around a 50% probability that global temperatures will plateau below a 2°C rise. In trillion tonne terms, based on a “business as usual” scenario, we will cross that point in about mid-2043. Shifting the bar such that we have a better than 75% chance of limiting the temperature rise to 2°C moves the crossover point back 15 years, to early 2028.

There is also no guarantee that we can collectively limit the temperature rise to below 2°C. Even if emissions stopped today, the range of possible outcomes from a 400 ppm CO2 level includes 2°C, albeit at quite a low probability. This is because the atmosphere is not in a state of heat equilibrium and will continue to warm at current levels of CO2.  As such, determining a target atmospheric CO2 concentration becomes difficult. 450 ppm is convenient in that it is above current levels, was feasible (at a stretch) when first raised and coincided with a 50% chance of limiting the temperature to 2°C. More recently James Hansen of GISS / NASA has argued for a target of 350 ppm, in that this would restore the current heat imbalance in the system and therefore stop the temperature rising. The problem with this goal is that we have already passed it and nobody really likes setting a target which can’t be met.

Further to the problem of determining a desired plateau for atmospheric CO2, comes the even more difficult task of translating this into a physical limit on global emissions. The task of halving emissions by 2050 is often discussed, but little mention is made of the fact that after 2050 the trajectory must head pretty rapidly to zero. Even the “half by 2050” goal has been obscured by a forgotten baseline year. For some it is 1990, others it is 2000 / 2005 or even just half compared to now. These are all very different. The original baseline when the “half” was first discussed was 1990, which for energy related emissions translates to a goal of 10 GT per annum – China today is at about 8 GT.

As already noted, CO2 acts like a “stock pollutant” in that it collects in the atmosphere. The best approach for this is to find a mechanism for limiting the cumulative CO2 emitted, in other words never emitting that trillionth tonne of carbon.

Framing the problem in this way then perhaps makes us think differently about the comments made by Todd Stern. Trying to carve up the space left in the atmosphere between 190+ nations may be a diplomatic stretch too far, so we should at least move with haste towards what we can do now. In the interim, as actions start to take root and countries realize that limiting emissions isn’t the end of life, the universe and everything, the door then opens to a more comprehensive approach. This would be an “evolving legal agreement” .

Such an approach isn’t the ideal, but given that “immediate global agreement” has very little chance of happening, it would appear to be the prudent way forward, but with the our sights still set on the 2°C goal.