The two degrees temperature target, favoured by the European Union and the G8, and mentioned in the Copenhagen Accord, has quite an interesting history. It can be understood in rather different ways (PDF), ranging from a threshold beyond which catastrophe looms, as a level at which costs and benefits of mitigation policies are optimised, or as a simplifying measure in a highly complex management process.
Not so long ago, my colleague Oliver Geden at SWP published two papers suggesting that sooner or later, politicians might have to reconsider the two degrees target because global greenhouse gas levels will have made it unlikely to stay below that threshold (see also the discussion at Die Klimazwiebel or at Roger Pielke Jr.’s). This was not welcomed by everyone, since some people thought Oliver had argued against the target as such. Far from it, he merely questioned its future viability as a political strategy once the risk of overshooting two degrees will have become obviously high.
In fact, the numerical risk of exceeding the target is a key variable, with possible GHG levels corresponding with a two degrees temperature increase ranging from 330 to 700 ppm, according to Boykoff et al. 2010. This huge range is due to the uncertainties regarding climate sensitivity, which the IPCC AR4 put at 2-4.5°C for a doubling of CO2.
The following graph from Malte Meinshausen nicely shows the relationship between GHG levels and probability of staying below or exceeding two degrees (taken from Meinshausen 2005, PDF):
It might therefore be sensible to encase the two degrees target with other measures that ultimately serve the same purpose of limiting the amount of warming we’ll likely have to bear. And indeed, climate negotiators have introduced such an encasing in the negotiation text.
In the 70 pages long negotiation text (PDF) of the UNFCCC Ad Hoc Working Group on Long-term Cooperative Action under the Convention (AWG-LCA) delegates are currently discussing in Tianjin, the temperature target is being followed by two other targets: A peak year for emissions, and a global greenhouse gas reduction target. The draft text reads as follows, with [xyz] marking passages that are so far contested among parties:
Temperature, Para.2: “… reducing global emissions so as to [maintain] [hold] [stay well below a 1.5 degree Celsius increase in global average temperature above preindustrial levels] the increase in global temperature below [1.5][/350ppm] degree Celsius above pre-industrial levels]…”
Peak year, Para.3: “…peaking of [global][their] greenhouse gas emissions [[in 2015 and no later than 2020] [no later than 2015] [by 2020 at the latest,] [in order to hold the increase in global temperature below [1.5]  degree Celsius] and the peaking of national emissions] [in 2015]…”
Reductions, Para.4: “[Developed country][Annex I] Parties as a group should [aim to] reduce their greenhouse gas emissions by [at least  per cent from 1990 levels by 2020 and] [80 per cent by 2035 and] [[75-85][around 80][at least 80-95][more than 95] per cent from 1990 levels by 2050] [more than 100 per cent from 1990 levels by 2040] [underpinned by a mid-term target of at least 40 per cent reductions from 1990 levels by 2020.”
As you can see, delegates seem well aware that the two degrees target should not be the only game in town. If two or more of these targets will make it through to a final text to be adopted by the COP in Cancún later this year, they would have to correspond with each other. It is technically unfeasible to agree upon, say, a 1.5 degree target, yet at the same time calling only for a 75% reduction of developed country emissions by 2050 while leving out any commitments for developing countries.
Likewise, if negotiators would agree upon a more than 95% reduction for developed country emissions by 2050 while also limiting developing country emissions, and finding consensus on 2015 as a peak year of global emissions, the two degrees target would not be central at all to such a fictitious (and fantastic, by the way) document anymore.
So, do we already witness the downgrading of the two degrees target in terms of centrality and prominence, just a few months after which it had finally made it into a (“taken note of”-)COP document? Is this encasing a first stept to get rid of the target at some future negotiation round, since it is ultimately unmanageable? Or do a peak year and a global emissions reduction target solidify the position of the two degrees target?