One of the most important targets that will be discussed in Copenhagen is to limit the global rise in temperatures to 2C.

Politicians like to talk about this, to have a clear picture of the extent of global warming, and 2C is widely accepted as the boundary where we cross into really dangerous territory. The problem with this argument is that we don’t have any control over global temperatures, we can only control our emissions.  
 

Sir David King rightly says in today’s FT that we mustn’t lose sight of the real objective, which is to keep the CO2 concentration under 450 ppm. (I would add to that: well under 450 ppm). Even at that level our chances of a temperature increase of 2C or lower will be less than 50%. If we continue with “business as usual”, we are adding 2 ppm per year to the existing concentration and we will reach and cross the 450 ppm threshold in the year 2045.  

I think it’s important to emphasise that curbing emissions is all we can do and we do not have the ability to curb temperatures, so setting a target at 2C is meaningless. 

For any given level of concentration of CO2 there’s a probability distribution for the resulting temperature rises. This looks like a bell-shaped curve where the tail on the right is “fatter” than on the left – this means that the likeliest temperature is not exactly at the peak but slightly shifted to the right. For the concentration of 450 ppm of GHG (in CO2-equivalent) the likeliest range (the bulk of the peak of the distribution) is between 2C and 3.5C, and the likeliest temperature increase is 2.5C (in order to have a fifty-fifty chance to reach 2C we should be at about 430 ppm).  

Even today, if all GHG emissions dropped to zero, the likelihood of a 2C would not be negligible: the probability is about 15%. This is low, but it is not within the realm of the impossible. Our constant excess of emissions shifts the probability curve constantly to higher values and the probability of reaching higher increases grows every year.  

I think we need to face the possibility that even within our best endeavours staying below 2C might be beyond our reach, and we need to plan ahead for a very different world in a few decades’ time, where we can anticipate that countries will struggle to get their necessary share of the planet’s resources, especially water.  

The most meaningful target to aim for is to restore the equilibrium of the planet’s carbon cycle. That is the long term goal. And to achieve that we need to curb emissions to remain not under 450 ppm, but under 350 ppm, which is the natural concentration that our atmosphere can sustainably cope with.