What is needed to keep the temperature rise to 2°C
Is it possible to limit the rise in emissions to 2°C? If we do nothing, emissions will roughly double by 2050, putting us on course for the most apocalyptic of the IPCC’s six scenarios, Category VI, which foresees temperatures rising by up to 6C.
The 2°C rise is at the very other end of the scale of scenarios. It is the lowest projected rise in the most optimistic scenario, Category I, which projects a temperature rise of between 2.0 and 2.4C, resulting from stabilising CO2 at between 350 and 400ppm. This is projected to require cuts of 50%-85% in global emissions by 2050, with a peak in emissions no later than 2015. CO2 currently stands at 380ppm so in fact the lower end of this range of projections means reducing emissions so far that the blanket gets thinner.
These projections use ranges of numbers because science cannot be exact, but inevitably that has given politicians room to spin, or at least fudge, the data.
The UK government position articulated in 2007, for example, is that “a 2°C maximum temperature rise equates to a reduction in global greenhouse gas emissions of at least 50% by 2050 (on 1990 levels).” [1] Tony Blair’s version is: “The panel (IPCC) says we must cut global emissions in half from 1990 levels with a peak in emissions by 2015 if we are to have a reasonable chance of avoiding a greater than 2 degree rise”.
These statements are technically accurate, but selective. They suggest that the least demanding target for emissions cuts (50% by 2050) can deliver the most optimistic target for temperature (2°C ). Turn the spin on its head and a pessimist could use the figures to say that we must cut emissions by 85% to have a good chance of preventing a rise of 2.4C. And note that the statements are silent on the fraught issue of the concentration at which CO2 needs to be stabilised.
In fact, the Category I scenario on which these statements are based foresees stabilisation happening at a maximum of 400ppm – and that is still far from ‘safe’. It is a figure which the Stern Review showed still carried a risk of between 26% and 78% of a rise of over 2 degrees. [2] It was also the level that Stern called “likely to be unachievable”.
Stern said that total GHGs should be stabilised between 400 and 490ppm of CO2. The middle of that range is 445ppm which in the IPCC analysis leads to a rise of over 2.8C. Stern has been widely drawn on by businesses and others. But if we’re serious about aiming for 2°C , then we have to be more ambitious.
Stern acknowledged that stabilising CO2 at the higher end of his recommended range, 490ppm, carries a risk of between 63% and 99% that temperatures will rise by more than 2°C . In fact he said 490pm carries a 21%-69% risk of a rise of over 3°C, which he admitted would entail “very damaging physical, social and economic impacts, and heightened risks of catastrophic changes.”
The UK Government’s 2007 statement said that the scenario of cutting GHG emissions by 50% by 2050 and keeping temperature rises to 2°C “ means that global emissions must peak in the next 10-15 years, then decline.” [3] This is now simply wrong if we follow the IPCC scenario to target 2°C , because its peak year in that projection is not 10-15 years after 2007 – ie 2017-2022 - but 2015, as indeed Blair has recognised.
The prevalence of ranges in scientific projections have allowed both politicians and business people to be selective with data in ways that mask the real urgency of the situation and obscure conflicts between sources of conventional wisdom such as Stern and the IPCC.
But if we look objectively at the data we can see certain facts stand out.
To be sure of avoiding disaster, there is a wide belief that we have to limit the temperature rise to 2°C
To limit the rise to 2°C , we have to be more ambitious than Stern who said that the stabilisation level required – 400ppm CO2 - was unlikely to be achieved and recommended aiming for targets that carried large risks of a rise over 2°C .
To limit the rise to 2°C , we need to aim for a 2015 peak in emissions and probably for cuts of well over 50% by 2050.