Although there is no safe level for increased temperatures caused by climate change, the Intergovernmental Panel of Climate Change has concluded that we need to limit a temperature rise to no more than 2°C (relative to pre-industrial 1900 levels) in order to avoid the irreversible effects of climate change.

The total amount of CO2 emitted from fossil fuel sources and land use changes affect the degree of global warming. CO2 levels are increasing each year, but removal of CO2 is much slower and it is accumulating in the atmosphere.

As there is a strong relationship between maximum warming and amassed greenhouse gas (GHG) emissions, German, British and Swiss researchers used a combined carbon cycle- climate model to assess thousands of combinations of scenarios and assumptions associated with global warming caused by man-made emissions of GHGs. This model could, for the first time, account for all known uncertainties, in addition to including the effects of all GHG emissions, such as methane and nitrous oxide.

The researchers calculated the chances of each emission scenario, with associated temperature rise, of occurring. The study suggests that there is a 75 per cent chance that 1000 billion tonnes of CO2 can accumulate in the atmosphere between 2000 and 2050 without increasing the temperature by more than 2°C. Or, put another way, there is a 25 per cent chance that the temperature rise will exceed 2°C if 1000 billion tonnes of CO2 accumulates in the atmosphere between 2000 and 2050.

If 1440 billion tonnes of CO2 accumulates in the atmosphere between 2000 and 2050, then there is a greater chance (a 50 per cent chance) that the rise in temperature will exceed the 2°C limit. As 234 billion tonnes have already been emitted to the atmosphere between 2000 and 2006 and emissions have continued to increase since then, more than a 50 per cent reduction in global emissions, compared with 1990 levels, is required by 2050, if global temperatures are to rise by no more than 2°C.

Hence, the study suggests that the only viable means of staying below 2°C is to peak global emissions by 2015. Otherwise the subsequent reduction rates will be extremely challenging. If global emissions peak later than 2020, emission reductions of more than 6 per cent a year are required. This equates to 'a Kyoto-Protocol per year'. In addition, the study suggests that less than a quarter of the fossil fuels that can feasibly be extracted today can be burnt between 2009 and 2050, if the temperature rise is to be limited to 2°C. GHG emissions, other than CO2, also increase the risk of a higher temperature rise.

The researchers estimated that there was a 25 per cent chance that 1500 billion tonnes (and a 50 per cent chance that 2000 billion tonnes) of CO2 equivalents (all GHGs, including CO2) could accumulate before the 2°C temperature threshold was exceeded. This means that there is a 75 per cent chance that the temperature rise will exceed 2°C if 1500 billion tonnes of CO2 equivalents accumulate in the atmosphere. This cumulative budget approach provides a useful means of understanding whether the 2020 and 2050 targets currently discussed in the UN Framework Convention on Climate Change negotiations are sufficient to avoid dangerous climate change.

Source: Meinshausen, M., Meinshausen, N., Hare, W. et al. (2009). Greenhouse gas emission targets for limiting global warming to 2°C. Nature. 458:1158-1163. Contact: malte.meinshausen@pik-potsdam.de