'The 1995 Second Assessment Report from the Intergovernmental Panel on Climate Change (IPCC), which brings together over 1,000 leading climate scientists, put forward evidence that the risk of severe climate change impacts would increase markedly beyond a temperature rise of 2°C above pre-industrial levels. Consequently the Council of Ministers set 2°C as the EU's target ceiling. Scientific studies since then, including the IPCC's Third Assessment Report in 2001 as well as the Fourth Assessment Report, have further underpinned the arguments for keeping within this limit. Significant impacts on ecosystems and water resources are likely even with a temperature increase of 1-2°C above pre-industrial levels. But once global warming exceeds 2°C, climate impacts on food production, water supply and ecosystems are projected to increase significantly and irreversible catastrophic events may occur.
The Council has noted that to respect the 2°C limit, atmospheric concentrations of greenhouse gases - currently around 425 ppmv CO2 eq and rising by 2-3 ppmv CO2 eq per year - would have to remain well below 550 ppmv CO2 eq. Stabilising concentrations at around 450 ppmv CO2 eq in the long term would give a 50/50 chance of staying within the 2°C ceiling. This is the long-term concentration that has guided the Commission in its most recent economic analysis. Concentrations are however likely to overshoot 450 ppmv CO2 eq. in the short term before deeper emissions reductions bring them back down to this level subsequently.'
Source: Questions and Answers on the Commission Communication Limiting Global Climate Change to 2 degrees Celsius