The US and China today announced what has been hailed as a historic deal on greenhouse gas emissions, with China agreeing to cap emissions around 2030 and the US committing to 26 – 28% reductions by 2025.

What does this actually mean for the climate and for the damage that climate change might bring?

Running the agreements through the PAGE09 integrated assessment model, and throwing in the EU’s pledge to cut emissions by 40% by 2030 for good measure, it appears that these agreements on their own give us less than a 1% chance of keeping the rise in global mean temperatures below the iconic 2 degC level in 2100. Most likely the rise will be about 3.8 degC . This assumes all other regions of the world continue to allow their emissions to grow along the IPCC’s A1B business as usual scenario. Annual mean climate change impacts will still rise to about $20 trillion per year by 2100, with about 2/3 of those impacts in poor countries.

So while the deal may be politically important, statements about its ambition should be treated with caution.

Update added 12:20 on 12 November 2014: Some have commented that assuming the rest of the world continues on the A1B business as usual path is unduly pessimistic. So I have repeated the analysis assuming the rest of the OECD matches the US’s actions of a 28% cut by 2025 (with the EU cutting by 40% as before), and the rest of the developing world matches China’s pledge to stop increasing emissions by 2030. The chance of staying below 2degC in 2100 rises to 1.1%, and the mean impacts in 2100 are now about $19 trillion. The underlying message remains the same: These pledges are only the first step on a very long road.