On 25 June 2013, President Obama made a major speech on climate change whose over-arching goal was to put the US on track to meet its commitment to cut carbon emissions 17% from 2005 levels by the end of the decade.
It is by no means clear that the plan will succeed in this aim, but if it does, we can reasonably ask how much the world would benefit, from lower temperature rises and less economic and non-economic damage.
Piers Forster (@piersforster) made some initial calculations that a 17% drop in US emissions by 2020 could reduce CO2 concentrations in 2100 by between 3 and 5 ppm, and reduce temperature rises by between 0.02 and 0.04 degC compared to where they would be without the US plan. But Piers’s calculation used just a single value for equilibrium climate sensitivity (ECS), of 3 degC.
The PAGE09 model uses a range of climate sensitivities consistent with the latest IPCC reports. Running Obama’s plan to reduce US emissions by 17% by 2020 through 10,000 runs of the PAGE09 model confirms that with a range of ECS values, the plan will reduce global mean temperatures by between 0.012 and 0.032 degC by 2100 (5-95% values), with a most likely reduction of 0.02 degC, compared to business as usual. The PAGE09 model can also do what Piers could not, and make an estimate of how much this drop in temperatures would reduce global damages from climate change. The answer is it will reduce them by between 0.5 and 10.5 trillion dollars (5-95% values), with a mean reduction in damage of 4.5 trillion dollars.
Details: US CO2 emissions assumed to be 17% below base year (2008) emissions from 2020 until 2075. Comparison with IPCC A1B business as usual scenario.