How much will the world benefit from Obama’s climate plan?

posted in: Uncategorized | 6

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.

Or, to put it another way, mean climate change damages reduce from about $400 trillion to about $395 trillion. As other commentators have said, it’s a start.

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.

6 Responses

  1. David Smith

    A reduction of just 0.02 degrees!
    Such a change in temperature is pretty much beyond the limits of what our current technology is capable of measuring.
    So, shutdown industry and restrict access to cheap electricity for next to no difference.

  2. Rich

    His estimation of a 3-5 reduction of CO2 by 2020 is a huge exaggeration by my calculations. The correct figure is about 0.9ppmv. That’s assuming that the US emissions currently stand at about 5.5 gigatonnes/year. Converting gigatonnes of CO2 to ppmv is a simple case of dividing it by 7.68. Furthermore when applying the IPCC’s logarithmic equation relating increments of CO2 to increments of radiatve forcing (RF) and then converting that RF to a temperature increase with the Stefan-Boltzmann law gave me a warming of 0.0003C.

    • cwhope

      Thanks for your comment. As you’ll see in the post, the 3-5 ppm reduction is in 2100, not 2020, from an emission reduction that is sustained from 2020 on. To check that you can’t just do your calculation. You need to have a model of CO2 removal from the atmosphere. It doesn’t seem too unreasonable to me.

      • Rich

        Thanks. Would I be right in assuming that the 17% reduction in US CO2-emissions that you say is “sustained from 2020 on” are progressively curtailed every year thereafter? The reason I ask is because the figure of 3-5ppmv seems very small. US CO2-emissions currently stand at 5.5 gigatonnes/year (See Wikipedia’s ‘List of countries by carbon dioxide emissions’ page). 17% of 5.5 gigatonnes is 0.935. You can convert that straightforwardly into ppmv by dividing it by 7.68 (or you can round it off to 8, since it has no significant knock-on effect to the calculation). Diving it by 7.68 gives us a decrease of 0.12ppmv/ year. Therefore by 2100 (in 83 years) that works out at about 10ppmv. Applying the IPCC’s logarithmic equation results in a ‘radiative forcing’ of about 0.1W/sq.m and converting that to temperature with the IPCC’s feedback-equation produces 0.08C of warming by 2100.

        • cwhope

          You haven’t accounted for the fact that the majority of the CO2 has a finite lifetime in the atmosphere, round about 100 years, so a lot of the emissions would have disappeared in any case by 2100. That is why you get a decrease of 10 ppm, rather than the 3-5 ppm that is given by a model containing the proper atmospheric dynamics.

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