Select Committee on Economic Affairs Minutes of Evidence

Supplementary memorandum by Dr Chris Hope, Judge Institute of Management, University of Cambridge

  1.  There has been some concern expressed about the accuracy of the emission scenarios from the IPCC Special Report on Emission Scenarios (SRES). All of the results from PAGE that use an unconstrained emission path are based upon scenario A2 from the SRES. This note shows the effect of making an extreme assumption about the inaccuracy of scenario A2, namely that global emissions of all greenhouse gases and sulphates in all future years are only half the values assumed in Scenario A2 from the SRES. The emissions of CO2 that result are shown in the figure below; note that they are below the year 2000 emissions until 2040.

  2.  How does this extreme assumption affect the concentration, radiative forcing, temperature and impacts in future years? The figure below shows the mean values from the PAGE model for these variables by date, expressed as a proportion of the mean values from running PAGE with the A2 scenario from the SRES.

  3.  The emissions are at 50 per cent of the A2 values throughout, as this is the assumption that we are making.

  4.  The concentration of CO2 in the atmosphere is 88 per cent of the A2 value in 2040, and stays above 66 per cent of the A2 value all the way through to 2200. This is because past emissions stay in the atmosphere for many decades, and so it takes a long time for the lower future emissions to have an effect.

  5.  The radiative forcing from the greenhouse gases is 79 per cent of the A2 value in 2040, and never drops below 75 per cent of the A2 value. It is below the CO2 concentration line for most of the next century because the radiative effects of the shorter-lived greenhouse gases such as methane disappear from the atmosphere much more quickly than CO2.

  6.  The global mean temperature is actually higher than the A2 value until 2020, as the short-term influence of the lower sulphates outweighs the longer-term influence of the greenhouse gases. Sulphates cool the atmosphere, so if there are less of them, the global mean temperature will be higher. By 2040 the global mean temperature is 92 per cent of the A2 value, and it never drops below 78 per cent of the A2 value.

  7.  The impacts of climate change are likewise higher than in the A2 scenario until 2020, and are 86 per cent of the A2 value in 2040. By 2200 they have dropped to 59 per cent of the A2 value, as impacts are more than a linear function of temperature.

  8.  The net result of all this can be seen in the mean total impacts aggregated over time and discounted back to the present day, which are shown in the table below. The mean value of $45 trillion is 59 per cent of the A2 mean value of $76 trillion. Of more immediate policy relevance is the social cost of carbon, which is the benefit of reducing today's emissions of carbon by one tonne. As the table shows, the mean value for the social cost of carbon is essentially identical to the A2 value.
Mean values of impacts and social cost of carbon, by scenario A2 from SRESHalf A2
Total climate change impacts$76 trillion $45 trillion
Social cost of carbon$43 per tonne $43 per tonne

  9.  This insensitivity of SCC to the emission path is rather counter-intuitive and is a strong argument for using an integrated assessment model, as neither a scientific nor an economic model would pick it up. I explained it more fully in paragraph 10 of my written evidence. It essentially means that any inaccuracy in the emissions of the SRES scenarios, even as extreme as an overestimation of emissions by a factor of two, is irrelevant for policy decisions that we need to take today.

11 February 2005

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