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 SRES | Half 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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