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The data for these figures was assembled from a subset of the
climateprediction.net ensemble of general circulation models. The ensemble is
based upon the Hadley Centre AM3 atmospheric model coupled to a thermodynamic
slab ocean, while the physical parameters representing processes in the model
are perturbed within the current range of uncertainty deemed appropriate by
experts in the respective fields.
The experiment consists of three stages:
Stage 1 - A calibration stage with fixed ocean temperatures to determine what
heat flux is necessary at the ocean surface to maintain a stable atmosphere
with pre-industrial greenhouse forcing.
Stage 2 - A control stage to examine whether the perturbed atmosphere can
remain stable when coupled to a thermodynamic ocean and pre-industrial
greenhouse gases
Stage 3 - A double carbon dioxide run to determine the model response to a
known greenhouse gas forcing
The published figures show the equilibrium response in surface temperature to
current greenhouse forcing. The current forcing (the radiative power per unit
area applied to the climate system, compared to pre-industrial times) was taken
as the most probable total forcing as listed in the IPCC third assessment
report as 2.25 Wm-2 global mean.
The equilibrium response to a double carbon dioxide forcing was inferred by
assuming the temperature would follow a decaying exponential function, and then
interpolating the trend obtained from the fifteen year Stage 3 simulation.
This temperature response was then scaled by the ratio of current greenhouse
forcing (compared to pre-industrial) to the double carbon dioxide forcing in
the experiment (2.25/3.74).
Some runs were discounted from the analysis:
- Those runs with large instabilities in the control simulation were ignored (drift greater than 2K)
- Some runs with apparently negative temperature response were ignored
because the cooling is due to a feedback between the tropical slab ocean and
the overlying cloud cover. This feedback would not exist in a model with a
dynamic ocean, or indeed in the real world where heat transport from elsewhere
in the ocean would compensate for the effect.
Thus 10482 of an original 12638 simulations are used and each remaining member
of the ensemble is allocated a scaled global mean temperature response for
present day greenhouse gas forcing. The distribution of these responses is
shown in the published histogram.
The figures representing global summer temperature response to present day
forcing in a high and low sensitivity case are constructed with the aid of
ERA-40 reanalysis data for the 1960's. The surface of the planet is divided up
into a 96x73 grid, and the mean of June, July and August for the years
1960-1969 is taken from the ERA-40 data for each grid-cell. This map is
assumed to represent approximately pre-industrial temperatures.
Two simulations from the climateprediction.net ensemble were then taken; one
from the 5th percentile of global mean temperature response to greenhouse
forcing, and one from the 95th percentile. An eight year mean taken between
years 7 and 15 of both control and double carbon dioxide is recorded by the
experiment for each grid-cell.
The temperature response in each grid-cell during the third stage is assumed to
follow an exponential decay with the same time constant as that of the global
mean temperature. Thus, a constant scaling factor converts the difference
between the 8 year mean temperatures into an equilibrium temperature response
in each grid-cell. This equilibrium response is again scaled by the ratio of
present day greenhouse forcing (compared to pre-industrial) to the double CO2
forcing.
Hence, the three globes shown are constructed as follows:
- 'Pre-industrial' - a mean of June, July and August temperatures taken for
the years 1960-1969 from the ERA-40 reanalysis dataset.
- 'Low sensitivity' - the above temperature map added to the equilibrium
temperature response evaluated for each grid-cell, and scaled to most probable
forcing due to present day greenhouse gas levels. The model shows a global
mean temperature response of 1.34 degrees to this greenhouse forcing.
- 'High sensitivity' - as a above, but using a model with a global mean
temperature response of 4.29 degrees
Ben Sanderson, Dave Frame, Myles Allen, 1/7/2005
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