Supplementary memorandum submitted by
the Met Office
During the oral evidence session on 23 June
2009, the Committee asked the Met Office to provide a supplementary
diagram to demonstrate the Committee on Climate Change's (CCC)
cautionary approach to climate sensitivity distribution. Also
included in this note is clarification on the issue of the inclusion
of coupled/non-coupled feedbacks in the models used by the CCC,
and a short summary of what we have done and what we intend to
do with respect to further use of a complex 3-dimensional earth
system model and the benefits of further work.
Following the oral evidence session, Mr Aubrey
Meyer wrote to Dr Jason Lowe at the Met Office requesting clarification
on some of the evidence presented to the Committee. Mr Meyer copied
his questions to the members of the EAC and we have, for completeness,
included a summary of our response at Annex A.
The models used by the Committee on Climate
Change did include the feedback of climate change on the carbon
cycleit was a coupled model as defined in section 10.4.1
of the IPCC AR4 study.
The CCC chose a particular climate sensitivity
estimate when deciding on emissions targets. The climate sensitivity
uncertainty estimate chosen was:
(a) derived from a combination of the most sophisticated
model available combined with a wide range of observations;
(b) a precautionary choice in that it provides
a lower probability of staying below 2C than other alternatives
climate sensitivity uncertainty estimates.
Although the CCC climate change scenarios were
produced using a simple earth system model its performance was
validated against a more complex model.
Since these scenarios have been produced the
simple model has been further evaluated against a more complex
model and proven to skilful.
Estimates of risk of exceeding a given temperature
(eg 2°C), by a certain year, for a given emissions pathway,
are a consequence of uncertainty in the climate projections.
The uncertainty in the simple earth system model
used to generate the global climate projections for the CCC was
expressed in turns of three key parameters. At present, we do
not know the precise value of these parameters, but we do have
information on their ranges:
Climate sensitivity: a measure of how
much the average global temperature will eventually rise if atmospheric
CO2 concentrations were to double.
Ocean diffusivity: a measure of how effectively
heat is mixed between the upper ocean and the deep ocean. This
has a significant impact in the rate of surface warming.
Carbon cycle-climate feedback: a measure
of how much climate change can alter the natural flows of carbon
between the atmosphere, the land and the ocean.
For a given stabilisation concentration of greenhouse
gases in the atmosphere, the climate sensitivity determines the
stabilisation temperaturealthough this temperature may
take several decades or longer to be reached. Climate sensitivity
and ocean diffusivity together determine how long it takes the
temperature to reach stabilisation once the concentration of greenhouse
gases and aerosols has stablised. For a given pathway of future
emissions, all three parameters determine the evolution of the
greenhouse gas concentration over time.
The modeling approach used by CCC can be described
as "coupled" in that it includes the feedback of climate
change onto the carbon cycle.
This is the definition of "coupled"
as used in section 10.4.1 of the IPCC AR4 WG1 and the C4MIP study.
Figure 1 below presents the probability of the
equilibrium warming exceeding 2°C (y-axis) for a range of
stabilisation greenhouse gas concentrations expressed as equivalent
carbon dioxide concentrations (x-axis). Each line on the plot
is for a different estimate of the uncertainty in climate sensitivity.
Eleven different estimates of climate sensitivity are shown, giving
eleven different estimates of the risk of exceeding 2°C.
Using any of the lines on this figure it is possible to quote
the stabilization equivalent CO2 concentration that gives a 50/50
chance of exceeding 2°C. As each climate sensitivity uncertainty
distribution leads to a different result, care must be taken when
interpreting the CCC results in terms of probability.
Figure 1. The probability of eventually exceeding
2°C for a range of different climate sensitivity estimates.
This figure is adapted from Fig 28.5 Meinshausen et al. 2006.
For clarity red squares have been superimposed on the Murphy et
The 50/50 result in the CCC simulations is obtained
using the Murphy et al. climate sensitivity distribution. The
Murphy et al. climate sensitivity uncertainty distribution was
chosen for two reasons. First, it combines our most sophisticated
type of model (complex 3-dimensional models) with a wide range
of observations. Second, for stabilisation around 450ppm, it provides
a lower probability of staying below 2°C than alternative
estimates of climate sensitivity uncertainty, ie it is a precautionary
choice. Above around 430ppm, it is clear that the Murphy et al.
distribution gives the highest chance of exceeding the temperature
target. Although this discussion is based on stabilisation temperature,
a similar argument can be developed for the temperatures in the
CCC scenarios at 2100.
What was done at the time of producing the CCC
A simple climate model (with coupled climate-carbon
cycle feedback) was set up and demonstrated to reproduce the response
of more complex earth system models for scenarios of increasing
greenhouse gas concentration. Having demonstrated model skill,
a range of simulations was made using the simple climate model.
Several alternative sets of emissions scenarios were used and
the results also contain information on uncertainty/risk.
What has been done since?
As part of the Met Office integrated climate
programme (ICP), an idealised simulation with a complex three
dimensional earth system model was carried out. This was idealised
in the sense that emissions reductions were very fast and only
carbon dioxide was treated. The simple climate model used in the
CCC simulations was then compared against this new complex model
simulation. The good agreement showed that the simple model has
skill for scenarios in which emissions are reduced significantly
(as well as that already demonstrated for cases when concentrations
of greenhouse gases continue to rise rapidly). This increased
our confidence in the suitability of the simple modeling approach.
What should still be done?
We recommend that a small number of simulations
be set up of the CCC emission scenarios using the complex three-dimensional
earth system model. This has several purposes. First, it provides
a further check of the simple model for the precise multi-gas
scenarios used in the CCC work. Second, it provides extra information
on climate variability and any sudden surprises, for instance,
changes in the ocean circulation. Third, it provides regional
information, so that it is possible to understand which regions
are warming most rapidly and to examine the local projected changes
in carbon cycle feedback. As a by-product, this approach may produce
information useful to the adaptation sub-committee of the CCC.
At present, we do not recommend repeating the
entire simple model experiment set with complex three-dimensional
models to estimate the risk. Such a project would be comparable
in scale to the recent UKCP09 analysis and is unlikely to provide
significantly better global risk estimates. However, this position
should be reviewed as climate science and/or model understanding
develops, and if the requirement for global adaptation information
RESPONSE TO MR AUBREY MEYER'S QUESTIONS ON
MET OFFICE EVIDENCE TO THE ORAL SESSION OF 23 JUNE 2009
Mr Meyer's questions centered on climate model
results shown in the IPCC AR4 WG1 report. The specific questions
and our responses are reproduced below and we have included some
background information, also supplied to Mr Meyer, to facilitate
a broader contextual understanding.
Question One: "As I pointed out
in the written evidence from GCI that you said that you looked
at, my reading of the figure from IPCC AR4 Chapter 10 is that
with `coupling' introduced, the image in fact shows the extent
of the need to reduce the full-term emissions contraction-event
associated with a given reference curve for concentrations. Can
you confirm that that is your understanding please?".
Met Office Response: The graph taken
from fig 10.21 of the IPCC AR4 WG1 report shows the results of
three models. The Hadley Centre curve shows a simple model set
up to replicate the more complex Hadley Centre model used in C4MIP.
The simple model was then used to study the emissions that lead
to a stabilisation level for CO2 of 450ppm for a single pathway.
For this particular pathway, and only considering CO2, the curve
does show when coupling of climate to the carbon cycle is included,
as it was by the CCC, emission levels would have to reduce further
to achieve a given stabilisation level of CO2 concentrations.
However, given that all the models in C4MIP and fig 10.21 are
considered credible we believe the appropriate scientific approach
is to include information from the full range of available models
not just the results of a single (worst case) model. To that extent
the Hadley SM curve on the graph is not, by itself, a good indication
of the need to reduce emissions targets further than was indicated
in the CCC simulations.
Question Two: "In the example graphic
taken from the IPCC AR4 in what is tagged as the C4 MIP 'Hadley
SM' model with runs for 450 ppmv it shows very clearly that what
in the IPCC image is called:
"uncoupled" for 450 ppm requires a
50% cut in carbon emissions globally by 2050 and "coupled"
for 450 ppmv requires an 80% cut in carbon emissions globally
Can you confirm that that is your understanding
of this image please?"
Met Office Response: As explained above,
fig 10.21 does not show results from C4MIP, rather it shows outputs
from three simpler climate models which also include interactions
between the carbon cycle and climate. Furthermore, using the results
of a single mode for a pathway of a particular shape and only
considering CO2 to make general conclusions about global emissions
reduction targets for a single year, 2050, is not appropriate.
It is also important when discussing percentage emission reductions
by 2050 to state the year to which they are relative. The CCC
expressed their recommendations for UK emissions relative to 1990.
Question Three: "You went on to
say, "The precise values we use to work out the magnitude
of the coupling comes from elsewhere in IPCC and from a study
referred to as a C4MIP study, which to date is the most comprehensive
analysis of that particular type of feedback onto the carbon cycle".
The runs in question and highlighted in the attached graphic from
the IPCC AR4 bear the tag "Hadley SM". Can you as a
member of the UKMO Hadley Centre please explain to me what `elsewhere
in the IPCC' refers to?"
Met Office Response: Chapter 7 of the
AR4 WG1 report summarises the results of the C4MIP project while
table 7.4 presents the range of coupling factors for all 11 of
the models used. C4MIP is mentioned extensively in section 10.4.1
of the IPCC AR4 WG1 report (the section from which you have taken
The C4MIP project, summarised in Chapter 7 of
the IPCC AR4 WG1 report, set out to understand the importance
of coupling the carbon cycle to climate change and its impact
on the evolution of atmospheric concentrations of CO2. Eleven
models that explicitly represented the interaction between climate
and the carbon cycle were used in the project.
Each model was driven by a single emissions
scenarioSRES A2and was run twice, once with climate
coupled to the carbon cycle and once without. Each model simulation
produced an evolving estimate of the total atmospheric concentration
of CO2. By comparing the coupled to the uncoupled simulations,
it was possible to gain an indication of the importance of feedback
between climate and the carbon cycle.
All of the models run in C4MIP demonstrated
that coupling the climate to the carbon cycle is important and
that by 2100 climate change leads to the biosphere being less
able to absorb CO2. A key result from the study was the significant
variation across the models in the size of this effect, demonstrating
significant uncertainty in representing the climate-carbon cycle
feedback. Although C4MIP found the Hadley Centre model showed
the strongest feedback effect, the other ten models are also credible
and their results cannot therefore be ruled out.
This parity between the eleven models meant
it was important, in the work carried out for the Committee on
Climate Change, that the results from all C4MIP models were used
to select the strength of interaction between the climate and
carbon cycle. Several different future emissions scenarios were
then run through a climate model (which has a treatment of the
carbon cycle), in each case estimating uncertainty in temperature
and greenhouse gas concentrations. The full uncertainty range
was due in part to the range of climate-carbon cycle feedbacks
in C4MIP. For each emissions scenario an output from the simulations
was a probability distribution showing how likely different amounts
of 21st century warming will be. The Committee on Climate Change
then selected the emissions scenario that showed a 50% chance
of limiting warming to approximately 2C above pre-industrial levels
at 2100, as well as reducing the risk of a 4C rise to very low
Before the simulations for the Committee on
Climate Change, the Hadley Centre and two other modeling centres
had already carried out studies specifically to evaluate the impact
of climate change on carbon cycle feedbacks, and therefore the
emissions required to reach atmospheric stabilisation at a number
of concentration levels. These are shown in fig 10.21 in the IPCC
AR4 WG1 report. Emissions pathways were based on CO2 only, unlike
the more realistic Committee on Climate Change simulations that
included aerosols and other Kyoto gases. Also relevant is that
the Hadley simple model simulations in fig 10.21 were constrained
so that atmospheric CO2 followed a particular pathway to 450ppm.
In the Committee on Climate Change simulations, the atmospheric
concentrations were not constrained in the same way. Instead,
the emphasis was placed on the pathway of global temperature rise.
It is important to recognise the limitation of the experiments
reported in fig 10.21which were largely to gain an understanding
of the nature of the coupling between climate and carbon cycle
rather than to provide definitive guidance on emissions reduction
The models used by the Committee on Climate
Change did include a coupling between climate and the carbon cycle
and took full account of the `coupled' model research presented
in the AR4 WG1 report, the C4MIP study and related research.
8 Meinshausen, M et al. Multi-gas emissions pathways
to meet climate target in Avoiding Dangerous Climate Chang, ch
28, edited by Schellnhuber, Cramer, Nakicenovic, Wigley &
Murphy, J M et al. Nature 430 768¸772 (2004). Back