Memorandum submitted by the Met Office
INTRODUCTION
1. Climate change is real and getting worse.
The earth is already nearly 0.8C warmer than it was in around
1900. Without large and rapid global emissions reductions it is
very likely that global warming will exceed 2°C over the
coming decades.[4]
2. The present day concentration of the main man-made
greenhouse gas, CO2, is already around 380 ppm with other greenhouse
gases adding an equivalent CO2 of around 70 ppm. Some estimates
suggest that greenhouse gases would have to be stabilised at or
below 500 ppm CO2-eq to give a good chance of limiting eventual
global temperature rises to between 2 and 3°C above pre-industrial
levels. The Met Office Hadley Centre models warn that an even
lower level of 450 ppm would most likely be required. Even if
we can limit global warming to between 2 and 3°C, and local
changes may be considerably larger over most of the globe, there
will be significant changes in the world's climate, some of which
may be irreversible.
3. Climate change is a global issue and the reduction
of greenhouse gases therefore requires a concerted global effort.
UK carbon budget targets must be aligned to global targets.
The frequency with which targets and budgets should
be reviewed and updated to take account of new scientific evidence
4. Currently, carbon budgets are specified in
detail for three periods, 2008-12, 2013-17, and 2018-22, with
additional discussion of the 2050 target. There may be some value
in extending the detailed budgets further into the future based
on existing knowledge. Later, the budgets should all be updated
when new information on climate science or significantly different
models becomes available. A pragmatic approach would be to link
it to the IPCC reporting cycle (approximately five to six years).
Additionally, if new technologies become available or costs of
mitigation technology changes significantly it may be useful to
update the budgets between IPCC reporting periods.
The suitability of the climate models and the
validity of the assumptions used by the Committee on Climate Change
in setting carbon budgets
5. The Met Office Hadley Centre worked with the Committee
on Climate Change to translate global multi-gas emissions scenarios
into temperature projections. This was carried out using a simple
climate model, set up to sample uncertainty in key climate parameters.
The ability of this modelling system to reproduce more complex
climate models was demonstrated during a study using scenarios
with increasing greenhouse gas concentrations. This validation
has since been extended to demonstrate that it also has skill
in reproducing more complex Earth System Models for emissions
scenarios with rapidly declining emissions.
6. The Committee on Climate Change's 2016:4%low scenario
corresponds to a CO2 emission reduction of around 50% on 1990
levels by 2050. Using the Met Office Hadley Centre model, it corresponds
to a median warming of a little over 2°C, with a probability
of around 50% of exceeding 2°C.
7. The Committee on Climate Change required
probabilistic scenarios of temperature projections for a range
of emissions scenarios. Doing this with the most complex three-dimensional
earth system model would be computationally expensive. Variants
of the simple model approach used for the Committee on Climate
Change have been used in mitigation studies in the United States
and the EU, and we believe this approach is suitable here.
8. However, we also recommend that in future
a small number of further simulations be carried out with a complex
three-dimensional Earth System Model. This would provide additional
validation and make available regional scenarios for estimating
the residual impacts after emissions reductions, which would be
useful for adaptation planning.
9. Some of the validation data on the use of
the simple model to give temperature projections, and the details
of the 2016:4%low scenario results, are reported in the technical
annex to the Committee on Climate Change's report. We would be
pleased to supply copies of the key validation plots and to provide
a longer submission or presentation on any aspects of our response.
Choice of climate sensitivity uncertainty distribution
in Met Office models
10. In projecting the response to emissions
reductions there are some important issues to consider, particularly
in the uncertainty in one of the key climate parameters—climate
sensitivity. Climate sensitivity can best be described as a measure
of the eventual amount of global average warming for doubling
of CO2.
11. The Met Office estimated the distribution of
climate sensitivity uncertainty from a study by Murphy et al.
This combined information on complex climate models with a wide
range of climate observations.
12. Alternative distributions of uncertainty
in this parameter do exist and a short set of test simulations
using some of these reveals that the Murphy et al distribution
tends to give a lower probability of staying under a 2°C
global warming target. The implication is that, for the type of
scenarios we consider here, we have taken a precautionary approach
to estimating the risk of exceeding a given peak temperature level
for a given evolution of greenhouse gas concentrations in the
atmosphere.
13. However, we also note that while the probability
of exceeding the 2°C level varies with the choice of distribution,
the spread in the central (median) estimate of warming by 2100
using different climate sensitivity distributions for various
mitigation scenarios (such as the 2016-4%low) tends to be much
less than the difference between a business-as-usual (do nothing)
scenario and the mitigation scenario.
14. We interpret this as there being a significant
degree of robustness in the median warming presented by the Committee
on Climate Change, whereas the estimate of the probability of
staying below 2°C represents a more precautionary approach.
Other recent scenarios
15. Alternative mitigation scenarios have
recently been published by Anderson and Bows, Meinshausen et
al, Allen et al, and Parry et al.
(a) The Anderson and Bows simulations suggests
that more rapid post peak reductions in emissions than that estimated
by the Committee on Climate Change will be required to achieve
similar temperature outcomes. We believe this is an artefact of
their method, and that this method is not as suitable as that
used by the Committee on Climate Change and Met Office Hadley
Centre.
However, we plan to examine the Anderson and
Bows scenarios in the modelling framework we applied to the Committee
on Climate Change scenarios and would be pleased to update the
Committee later.
(b) The Meinshausen et al scenarios appear
to give a median warming of around 1.8°C for a 50% emissions
reduction by 2050, and estimates a lower probability of exceeding
2°C. This appears to be mainly a consequence of the climate
sensitivity treatment—less weight is given to the Murphy
et al distribution (see points 7 to 11).
(c) The Allen et al scenarios treat only
CO2 but appear to be largely consistent with the Committee on
Climate Change warming projections. This work also demonstrates
why it is useful to have some consideration of the target emissions
trajectory even beyond 2050.
(d) The Parry et al scenarios use a similar
approach to the Committee on Climate Change work and give a consistent
result.
Future work
15. AVOID is a DECC/Defra funded research programme
led by the Met Office in a consortium with the Walker Institute,
Tyndall Centre and Grantham Institute. It was set up to provide
UK stakeholders, especially those in Government, with the latest
mitigation advice, tailored to their specific needs. AVOID will
make the latest climate science accessible to decision makers,
building a framework that will encourage integration between climate
scientists, social scientists and economists to inform policy.
16. The Met Office Hadley Centre is currently extending
the range of scenarios used by the Committee on Climate Change,
and later this year we will provide a new estimate of global impacts
and the costs of achieving mitigation policy. Early indications
are that the AVOID scenarios are consistent with the Committee
on Climate Change scenarios. We would be happy to provide further
information from this programme to the Committee.
Further details about the AVOID programme can
be found here Met Office: Avoiding dangerous climate change
The Met Office
17. The Met Office has a world-leading standing:
because of its scientific excellence in both Numerical Weather
Prediction and Climate Research and because—uniquely—both
activities are carried out within one organisation using a single
modelling suite. This combination of scientific expertise and
operational capability means that the Met Office can provide "seamless"
prediction—on timescales from an hour to 100 years.
18. The Met Office supports the UK's high profile
policy role on climate change issues—scientists from the
Met Office's Hadley Centre made a significant contribution to
the Intergovernmental Panel on Climate Change (IPCC) assessment
reports and to the internationally recognised UK Stern review
on the economics of climate change. We are at the forefront of
world leading climate research, funnelling data from diverse natural
sciences into climate prediction models that will produce, for
example, the UKCP09 projections.
21 May 2009
REFERENCESAllen,
M R et al. Nature doi:10.1038/climate. 2009.38 (2009).
Anderson, K and Bows, A Phil Trans R Soc A 366,
3863-3882 (2008).
Committee on Climate Change Technical Annex to
Chapter 1, Building a low-carbon economy—the UK's contribution
to tackling climate change (2008).
Meinshausen, M et al. Nature 458,
1158-1162 (2009).
Murphy, J M et al. Nature 430,
768-772 (2004).
Parry, M et al. Nature 458,
1102-1103 (2009).
4 Intergovernmental Panel on Climate Change Working
Group 1 4th Assessment Report. Back
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