The Interpretation and Design of Ensembles of Complex...
Transcript of The Interpretation and Design of Ensembles of Complex...
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The Interpretation and Design of Ensembles of Complex Models for Climate Change Decision MakingDave Stainforth (Leonard Smith, Dave Frame, Myles Allen, Carl Christensen, Tolu Aina)
MPI PKS Workshop30th July 2009
1. Sources of Uncertainty2. Approaches to Ensemble Interpretation3. Influence on Ensemble Design4. Weighting and Excluding5. Communication
Grantham Research Institute & Centre for the Analysis of Timeseries, London School of Economics and Political Science
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A Comment on Computing
• Opportunities for massive increase in computing power for modelling. But what for:
– Increased resolution 1km globally?– Increased complexity (earth system models).– Increased ensemble size
• What is sufficient for any specific purpose?– Academic understanding.– Informing decision makers.
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Sources of Uncertainty
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Sources of Uncertainty In Climate Forecasts
• External Influence Uncertainty.• Initial Condition Uncertainty
– Microscopic Initial Condition Uncertainty. (Aleatory)– Macroscopic Initial Condition Uncertainty. (Epistemic)
• Model imperfections (Epistemic)– Model Inadequacy. – Model Uncertainty.
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Sources of Uncertainty
• External influences uncertainty:Changes due to factors external to the climate system e.g. greenhouse gas emissions (natural and anthropogenic), solar radiation, volcanic emissions etc.Response: Scenarios for possible futures.
Source: IPCC, Fourth Assessment Report, Summary for Policy Makers
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Sources of Uncertainty and How to Include Them In a Climate Forecast
• Microscopic Initial Condition UncertaintyHow is the prediction is affected by our imprecise knowledge of the current state of the system at small, rapidly mixing, scales?Response: Initial Condition Ensembles
Source: 50 member IC ensemble from climateprediction.net.
Source: IPCC, Third Assessment
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Sources of Uncertainty and How to Include Them In a Climate Forecast
• Macroscopic Initial Condition UncertaintyHow is the prediction is affected by our imprecise knowledge of the current state of the system on relatively large, slowly mixing, scales?
• Response: Better Observations / Directed Observations (informed by model ensembles?)
• Ocean temperature and salinity structure.Sutton and Hodson, Science, 2005
• State of the quasi-biennial oscillation.
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Sources of Uncertainty and How to Include Them In a Climate Forecast
• Model InadequacyAll models are unrealistic representations of many relevant aspects of the real world system.
• Response: A context for all climate forecasts.
• Processes known to be important are absent.e.g. ice sheet dynamics, atmospheric and oceanic chemistry, stratosphere circulation.
• Parameterized processes are unlikely to capture small scale feedbacks.
• Inadequate simulation of some processes which should result from the fundamental processes included.e.g. hurricanes, diurnal cycle of tropical precipitation.
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Sources of Uncertainty and How to Include Them In a Climate Forecast
• Model uncertainty:Climatic processes can be represented in models in different ways e.g. different parameter values, different parameterization schemes, different resolutions. What are the most useful parameter values and model versions to study within the available model class? What is the range of possibilities?Response: Perturbed-Physics Ensembles
Stainforth et al.2006
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Approaches to Model Interpretation
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Available Simulations Which Explore Model Uncertainty
• Climateprediction.net – perturbed physics ensemble• QUMP – perturbed physics ensemble• Multi-model ensembles: CMIP IV / IPCC.
• Differences:– Numbers of simulations.– Interpretational approach.
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The Climateprediction.net Experiment
Stan
dard
m
odel
set
-up
Perturbed Physics Ensemble
Initial Condition Ensemble
Forcing Ensemble
Overall G
rand Ensem
ble
10000s 10s10s
Statistics• > 300,000 participants.• > 24M years simulated.• > 150,000 completed simulations.
(Each 45years of model time)• 10000 years of computing time.
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Climateprediction.net 1st Experimental Design
First Experiment: Unified Model with thermodynamic ocean. (HadSM3) [Full climate resolution atmosphere: HadAM3; 3.75o x 2.5o]
Aim: To identify parameter combinations which have little effect on the mean climate but a large effect on climate sensitivity.Evaluate perturbations to atmosphere/ocean fluxes.
15 yr spin-up 15 yr, base case CO2
15 yr, 2 x CO2
Derived fluxes
Diagnostics from final 8 yrs.
Calibration
Control
Double CO2
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Grand Ensemble Frequency Distribution of Climate Sensitivity
From Stainforth et al. 2005
Interpretational Approaches 1: A range of plausible possibilities
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Interpretational Approaches 2: Deduce (subjective) Probabilities
Change in mean summer precip:10% 90%
Murphy et al, 2004
UKCIP, 2009
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Interpretational Approaches 3: Maximum Consistency / Likelihood
Allen et al. 2008, not yet published. Stainforth et al. 2007
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Interpretational Approaches 4: Transfer Functions
Stainforth et al. 2007
Piani et al. 2005
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Approaches to Model InterpretationIssues and Concerns
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Interpretational Approaches 2: Deduce (subjective) Probabilities
Change in mean summer precip:10% 90%
Murphy et al, 2004
UKCIP, 2009
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UK Climate Projections 2009:Change in Wettest Day in Summer Medium (A1B) scenario
2080s: 90% probability level:very unlikely to be greater than
2080s : 67% probability level:unlikely to be greater than
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A (Very) Basic Summary of My Understanding of the Process
• sample parameters, • run ensemble, • “emulate”, • weight by fit to
observations
Emulate
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Issues
• Size of ensemble given size of parameter space.• The ability of the emulator to capture non-linear effects.• The choice of prior i.e. how to sample parameter space.• The justification for weighting models.• On what scales do we believe the models have information?
Choice of parameter definition
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A Bayesian Method. But is it Subjective
Murphy et al, 2004Ratio added by Myles Allen
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Adopting alternative plausible parameter sampling designs has a big impact on results
Plot: Myles AllenRefs: Frame et al., 2005, Allen et al.,
2005
Who has a prior “belief” that cloud parameterisations can have a parameter for ice fall rate but not for ice residence time?
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Interpretational Approaches 3: Maximum Consistency / Likelihood
Allen et al. 2008, not yet published.
Stainforth et al. 2007
• Informative for global scale variables.• For regional variables – how do we interpret
models which are consistent for some variables in some regions but not for others?
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Interpretational Approaches 4: Transfer Functions
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Okay as far as they go
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Grand Ensemble Frequency Distribution of Climate Sensitivity
From Stainforth et al. 2005
Interpretational Approaches 1: A range of plausible possibilities
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Combined Distributions of Regional, Seasonal Temperature and Precipitation:
Non-discountable Envelopes of Possibilities? Useful?
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weADAPT.org: Climate Change Explorer: Future rainfall - Mali
Max Min
BOUGOUNI.cccmacgcm.futurea.Anomaly BOUGOUNI.echam5.futurea.Anomaly
BOUGOUNI.giss.futurea.Anomaly
0.00
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20.00
30.00
40.00
-10.00
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prec
ip to
tal
Jan 31 Mar 31 May 31 Jul 31 Sep 30 Nov 30
Total monthly precipitation anomaly
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Implications for Model Design
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Exploration of Parameter Space
P1Low HighStnd
Stnd
Low
HighP2• There are hundreds of uncertain parameters in a GCM.
• To study them one at a time is relatively easy.• But they interact non-linearly so we need to explore
multiple perturbations simultaneously.
No. of parameters
One at a time
All combinations
1 3 32 5 93 7 276 13 72921 42 1010
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Exploration of Parameter Space (2)
Is a latin hypercube design of value?
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Remember the size of parameter space
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Options for the aims of Parameter Space Exploration
• Understanding the influence, implications and interactions of parameters.
• Push out the bounds of:– Likelihood / consistency distributions.– Increase the bounds of plausible envelopes.– Find examples which contradict previously deduced transfer functions.– (Preferably do this in terms of variables relevant to decisions or specific
questions of understanding.)
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Weighting and Excluding
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Weighting and Excluding
• How do we judge these large ensembles?• What is sufficiently unphysical, unreasonable, implausible?• Given that all models are wrong, are inconsistent with reality, what’s
the basis for giving them relative weights • Or for excluding models entirely?
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Frequency Distribution of Simulations
15 yr spin-up 15 yr, base case CO2
15 yr, 2 x CO2
Derived fluxesDiagnostics from final
8 yrs.Calibration
Control
Double CO2
From Stainforth et al. 2005
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Frequency Distribution of Simulations and Model Versions
To find potentially realistic model versions we remove those which are unstable in the control.
The remaining negatively drifting 2xCO2 model versions are an unrealistic consequence of using a slab ocean.
From Stainforth et al. 2005
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Rejection of Negative Sensitivities
From Stainforth et al. 2005
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CMIP-2 coupled models
Single perturbationsSingle perturbationsOriginal model
Can observations constrain the distribution?
From Stainforth et al. 2005
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The Climate Prediction Index
Murphy et al, 2004
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Sensitivity Analysis – Does a constraint change the result? Global Mean Heat Flux
• Yet models and model versions are not independent.
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In-Sample .vs. Out of Sample Analysis
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Grand Ensemble Frequency Distribution of Climate Sensitivity
From Stainforth et al. 2005
In-Sample .vs. Out-of-Sample Analysis
• Not only do we not have the possibility to verify a climate forecast for the real world
• we also run risks relating to conclusions about particular models due to:– difficulties in running old
model versions,– difficulties repeating
large ensembles.
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Communicating / Informing
• Communicating what we understand• Researching what is relevant to society.• Developing models and understanding what will be valuable in 10
years time.(Focus on developing a more useful model rather than a better model.)
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The Speed of Light
“Thus, after a long and, at times, hectic history, the value for c has at last settled down into a fairly satisfactory steady state’’R.T. Birge, 1942
Recommended values of the speed of light: 1929-1983From Henrion and Fischhoff, 1985.
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Let’s Be Careful Out There