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Of what use is a statistician in climate modeling?
Peter GuttorpUniversity of Washington
Norwegian Computing [email protected]
http://www.stat.washington.edu/peter
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Acknowledgements
ASA climate consensus workshopKevin TrenberthBen SanterMyles Allen
NCAR IMAGe/GSPPeter CraigmileBarnali DasFinn LindgrenHåvard Rue
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Outline
Difference between weather and climateModeling climateAnalyzing trendsLooking at extremesComparing climate models to weather data
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Weather and climate
Climate is –average weather
WMO 30 years (1961-1990)–marginal distribution of weather
temperaturewindprecipitation
–classification of weather typestate of the climate system
Weather is–current activity in troposphere
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Models of climate and weather
Numerical weather prediction:–Initial state is critical–Don’t care about entire distribution, just most likely event–Need not conserve mass and energy
Climate models:–Independent of initial state–Need to get distribution of weather right–Critical to conserve mass and energy
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The heat engine
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Greenhouse effect
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A simple climate model
What comes in
must go out
Sπr2 (1−a)
4πr2εσT4
Solar constant1367 W/m2
Earth’s albedo0.3
Effective emissivity(greenhouse, clouds)0.64
Stefan’s constant5.67×10-8 W/(K4·m2)
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Solution
Average earth temperature is T=285K (12°C)One degree Celsius change in average earth temperature is obtained by changingsolar constant by 1.4%Earth’s albedo by 3.3%effective emissivity by 1.4%
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But in reality…
The solar constant is not constantThe albedo changes with land use changes, ice melting and cloudinessThe emissivity changes with greenhouse gas changes and cloudinessNeed to model the three-dimensional (at least) atmosphereBut the atmosphere interacts with land surfaces……and with oceans!
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The solar constant
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Sources of uncertainty
ForcingsSea surface temperature is uncertain, especially for early yearsGreenhouse gases vague estimates for early part
DataGlobal mean temperature is not measuredUncertainty in estimates may be as big as 1°C
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Estimating global mean temperature
Global Historical Climatology Network5206 stations with homogenized data of
at least 20 years
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Global covariance
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The MRF approach
κ2 (s) − Δ( )α /2 x(s) = ε(s)
Solve this stochastic differential equation on a triangulation. Covariance is nonstationary Matérn on the manifold.
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Temperature trends
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Is the trend significant?
Many analyses use OLSMore sophisticated use AR(1)Annual averages show evidence of long-term dependence:
Thus standard errors from simple time series model can seriously overstate significance
f(λ) ∝ bλ−2d , λ → 0
d=0.4
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Oxygen isotope in coral cores at Malindi, Kenya
Cole et al. (Science, 2000): 194 yrs of monthly d18O-values in coral core.Decreased oxygen corresponds to increased sea surface temperatureDecadal variability related to monsoon activity
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Multiscale analysis of coral data
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Coral data correlation
d̂ =0.359 (CI[0.143,0.597])
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Malindi trend
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Finding climate change effects in extreme
temperaturesClimate model runs indicate increasing minimum temperatures, decreasing range of temperatures at mid latitudesCan we see this in long temperature series?Stockholm daily 1756-2004Moberg et al. (2002)Clim. Sci. 53: 171-212
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Annual minimum temperature and range
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Comparing climate model output to weather data
Global models are very coarseRegional models are driven by
boundary conditions given by global model runs
In either case, describe distribution of weather, not actual weather
Consider a regional model driven by “actual weather”
Stockholm 50 km x 50 km grid, 3 hr resolution (SMHI-RCA3; ERA40)
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How well does the climate model reproduce data?
Shift function
F(x+Δ(x))=d
G(x)
Δ(x)
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Resolution in a regional climate model
50 x 50 km
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Microclimate
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Model problem?
Cloud water contentDownward longwave radiationMean annual temperature about 1.7°C
higher in model than Stockholm series
Correct comparison is to interpolation to same grid
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Philosophical issues
How well must a model describe current climate in order to produce believable forecasts?What is the probability model that allows ensemble methods?Is there a selection bias in the models that are used in the IPCC assessments?