DJF Warsaw Sept05
Transcript of DJF Warsaw Sept05
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Predicting and Attributing Climate Change
Dave FrameDepartment of Physics, University of Oxford
Oxford University Centre for the Environment
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Defining Climate
Climate is the statistics of the weather Global mean, annual mean surface temperature
East Pacific summer sea-surface temperatures Mean annual Indian Rainfall
Average July humidity in Toru
Return period of Florida hurricanes
Wide range of spatial and time scales involved Climate is what we expect; weather is what we get
Ed Lorenz
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Climate responds due to:
Factors internalto the climate system: Variability in the atmosphere
Variability in the oceans Variability in the biosphere
Factors externalto the climate system: Rising levels of greenhouse gases
Volcanoes Fluctuations in solar output
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Climate as a predictable system
Climate is to weather as the bank is to the roulette
wheel:
The statistics of the system are simpler than the
system itself
Easier to be right in the long run than in the short
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Factors governing predictability
Initial conditions
are the state and trajectory of the climate system at thebeginning of the forecast
Boundary conditions
are the external factors that control the weather we
should expect on average
Necessary for predicting weather
Crucial in predicting climate
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Predictive skill over time
Skill diminishes as natural anomalies in climate
wash out of the system (as the roulette wheel
relaxes back to its statistical norm)
Skill increases over time as the boundary conditions
start to drive the statistical norms (as the roulette
wheel gums up)
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Sources of predictability
Time (yrs)
P r e
d i c
t i
v e
S k i l
l
Initial Conditionpredictability
Boundary ConditionPredictability
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Boundary conditions and
global climate
Climate is determined by the boundary conditions ofthe atmosphere-ocean system:
solar irradiance (power output of the sun)
atmospheric composition (greenhouse gases, volcanic
activity, etc.)
positions of continents, ice-sheets etc.
If these change, climate is likely to change
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Factors in the climate system
Kiehl and Trenberth, 1996
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SUN
Sunlight
passesthrough the
atmosphere..
..and warms the earth.
..most escapes to outer space
and cools the earth...
Infra-red radiationis given off by the earth...
but some IR is
trapped by somegases in the air,
thus reducing the
cooling.
Source: Ellie Highwood
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Energy in the climate system
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Climate varies on geological timescales
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Global Temperature last 1000 yr
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In the light of new evidence and taking into account the remaining uncertainties,
most of the observed warming over the last 50 years is likely to have been dueto the increase in greenhouse gas concentrations
Source: IPCC Third Assessment Report, 2001
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Model hierarchy
Physics constraints operate at all scales:
energy balance
energy transport
geostrophic balance
Moisture availiability
Cloud condensation principles
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Model hierarchy
And we can usefully model the climate system at a
similarly wide range of scales
1. zero-dimensional energy balance models (EBMs);
2. one dimensional radiative-convective models (RCMs);
3. two-dimensional statistical-dynamical models (SDMs);
4. three-dimensional general circulation models (GCMs).
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Energy Balance Models
TF
dt
Tdceff =
Treat the climate system as an energy balance
problem: what goes in must come out
Can write an equation that looks at temperatureresponse to forcing (changes in incoming or
outgoing radiation)
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Energy Balance Models
TF
dt
Tdceff =
Treat the climate system as an energy balance
problem: what goes in must come out
Heat uptake
of the system
Climate
forcing
Temperature
response
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Energy Balance Models - Ensembles
TF
dt
Tdceff =
Ideally, wed take an unbiased sample of all viable
climate models, but we cant do that
Best we can do is take this scatter-gun approach
Repeat with other models
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General Circulation Model of the Atmosphere:
3 Equations of Motion
Equation of State
Energy EquationMass Conservation }
3D wind field
Temperature
PressureDensity
Convection scheme
Cloud scheme
Radiation scheme Sulphur cycle
Precipitation
Land surface and vegetation
Gravity wave drag scheme
The Model also includes:
Each of these equations is evaluated at each point in the model [96
longitudes by 73 latitudes by 19 vertical levels] every half hour
timestep
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Climate modelling (1990)
General Circulation Models (GCMs)
Atmospheric GCMs
Ocean GCMs
Ocean only Model
Atmosphere only Model
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Climate modelling (2000)
Coupled Ocean-Atmosphere GCMs
Ocean Model
Atmosphere Model
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Climate modelling (2005?)
Coupled GCMs with biogeochemical cycles
Ocean Model
Atmospheric Model
CouplerCryosphere Model
Chemistry Model
Biosphere Model
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GCM Performance
Modern Coupled GCMs
Perform well at continental scales Perform well at interannual -> climatological scales Perform less well at short time scales Perform less well at regional scales
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Model simulation of recent climate
Natural forcings only(solar, volcanic etc. variability)
Anthropogenic forcings only(human-induced changes)
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Model simulation of recent climate
Natural + Anthropogenic forcings
Natural forcings
Anthropogenic forcings
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Solar forcing in models
Stott et al, 2001
Combined forcing, doubling solar response
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Increasing greenhouse gases:
Increases the infrared opacity of the atmosphere.
Raises the mean altitude of air radiating to space.
Higher air is colder (by ~6K/km) and so emits less. Net radiation to space is reduced, by ~4W/m2 for a
doubling of CO2.
Climate system adjusts to restore balance.
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Forcing Uncertainties
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Warming rates in different models (Model
Spread)
Different
models yield
differentwarmings
under the same
scenarios
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Net ranges under various scenarios
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Developed Country Per capita Emissions far
Exceed Developing Country Per Capita Emissions
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Ri k f l b l i fi t di 1 5K b
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Risk of global warming first exceeding 1.5K by a
given date
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Global model predictions
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Zonal mean precipitation changes at time of CO2 doubling in CMIP-2
models
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How uncertain are these model predictions?
Models depend on parameterisations of processes too small
to resolve.
Parameterisations represent the feedbacks between smaller
and larger scales.
Many prescribed parameters (e.g. ice fall speed in clouds)
are poorly constrained.
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GCM resolution ~ 2.5 in lat,lon
Explicit representation of larger
scale features;Sub-grid scale processes need
to be parameterized
Arbitrarily small scales affect
arbitrarily large scales in finite
time (Lorenz 1969)
The Met Office
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Uncertainty in climate forecasts
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R i l t t d i it ti
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Standard model version
Low sensitivity model
High sensitivity model
Regional responses: temperature and precipitation
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Uncertainty in climate forecasts
Combining physical uncertainty with economic
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Temperature Change (Degrees C) 2000-2100
0 1 2 3 4 5 6 7
Proba
bilityDensity
0.0
0.1
0.2
0.3
0.4
0.5
Median: 2.3
Lower 95%: 0.9
Upper 95%: 5.3
Combining physical uncertainty with economic
uncertainty: the Integrated Assessment problem
Source: Webster et al, 2001
El t f S t i bl D l t
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Elements of Sustainable Development
Courtesy of The World Bank
S
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Carbon
Trading
JI
More
Renewables
More
GEF
Clean
Technology
Clean
Fuel
Economic
InstrumentsEnvironmental
Standards
Regional
Agreements
Sector
Reform
Energy
EfficiencyRural
Energy
Internalizing
Global Externalities
(supporting the post-
Kyoto process)
Local/Regional
PollutionAbatement
(to be
strengthened)
Win-Win
(in place)
World Bank Strategy
Regional Behaviour European Precipitation
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Mediterranean Basin Northern Europe
Winter
Winter
Summer
Summer
Annual Annual
Unpublished analysis from climateprediction.net: Source: David Stainforth
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Record hot events are more likely in a generally warmer world
Summer 2003 temperatures relative to 2000 2004
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Summer 2003 temperatures relative to 2000-2004
From NASAsMODIS - Moderate
Resolution Imaging
Spectrometer,
courtesy of Reto
Stckli, ETHZ
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Excess mortality rates in early August 2003 indicate 22,000 - 35,000 heat-related deaths
Daily mortality in Baden-Wrttemberg
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Was the hot summer of 2003 due to climate change?
Anthropogenic emissions of greenhouse gases have doubled the risk of a
summer like 2003
By 2050, it could be that hot every other summer
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Standard Visualisation Package
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http://www.climateprediction.net
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Since September 2003,
100,000 participants in 142 countries havecompleted 100,000 45 -year GCM runs
computed 3 million model years
donated 8,000 years of computing time