Terrestrial and Vegetation Processes in ESMs

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Terrestrial and Vegetation Processes in ESMs
Peter Cox University of Exeter Met Office Chair
in Climate System Dynamics Mathematics Research
Institute
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Outline

• Terrestrial Processes and Climate
• The Changing Role of the Land-Surface Scheme
• in GCMs / ESMs.
• Climate-Carbon Cycle Feedbacks.
• New Frontiers in the Modelling of Terrestrial
  Processes.

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Terrestrial Processes and Climate
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Effects of Land Processes on Climate

• Physiological Controls (minutes-days) stomatal
  pores on plant leaves open and close in response
  to environmental factors (light, temperature,
  CO2, soil water etc.), controlling
  surface-atmosphere fluxes of CO2 and water.

Source Mike Morgan (www.micscape.simplenet.com/ma
g/arcticles/stomata.html)
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Effects of Land Processes on Climate

• Biophysical Vegetation Feedbacks
  (years-centuries) Changes in the distribution
  and structural characteristics of vegetation
  (e.g. height, leaf area index), change the
  physical properties of the land surface (e.g.
  albedo, aerodynamic roughness).

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Effects of Land Processes on Climate

• Carbon Cycle (years-centuries) Currently only
  about half of human emissions of CO2 remain in
  the atmosphere - the ocean and land ecosystems
  are absorbing the remainder.

Atmospheric Increase 4.1 /-
0.1 GtC/yr (57) Emissions (fossil fuel,
cement) 7.2 /- 0.3 GtC/yr
(100) Ocean-atmosphere flux
-2.2 /- 0.5 GtC/yr (30) Land-atmosphere
flux -0.9 /- 0.6 GtC/yr
(13)
Land carbon sink takes up about 25 of Global CO2
emissions
Estimated Global Carbon Balance for 2000-2005
(IPCC AR4)
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The Changing Role of the Land-Surface Scheme in
GCMs / ESMs
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The Role of the Land-Surface Scheme in GCMs

• Provides the boundary conditions at the
  land-atmosphere interface.
• Partitions the rainfall into runoff and
  evaporation. Evaporation provides
  surface-atmosphere moisture flux. River runoff
  provides freshwater input to the oceans.
• Partitions the available energy at the surface
  into sensible and latent heat components.
• Updates the surface state variables which affect
  these partitionings, e.g. snowcover, soil
  moisture, soil temperature .

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The Role of Land-Surface Schemes in GCMs

Atmospheric Model
Near Surface Conditions Temperature, Humidity,
Wind speed Surface Radiation Fluxes Downward LW,
SW
Surface Fluxes Sensible Heat Latent Heat
Land-Surface Scheme
Surface Parameters Albedo Roughness
length Rootdepth etc.
State Variables Soil Temperature Soil
Moisture Lying Snow Canopy Water
Prescribed Vegetation
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Status of GCM land-surface schemes in the 10
years ago

• Second generation land-surface schemes were
  making their way into climate and weather models.
• These included multi-layer soil moisture models,
  and plant physiological controls on
  evapotranspiration (through stomata).
• But neglected
• Subgridscale patchiness (heterogeneity) of the
  land-surface
• Vegetation dynamics and carbon cycle feedbacks
• What has happened since ?

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MOSES 2 Tile SchemeSeparate Surface Fluxes and
Surface Layers for each Surface Type9 surface
typesBroadleaf treeNeedleleaf treeC3
grassC4 grassShrubUrbanOpen water Bare
soilLand ice
Improved Treatments of Land-Cover Heterogeneity
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Quantification of uncertainties
inLand-atmosphere feedback
Koster et al., 2004
Models agree on location of hotspots, but
disagree on the importance of lands influence on
rainfall
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The Role of Land-Surface Schemes in GCMs (1997)

Atmospheric Model
Near Surface Conditions Temperature, Humidity,
Wind speed Surface Radiation Fluxes Downward LW,
SW
Surface Fluxes Sensible Heat Latent Heat
Land-Surface Scheme
Surface Parameters Albedo Roughness
length Rootdepth etc.
State Variables Soil Temperature Soil
Moisture Lying Snow Canopy Water
Prescribed Vegetation
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The Role of Land-Surface Schemes in ESMs (2008)

Atmospheric Model
Near Surface Conditions Temperature, Humidity,
Wind speed Surface Radiation Fluxes Downward LW,
SW
Surface Fluxes Sensible Heat Latent Heat
Land-Surface Scheme
Surface Parameters Albedo Roughness
length Rootdepth etc.
Surface Fluxes Carbon Dioxide
State Variables Soil Temperature Soil
Moisture Lying Snow Canopy Water
Vegetation Dynamics Model
Greening of the land-surface
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Climate-Carbon Cycle Feedbacks
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Standard Climate Change Predictions
Online
CLIMATE
Offline
Greenhouse Effect
CO2
CO2 Uptake by Land / CO2-fertilization of plant
growth
CO2 Uptake by Ocean / CO2 buffering effect
OCEAN
LAND
Fossil Fuel Net Land-use CO2 Emissions
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Climate Change Predictions including Carbon
Cycle Feedbacks
Online
CLIMATE
Offline
Greenhouse Effect
Climate Change effects on Solubility of
CO2 Vertical Mixing Circulation
Climate Change effects on plant productivity,
soil respiration
CO2
OCEAN
LAND
Fossil Fuel Net Land-use CO2 Emissions
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Hadley Centre climate-carbon GCM simulation shows
climate change suppressing land carbon uptake..
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Predictions of extra CO2 due to climate effects
on the carbon cycle
All models simulate a positive feedback, but with
very different magnitudes..
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Uncertainties in Carbon Cycle Feedbacks
Terrestrial Processes Dominate Uncertainty
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New Frontiers in the Modelling of Terrestrial
Processes
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New Frontiers in the Modelling of Terrestrial
Processes

• Statistical-dynamical Vegetation Models.

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TRIFFID Competition between Plant Functional
Types and Coupling to Climate Model
Rate of change of Area Fraction of PFT
Competition with other PFTs
Loss of PFT Area due to Disturbance
Carbon Flux available for reproduction of PFT
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Statistical Dynamics approach to large-scale
Vegetation Dynamics
Explicit simulation of rainforest regrowth on
multiple patches
Moment Equations for Statistics of Vegetation
State
Morecroft et al., 2001
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New Frontiers in the Modelling of Terrestrial
Processes

• Statistical-dynamical Vegetation Models.
• Direct Impacts of Changing Atmospheric
  Composition on Ecosystem Functioning
• - CO2 effects on water resources.

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Trends in Global River Runoff and Precipitation
Continental Runoff has Increased (Labatt et al.)
Continental Rainfall has Decreased in recent
decades (CRU data)
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Why might runoff have increased ?

• Climate change.
• Aerosol effects on surface radiation (Solar
  dimming).
• Land-use change.
• CO2-induced stomatal closure.

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Attribution of Trend in Global Runoff to Forcing
Factors
CO2 effect on water use efficiency detected at
the global scale
Gedney et al., Nature, 2006
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New Frontiers in the Modelling of Terrestrial
Processes

• Statistical-dynamical Vegetation Models.
• Direct Impacts of Changing Atmospheric
  Composition on Ecosystem Functioning
• - CO2 effects on water resources.
• - O3 effects on the land carbon sink.

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Effects of Ozone Exposure on Plants

• O3 reduces plant production
• causes cellular damage inside leaves
• reduced photosynthetic rates
• Increased C-allocate to detoxify
• and repair leaves
• O3 reduces stomatal conductance
• lowers internal leaf CO2 reducing rates of
  photosynthesis
• reduces O3 uptake.

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Simulated Ozone Effects on Global Plant
Productivity, 1901-2100
Large reductions in Production
across Temperate (Agricultural) and Tropical
Ecosystems
Sitch et al., Nature, 2007
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Ozone Effects on Land Carbon Sink and Climate
Sitch et al., Nature, 2007
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The value of multiple constraints
Land Carbon Sink
X
CO2 Fertilization
X
N Fertilization
River Runoff
X
O3 Damage
all radiative forcings are not created equal !
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Conclusions

• In the last 10 years the role of Land-Surface
  schemes in climate modelling has expanded
  considerably, as Climate Models begin to evolve
  into Earth System Models.
• Land-surface schemes still provide the lower
  boundary conditions to atmospheric models for
  heat and water fluxes, but they are now also
  expected to provide land-atmosphere CO2 fluxes
  and include treatments of vegetation dynamics.
• First generation coupled climate-carbon cycle
  models show that Terrestrial Processes are
  amongst the greatest uncertainties in 21st
  century climate change.
• New frontiers in Terrestrial Modelling for ESMs
  are being explored including improvements to
  vegetation dynamics, and the treatment of direct
  impacts of atmospheric pollutants on the
  functioning of terrestrial ecosystems.