Vegetation dynamics in simulations
of radiatively-forced climate change
Richard A. Betts, Chris D. Jones, Peter M. Cox
[chris.d.jones@metoffice.com]
Met Office
Hadley Centre for Climate Prediction and Research
Terrestrial Carbon Sinks Workshop, Wengen, Sept. 2002
1
Hadley Centre for Climate Prediction and Research
Simulating global vegetation in the Hadley Centre
coupled climate-carbon cycle model

Compare simulated vegetation with global observational
datasets

In simulations of future global change, investigate
interactions and feedbacks:
– direct effects of CO2 on vegetation
– biogeophysical feedbacks (through water cycle)
– biogeochemical feedbacks (through carbon cycle)
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Hadley Centre for Climate Prediction and Research
Hadley Centre Coupled Climate-Carbon Cycle Model
(Biogeophysical)
(Biogeochemical)
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Hadley Centre for Climate Prediction and Research
TRIFFID vegetation model

Competition between 5 plant functional types
– Broadleaf tree, Needleleaf tree, C3 grass, C4 grass, shrub
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
Carbon balance computed within GCM land surface
scheme

Interacts with atmospheric CO2

Vegetation distribution and leaf area determine land
surface characteristics in atmosphere model
Hadley Centre for Climate Prediction and Research
TRIFFID-GCM coupling
Photosynthesis,
respiration,
transpiration
(30 minutes)
Litter (1 day)
Broadleaf Tree
Shrub
C3 Grass
Soil
Competition (10 days)
5
Hadley Centre for Climate Prediction and Research
LAI,
albedo,
roughness
(1 day)
Coverage of vegetation types, control simulation
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Hadley Centre for Climate Prediction and Research
Fraction of gridbox
Vegetation cover: simulated - observed (IGBP-DIS)
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Hadley Centre for Climate Prediction and Research
Fraction of gridbox
Surface
temperature
changes (K)
relative to 2000
30-year means
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Hadley Centre for Climate Prediction and Research
2020
Precipitation
changes
relative to 2000
mm day-1
30-year means
2050
2080
9
Hadley Centre for Climate Prediction and Research
Changes in
tree cover
Gridbox fraction
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Hadley Centre for Climate Prediction and Research
What is the role of plant physiological responses to
CO2?

TRIFFID includes direct effects of CO2 on vegetation
– CO2 fertilization
– size of stomatal openings

3 simulations, IS92a concentration scenario
– (a) CO2 exerts radiative forcing only
(vegetation given constant present-day CO2)
– (b) CO2 exerts radiative and physiological forcings
(vegetation responds directly to rising CO2)
– (c) Other GHGs included as well as CO2
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Hadley Centre for Climate Prediction and Research
Broadleaf tree Net Primary Productivity(NPP)
in central Africa
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Hadley Centre for Climate Prediction and Research
Changes in
broadleaf tree
cover due to
physiological
responses to
CO2
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Hadley Centre for Climate Prediction and Research
Precipitation difference (mm day-1)
due to plant physiological responses to CO2
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Hadley Centre for Climate Prediction and Research
How do biogeophysical feedbacks affect Amazon
drying?

Changes in land surface characteristics
– albedo
– moisture availability (roots, canopy)
– aerodynamic roughness

2 simulations, IS92a GHG concentration scenario
(prescribed CO2 and other GHGs)
– (a) Vegetation fixed at present-day state
– (b) Dynamic vegetation updates land surface characteristics
– NB. No direct anthropogenic deforestation- “natural” responses only
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Hadley Centre for Climate Prediction and Research
Precipitation changes (mm day-1) due to
biogeophysical feedbacks
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Hadley Centre for Climate Prediction and Research
How do carbon cycle feedbacks affect
Amazon drying and dieback?

Further simulation: fully interactive carbon cycle

IS92a emissions scenario

atmospheric CO2 calculated within GCM
– (other GHGs prescribed)
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
vegetation and soil feedbacks on CO2

physical and biological ocean carbon feedbacks on CO2
Hadley Centre for Climate Prediction and Research
Vegetation &
soil carbon
changes
GtC
Interactive CO2
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Hadley Centre for Climate Prediction and Research
Effects of climate-carbon cycle feedbacks on
atmospheric CO2 rise
CO2 concentrations (ppmv)
1000
800
600
400
200
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with CO2-climate feedbacks
without CO2-climate feedbacks
1900
Hadley Centre for Climate Prediction and Research
1950
2000
2050
2100
Effects of climate-carbon cycle feedbacks on land
temperature rise
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with CO2-climate feedbacks
without CO2-climate feedbacks
Temperature rise (°C)
6
4
2
0
–2
1850
20
1900
Hadley Centre for Climate Prediction and Research
1950
2000
2050
2100
Further
precipitation
changes with
CO2-climate
feedback
(compared to
prescribed CO2
dynamic veg
simulation)
mm day-1
30-year means
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Hadley Centre for Climate Prediction and Research
Further changes
in tree cover
with CO2-climate
feedbacks
Gridbox fraction
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Hadley Centre for Climate Prediction and Research
Broadleaf tree cover
(gridbox fraction) in
coupled climatecarbon cycle
simulation
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Hadley Centre for Climate Prediction and Research
Conclusions

CO2 physiological effects enhance NPP through
fertilization but also exert climatic effect
– relative importance for vegetation varies from place to place

Biogeophysical feedbacks modify local climate change
– enhance Amazon drying

Carbon cycle feedbacks accelerate global climate and
vegetation change
– enhance Amazon drying and dieback

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Vegetation carbon sink may not be robust to climate
change
Hadley Centre for Climate Prediction and Research