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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)
2
Hadley Centre for Climate Prediction and Research
Hadley Centre Coupled Climate-Carbon Cycle Model
(Biogeophysical)
(Biogeochemical)
3
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
4

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
8
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
10
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
12
Hadley Centre for Climate Prediction and Research
Changes in
broadleaf tree
cover due to
physiological
responses to
CO2
13
Hadley Centre for Climate Prediction and Research
Precipitation difference (mm day-1)
due to plant physiological responses to CO2
14
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
8
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
21
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
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