Vegetation dynamics in simulations of radiatively-forced climate change

1Hadley Centre for Climate Prediction and Research

Vegetation dynamics in simulations of radiatively-forced climate change

Richard A. Betts, Chris D. Jones, Peter M. Cox[chris.d.jones@metoffice.com]

Met OfficeHadley Centre for Climate Prediction and Research

Terrestrial Carbon Sinks Workshop, Wengen, Sept. 2002

2Hadley 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)

3Hadley Centre for Climate Prediction and Research

Hadley Centre Coupled Climate-Carbon Cycle Model

(Biogeophysical)

(Biogeochemical)

4Hadley Centre for Climate Prediction and Research

TRIFFID vegetation model

Competition between 5 plant functional types

– Broadleaf tree, Needleleaf tree, C3 grass, C4 grass, shrub

Carbon balance computed within GCM land surface scheme

Interacts with atmospheric CO2

Vegetation distribution and leaf area determine land surface characteristics in atmosphere model

5Hadley Centre for Climate Prediction and Research

TRIFFID-GCM coupling

Photosynthesis, respiration,

transpiration (30 minutes)

Litter (1 day)

Competition (10 days)

LAI, albedo,

roughness(1 day)

Broadleaf Tree

C3 Grass

Shrub

Soil

6Hadley Centre for Climate Prediction and Research

Coverage of vegetation types, control simulation

Fraction of gridbox

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Vegetation cover: simulated - observed (IGBP-DIS)

Fraction of gridbox

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Surface temperature changes (K)relative to 2000

30-year means

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Precipitation changes relative to 2000

mm day-1

30-year means

2020

2050

2080

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Changes in tree cover

Gridbox fraction

11Hadley 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

12Hadley Centre for Climate Prediction and Research

Broadleaf tree Net Primary Productivity(NPP) in central Africa

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Changes in broadleaf tree cover due to physiological responses to

CO2

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Precipitation difference (mm day-1)due to plant physiological responses to CO2

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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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Precipitation changes (mm day-1) due to biogeophysical feedbacks

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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)

vegetation and soil feedbacks on CO2

physical and biological ocean carbon feedbacks on CO2

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Vegetation & soil carbon changes

GtC

Interactive CO2

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Effects of climate-carbon cycle feedbacks on atmospheric CO2 rise

1000

800

600

400

200

with CO2-climate feedbackswithout CO2-climate feedbacks

1900 1950 2000 2050 2100

CO

2 c

on

cen

trati

on

s (

pp

mv)

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with CO2-climate feedbackswithout CO2-climate feedbacks

1850–2

0

2

4

6

8

1900 1950 2000 2050 2100

Tem

pera

ture

ris

e (

°C)

Effects of climate-carbon cycle feedbacks on land temperature rise

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Further precipitation changes with CO2-climate feedback

(compared to prescribed CO2 dynamic veg simulation)

mm day-1

30-year means

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Further changes in tree cover with CO2-climate feedbacks

Gridbox fraction

23Hadley Centre for Climate Prediction and Research

Broadleaf tree cover (gridbox fraction) in

coupled climate-carbon cycle

simulation

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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

Vegetation carbon sink may not be robust to climate change