Mechanisms of Current
Terrestrial Carbon Sinks and
Future Persistency
Josep Canadell
GCP and GCTE International Office
Canberra, Australia
[Email: pep.canadell@csiro.au]
Outline
• Distribution and strength of terrestrial sinks
• Candidate sink mechanisms
• Where IPCC-2001 left the issue
• US terrestrial sink case study
• Mechanisms: present and future stability
• Land use change legacy
• Fire suppression
• Woody encroachment
• Climate change
• CO2 fertilization
• Nitrogen fertilization
• Reforestation
• Surprises through changes in biodiversity
Terrestrial Carbon Sources and Sinks [1990’s]
Pg C/yr
- 0.8
+ 0.7
- 1.7
+ 0.3
+ 0.7
IPCC 2001
Schimel 2001
Achard et al. 2002
Malhi& Grace 2000
Terrestrial Carbon Sinks [1990’s]
Net Sink
Pg C/yr
- 0.8
- 0.7
- 1.7
- 0.3
- 0.7
Gross Sink
Why do we need to know the mechanisms?
Future atmospheric
CO2 concentrations and
stabilization scenarios
Terrestrial Biosphere C Sink
Cramer et al. 2000
IPCC 2001
Sink Mechanisms – The 90’s understanding
Early 1990’s:
All due to CO2 fertilization (biogeochemistry models/Physiological)
Mid 1990’s:
There was more than CO2. N deposition with unrealistic
uptake rates of up to 80%.
Late 1990’s:
Cropland establishment and abandonment, CO2 and
Climate (IPCC 2001).
Early 2000’s:
All due to past land use practices (US-lead), (using forest
demography and age structure). CO2 no effect.
Candidate Mechanisms of Current Terrestrial Sinks
•
•
•
•
•
•
•
•
•
•
CO2 fertilization
Direct
Nitrogen fertilization
human induced
Climate change
Regrowth in abandoned croplands
Regrowth in previously disturbed forests
– Logging, fire, wind, insects
Fire suppression (woody encroach., forest thickening)
Decreased deforestation
Improved agriculture
Sediment burial
Future: Carbon Management (e.g., reforestation)`
Global Sink Attribution by IPCC 2001 [1920-1992]
*Climate effect is inferred
by S2 - S1
** Land-use effect is inferred
by S3 - S2
CO2/O2 Budgets
Net Biota-to-Air
HRBM
IBIS
LPJ
TEM
1.5
S3 Net Flux (Pg C yr-1)
S1 = CO2
S2 = CO2 + Climate*
S3 = CO2 + Climate +
Cropland Establishment
and Abandonment**
McGuire et al. (2001)
1.0
0.5
0.0
-0.5
-1.0
-1.5
-2.0
1920
1930
1940
1950
1960
1970
1980
1990
2000
In the 1980s
• North extra-tropics: CO2: -0.2 to -1.6; Climate: +0.4 to –0.2; Land use: 0.0 to -0.4
• Tropics: CO2: -0.6 to -1.4; Climate:+ 0.7 to -0.1; Land use: +0.5 to +1.2
• The analyses included only 3 out of 10 sink mechanisms thought to be important.
Sinks in the Coterminous U.S. [1980-90]
Forest trees
0.15
Other forest
0.15
Cropland soils
0.04
Woody encroach.
0.13
Wood products
0.07
Reservoirs
0.04
Exports - Imports
0.09
PgC yr-1
0.71 PgC yr-1 apparent U.S.
US-Fixed expt.rivers 0.04
Pacala et al. 2001
Sinks in the Coterminous U.S. [1980-90]
PgC yr-1
Forest trees
0.15
Other forest
0.15
21 %
Cropland soils
0.04
Woody encroach.
0.13
of the total Sink
due to trees
Wood products
0.07
Reservoirs
0.04
Exports - Imports
0.09
US-Fixed expt.rivers 0.04
Sinks in the Coterminous U.S. [1980-90]
Forest trees
0.15
Other forest
0.15
Cropland soils
0.04
Woody encroach. 0.13
Wood products
0.07
Reservoirs
0.04
Exports - Imports
0.09
US-Fixed expt.rivers 0.04
PgC yr-1
35%
of the sink is susceptible
CO2 and N deposition
fertilization
Sinks in the Coterminous U.S. [1980-90]
Forest trees
0.15
Other forest
0.15
Cropland soils
0.04
Woody encroach.
0.13
Wood products
0.07
Reservoirs
0.04
Exports - Imports 0.09
US-Fixed Ex.Rivers 0.04
PgC yr-1
32%
of total Sink due to other
less commonly
accounted mechanisms
Future Dynamics of Carbon Sink Mechanisms
1. Are the sink mechanisms 2. Will they increase
permanent features?
3.
time
Will they saturate?
time
in strength?
time
4. Will they disappear?
time
Forest Regrowth in Abandoned Croplands
1980’s-1990’s
Eastern United States (5 states)
98% of the C sink attributed to land use change:
2%
•
•
•
Forest regrowth after crop abandonment
Reduced harvesting
98%
Fire suppression
2% remainingForest
attributedtimeto: Forest
Inventory 1
• Increasing CO
2
• Nitrogen Deposition
• Climate Change
Inventory 2
Caspersen et al. 2000
Net Ecosystem Productivity (Mg.ha-1)
Sink Strength due to Forest Regrowth
4
t2
2
t3
0
t4
t1
-2
0
20
40
Years
60
80
100
Jiquan Chen, Univ of Toledo
Climate as a Driver of C Sinks in the U.S.
1950-1993/Biome-BGC
2/3 of forest growth rate
explained by increased
precipitation and extension
of growing season due to
warming
8% increase in precipt.
[1.39 mm yr-1]
No continental T change
[increased in west and decreased
on East]
Decrease annual vapor deficit
Nemani et al. 2002
Carbon Sink: Fire suppression
Fire exclusion has increased C storage in forests [last 100 yrs]
Total Area Burned (US)
Annual Flux of C (TgC yr-1)
Eliminating fire completely,
US forest could accumulated
2.6 Pg C by 2140
Sinks, for how long and at which cost?
Time Bomb
Swetnam et al.
Disturbances in Canada’s forests [1920 – 1995]
Increase after 1970
Area (million ha)
10
8
6
4
2
0
1920
1940
ClearCut
1960
Fire
1980
Insects
2000
Total
Kurz & Apps 1999
Net ecosystem C fluxes in Canada [1920 – 1995]
Decrease after 1970
400
Sink
Tg C / yr
300
200
100
0
-100
-200
1920
Source
1940
Variable Temp
1960
1980
2000
Constant Temp
Kurz & Apps 1999
Woody Encroachment
Woody plant encroachment has promoted C sequestration
in grassland and savanna ecosystems of N and S America,
Australia, Africa, and Southeast Asia over the past century.
CO2insink:
Maximum PotentialEstimated
C sequestration
the absence of fire =
Scholes and Hal 1996
2 Pg C yr-1 (upper value)
Photo: Martin 1975, Arizona 1903 & 1941
USA: 0.17 PgC/yr for the 1980s (Houghton et al., 1999)
NE Australia: 0.03 PgC/yr (Burrows, 1998)
Carbon accumulation due to woody encroachment
• There is a
Maximum limit.
• We may be overEstimating C gain
in wet regions.
Jackson et al. 2002
Goodale and Davidson 2002
Biomass Stimulation (%)
Biomass Responses to Elevated CO2
Increasing aCO2 Effects on Plant Growth
200
400
600
800
1000
200
400
Photo: R. Jackson [Texas, USA]
CO2 concentration (ppm)
600
800
1000
Canadell et al. (in preparation)
Saturation of CO2 Increased Water Use Efficiency
Stomatal acclimation - Solanum (C3 forb)
CO2
H2O
gS (mol m-2 s-1)
3
.
0
2
.
5
600 ppm
2
.
0
1
.
5
1
.
0
0
.
5
0
.
0
0
1
5
0
3
0
0
4
5
0
6
0
0
7
5
0
9
0
0
Intercellular [CO2]
5
5
0
p
p
m
3
5
4
p
p
m
4
7
6
p
p
m
2
9
4
p
p
m
2
5
0
p
p
m
4
1
5
p
p
m
Jackson et al. 2002
Nitrogen Deposition
NPP Responses to N fertilization
Fossil-fuel N Deposition on Land (kg/km2)
Net primary production
(g C m-2 y–1)
10,000
1000
100
10
Schlesinger 1997
1
10-2
10-1
100
101
102
1990
Townsend et al. 1996
103
Nitrogen input
(g N m-2 y-1)
• N deposition explains 100% of current sink
80%
20%
15%
(Holland et al. 1995, 97, Nadelhoffer et al. 1999, McGuire (in preparation)).
• The fertilization effect reaches a saturation.
• N deposition will not stimulate C uptake in the tropics (Hall & Matson 1999)
Annu
-2.00
Reforestation: Annual Flux of Carbon in China
-3.00
-80
-130
[1850-2000]
China
350
Annual flux of carbon (Tg C yr-1)
300
250
200
Degradation
Croplands
Industrial harvest
Fuelwood harvest
Plantations
150
100
50
0
1850
-50
1870
1890
1910
1930
1950
1970
1990
-100
-150
Houghton 2002
Maximum potential of C sink with reforestation
Historically,
450 Pg of C emitted (ff+lucc)
(200 Pg from deforestation)
90 ppm
(40 ppm from deforestation)
Ramakutty & Foley 1999
Nothing-to-eat Scenario:
700 ppm (by 2100) down to 660 ppm
More realistic scenario:
Half of the cropland returns to native
20 ppm
700 ppm (by 2100) down to 680 ppm
Prentice et al. 2001
Future Dynamics of C Sink Mechanisms
1. Are the sink mechanisms 2. Will they increase
permanent features?
3.
time
Will they saturate?
time
in strength?
time
4. Will they disappear?
time
Surprises
Increasing Dominance of Lianas in Amazonian Forest
Lianas have increased 1.7-4.6% yr-1
relative to trees (over last two decades).
Lianas increase mortality and decrease
tree growth.
Tropical sink may decrease sooner than
predicted.
Phillips et al. 2002
Invasive Bromus takes over at elevated CO2
Ambient CO2
4
3
2
Native annuals
Bromus
FACE - Nevada Desert
1
Density
0
Smith et al. 2000
550 ppm
Conclusions (i)
1. Major terrestrial biospheric sinks are in mid-latitudes (net sink)
and in the tropics (gross sink).
2. Legacy of past land use practices is a major driver of the current
Northern hemisphere C sink, and CO2 and N fertilization may
play a much smaller role than previously thought.
3. Management practices and disturbances that affect the age
structure and demography of ecosystems are critical for
understanding current and future C sinks. Both need to be
coupled to biogeochemical and ecophysiological models.
4. The causes of the tropical gross sink are less clear but CO2
fertilization may drive part of the sink. Why CO2 should
increase NEP in the tropics and not in temperate forests?
Conclusions (ii)
5. CO2 fertilization is likely to have a larger effect in the coming
decades but not beyond 600 ppm.
6. Globally, N deposition is responsible for less than 15% of the
current sink, much less than previously thought.
7. Timing of precipitation and temperature will determine the net
effect of climate change on C sinks.
8. Surprises in sink strength may arise in the future via changes
in biodiversity.
Conclusions (iii)
9. There are no permanent sink mechanisms that will ensure
indefinite terrestrial sinks. Many of the current sinks are
likely to decrease or disappear over the next half a century.
End