Mechanisms of Current Terrestrial Carbon Sinks and Future Persistency Josep Canadell GCP and GCTE International Office Canberra, Australia [Email: [email protected]]
Jan 14, 2016
Mechanisms of Current Terrestrial Carbon Sinks and
Future Persistency
Josep CanadellGCP and GCTE International Office
Canberra, Australia[Email: [email protected]]
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
- 0.8 - 1.7
IPCC 2001Schimel 2001
Achard et al. 2002Malhi& Grace 2000
Terrestrial Carbon Sources and Sinks [1990’s]
Pg C/yr
+ 0.7+ 0.3 + 0.7
Terrestrial Carbon Sinks [1990’s]
Pg C/yr
- 0.7- 0.3 - 0.7
- 0.8 - 1.7
Net Sink
Gross Sink
Terrestrial Biosphere C Sink
Cram
er et al. 2000Why do we need to know the mechanisms?
IPCC 2001
Future atmospheric CO2 concentrations andstabilization scenarios
Late 1990’s:Cropland establishment and abandonment, CO2 and Climate (IPCC 2001).
Mid 1990’s:There was more than CO2. N deposition with unrealistic uptake rates of up to 80%.
Early 2000’s:All due to past land use practices (US-lead), (using forest demography and age structure). CO2 no effect.
Early 1990’s:All due to CO2 fertilization (biogeochemistry models/Physiological)
Sink Mechanisms – The 90’s understanding
• CO2 fertilization• Nitrogen fertilization• 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)`
Candidate Mechanisms of Current Terrestrial Sinks
Direct human induced
Global Sink Attribution by IPCC 2001 [1920-1992]
S1 = CO2
S2 = CO2 + Climate*S3 = CO2 + Climate + Cropland Establishment and Abandonment**
*Climate effect is inferred by S2 - S1** Land-use effect is inferred by S3 - S2
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.
1920 1930 1940 1950 1960 1970 1980 1990 2000
S3
Ne
t F
lux
(Pg
C y
r-1
)-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
CO2/O2 Budgets
Net Biota-to-Air
HRBM IBIS LPJ TEM
McGuire et al. (2001)
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
Sinks in the Coterminous U.S. [1980-90] PgC yr-1
0.71 PgC yr-1 apparent U.S.
Pacala et al. 2001
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
21 % of the total Sink
due to trees
Sinks in the Coterminous U.S. [1980-90] PgC yr-1
35%of the sink is susceptible
CO2 and N depositionfertilization
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
Sinks in the Coterminous U.S. [1980-90] PgC yr-1
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
32%of total Sink due to other
less commonly accounted mechanisms
Sinks in the Coterminous U.S. [1980-90] PgC yr-1
1. Are the sink mechanisms permanent features?
time
Sink
Str e
ngt h
4. Will they disappear?
time
Sink
Str e
ngt h
3. Will they saturate?
time
Sink
Str e
ngt h
2. Will they increase in strength?
time
Sink
Str e
ngt h
Future Dynamics of Carbon Sink Mechanisms
Forest Regrowth in Abandoned Croplands
Eastern United States (5 states)
98% of the C sink attributed to land use change:• Forest regrowth after crop abandonment• Reduced harvesting• Fire suppression
2% remaining attributed to:• Increasing CO2
• Nitrogen Deposition• Climate Change
Caspersen et al. 2000
1980’s-1990’s
ForestInventory 1
ForestInventory 2
Gro
wth
Rate
2%
98%
time
Sink Strength due to Forest Regrowth
Jiquan Chen, Univ of Toledo
0 20 40 60 80 100
0
2
4
-2
Net E
cosy
stem
Pro
duct
ivity
(Mg.
ha-1
)
t3
t4t1
t2
Years
Nemani et al. 2002
Climate as a Driver of C Sinks in the U.S.
2/3 of forest growth rateexplained by increased
precipitation and extensionof growing season due to
warming
1950-1993/Biome-BGC
8% increase in precipt.[1.39 mm yr-1]
No continental T change [increased in west and decreased on East]Decrease annual vapor deficit
Fire exclusion has increased C storage in forests [last 100 yrs]
Carbon Sink: Fire suppressionP h
otos
: M. F
lan n
igan
[Can
ada]
Total Area Burned (US)
Houghton et al. 2000
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.
0
2
4
6
8
10
1920 1940 1960 1980 2000
Area
(m
illion
ha)
ClearCut Fire Insects Total
Increase after 1970
Disturbances in Canada’s forests [1920 – 1995]
Kurz & Apps 1999
Variable Temp Constant Temp
-200
-100
0
100
200
300
400
1920 1940 1960 1980 2000
Tg C
/ yr
Source
SinkDecrease after 1970
Net ecosystem C fluxes in Canada [1920 – 1995]
Kurz & Apps 1999
Phot
o: M
artin
197
5, A
rizon
a 19
03 &
194
1
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.
Maximum Potential C sequestration in the absence of fire = 2 Pg C yr-1 (upper value) Scholes and Hal 1996
Estimated CO2 sink:
USA: 0.17 PgC/yr for the 1980s (Houghton et al., 1999)
NE Australia: 0.03 PgC/yr (Burrows, 1998)
Woody Encroachment
Jackson et al. 2002Goodale and Davidson 2002
Carbon accumulation due to woody encroachment
• There is a Maximum limit.
• We may be over-Estimating C gain in wet regions.
Biom
ass
Stim
ulat
ion
(%)
GC
TE Sy nthesis. Mooney et al . 1999
Biomass Responses to Elevated CO2
Gro
wth
Enha
ncem
ent
CO2 concentration (ppm)200 400 600 800 1000200 400 600 800 1000
Phot
o: R
. Jac
kson
[Tex
as, U
SA]
Increasing aCO2 Effects on Plant Growth
Canadell et al. (in preparation)
550 ppm476 ppm415 ppm
354 ppm294 ppm250 ppm
0 150 300 450 600 750 9000.0
0.5
1.0
1.5
2.0
2.5
3.0
g S (
mol
m-2 s
-1)
Jackson et al. 2002
Stomatal acclimation - Solanum (C3 forb)
Intercellular [CO2]
600 ppm
CO2
H2O
Saturation of CO2 Increased Water Use Efficiency
Fossil-fuel N Deposition on Land (kg/km2)
1990 Townsend et al. 1996
Net
prim
ary
prod
uctio
n(g
C m
-2 y
–1)1000
100
10
1
10,000
10310210110010-110-2
Nitrogen input(g N m-2 y-1)
NPP Responses to N fertilization
Schlesinger 1997
Nitrogen Deposition
• 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)
Houghton 2002
Reforestation: Annual Flux of Carbon in China
-150
-100
-50
0
50
100
150
200
250
300
350
1850 1870 1890 1910 1930 1950 1970 1990
An
nu
al f
lux
of
carb
on
(T
g C
yr
-1)
Degradation
Croplands
Industrial harvest
Fuelwood harvest
Plantations
China
[1850-2000]
Historically,450 Pg of C emitted (ff+lucc)(200 Pg from deforestation)
Nothing-to-eat Scenario:700 ppm (by 2100) down to 660 ppm
Maximum potential of C sink with reforestation
Prentice et al. 2001
Ramakutty & Foley 1999
90 ppm(40 ppm from deforestation)
More realistic scenario:Half of the cropland returns to native
20 ppm700 ppm (by 2100) down to 680 ppm
1. Are the sink mechanisms permanent features?
time
Sink
Str e
ngt h
4. Will they disappear?
time
Sink
Str e
ngt h
3. Will they saturate?
time
Sink
Str e
ngt h
2. Will they increase in strength?
time
Sink
Str e
ngt h
Future Dynamics of C Sink Mechanisms
CO 2 fertili
zation
Forest Regrowth
NoneIncreased Precipitation
(depending on timing of warming)
Woody Encroachment
N deposition
Cropland Soils
Surprises
Increasing Dominance of Lianas in Amazonian Forest
Phillips et al. 2002
Phot
o : R
. Hay
s C u
mm
ins
Lianas have increased 1.7-4.6% yr-1 relative to trees (over last two decades).
Tropical sink may decrease sooner than predicted.
Lianas increase mortality and decrease tree growth.
Ambient CO2
550 ppmNativ
e an
nual
sBr
omus
Rela
tive
rati o
(ele
vate
d/am
bien
t CO
2)
1
2
3
4
0
Dens
ity
Seed
rain
Biom
ass
Invasive Bromus takes over at elevated CO2
FACE - Nevada Desert
Smith et al. 2000
1. Major terrestrial biospheric sinks are in mid-latitudes (net sink) and in the tropics (gross sink).
Conclusions (i)
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?
5. CO2 fertilization is likely to have a larger effect in the coming decades but not beyond 600 ppm.
Conclusions (ii)
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.
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.
Conclusions (iii)
End