Transcript The past, present and future of carbon on land

The past, present and
future of carbon on land
Bob Scholes
[[email protected]]
CSIR
Div of Water, Environment and Forestry Technology
South Africa
The global carbon budget, 1990-1999
Flux
PgC/y*
Increase in atmosphere
3.2+0.1
Emissions from burning fossil fuels
6.3+0.4
Ocean to atmosphere
-1.7+0.5
Land to atmosphere
-1.4+0.7
From atmospheric measurements. Prentice et al 2001, IPCC TAR Ch 3
* 1 Pg = 1 billion tonnes
The terrestrial carbon sink helps to
control the rise of atmospheric CO2
Currently averages around 3 PgC/y
Varies between years, following climate
Globally distributed
strong in the northern hemisphere
temperate region
Has grown since 1950*
Will saturate; perhaps this century
*model result, measurements confirm for 1980s
Mechanisms for the land C sink:
the proportional contribution by each is unknown
CO2 fertilisation
N fertilisation (from atmospheric deposition)
Regrowth of forest land cleared 18001940
Differing functional response of
photosynthesis and respiration to global
change
Biogeochemical cycles mesh like cogs…
…but this is only a metaphor. There is slippage.
Why do they link?
• ecosystem stoichiometry
• co-factors in shared
processes
The limitations of Liebig’s Law
Adaptation causes organisms in natural
ecosystems to be close to limitation by
several factors simultaneously
Factors interact such that one changes
the availability of others
Limitation can alternate in time, space
or process
Global biogeochemical models will need to be more
sophisticated in how they treat limitation
Human activity has altered all the cycles
Cycle
Carbon
% change*
+13
Nitrogen
Phosphorus
Sulphur
Water
+108
+400
+113
+16
Sediments
+200
Falkowski et al 2001 Science 290, 291-296
*100 x (perturbed-natural)/natural
C,N,P and H2O in terrestrial systems
CO2
N2
Rubisco
Stomata
Leaf
Fire
Allocation
Wood
Biological N Fixation
N2O, N2
Decomposition
Denitrification
P required
Soil water
Soil
Leaching
Why are African savannas nitrogen-poor?
Fires in Africa, May-Oct 1992
Scholes et al JGR 101, 23677
Infertile savannas and grasslands
Van Wilgen & Scholes 1997 In ‘Fires in
African savannas’ ch 3.
Does N deposition increase C storage?
Stoichiometry suggests that the C sink
due to N deposition is 0.6+0.3 Pg/y
(Hudson et al 1994 GBC 8, 307-33)
15N
data suggests that only about half
of the N is incorporated in organic
compounds (Nadelhoffer et al 1999 Nature 398,145-7)
Most N deposition is occurring in areas
approaching N saturation
Land-ocean biogeochemical link
Biological C pump is key to ocean sink
Complex limitation of ocean NPP by N, P, Fe
C sinking to deep ocean controlled by body
size, which is influenced by N, Fe, Si supply
Main sources of P, Fe, Si (and indirectly, N)
are on land
Land source strength is controlled by climate
(wind, drought/floods, vegetation cover)
Fertilisation of the southern Indian
ocean from Africa: Fe, Si, N and S
Piketh, S et al 2001 South African Journal of Science, 96, 244-246.
What message does this signal carry?
Periodicity at 110 000 years
Ceiling at 270 ppm
Floor at 180 ppm
Rapid rise
Slow draw-down
fine control
Petit et al Nature 399, 439-46
An Earth System hypothesis
180 ppm is the ground state.
Fine control by
‘biospheric compensation point’, mainly on land
Orbital forcing triggers ocean reorganisation,
releasing deep sea CO2. Amplified by other
greenhouse gases and retreating ice
250 ppm is a quasi-equilibrium, including
biological storage on land
Slow release of N, P and Fe from land
activates ocean biological pump, leading to
draw-down of atmospheric CO2
Falkowski et al 2001 Science 290, 291-6 (integrating other sources)
Leaf
compensation point
~50 ppm
[CO2] in atmosphere
Whole plant
compensation point
~120 ppm
[CO2] in atmosphere
[Hypothesis]
• respiration
•water use, nutrient supply
•fire
C assimilation
C assimilation
C assimilation
The biospheric carbon compensation point
Biosphere
compensation point
~180 ppm
[CO2] in atmosphere
Implications of the past Earth
System behavior
Return to the pre-industrial CO2 level
and climate will take millennia, and will
require reduction of emissions to some
small number
There is no known system attractor
above 250 ppm
Where have we come from, and
where are we going?
A purely physico-chemical view of the
climate system is no longer defensible
Greater integration of the carbon cycle
with the water, nitrogen, phosphorus
and other cycles is essential
Land-ocean links involving dust and
rivers are an important part of the
ecological metabolism of the earth