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Land sources and sinks of atmospheric CO2.
History of land use change.
Distribution of sources and sinks
Why is there a NH mid-latitude sink?
Tropical Sources and sinks
Future of the land sink.
0
10
20
30
40
50
60
70
1000 1200 1400 1600 1800 2000
Year
Are
a (m
illl
ion
sq
km
)
Historical estimated areas of land use
Forest
grassland
Pasture
crops
Courtesy John Grace, U. Edinburgh
“Pioneer” effect
tropical deforestation
Present distribution of Land sources and sinks
Firm conclusions:• A substantial sink in the Northern Hemisphere mid-latitudes.
– Unknown distribution among the continents• The tropical land areas are thought to be nearly neutral. • All sinks are variable from year to year and decade to decade.
-180 -120 -60 0 60 120 180
90
30
-30
-90
Longitude
Latitude -0.5±0.6
0.1±0.6
-0.2±0.1
-0.8±0.2
-0.3±0.2
0.5±0.1
-0.7±0.3
-1.3±0.5
0.1±0.7
-0.3±0.5
0.8±0.4
-0.1±0.3
N. hemisphere
Tropics
S. hemisphere
Variation in the growth rate of atmospheric CO2, 1957-1999
2
4
6
8
Global(NOAA)
Cape Grim(CSIRO)
0
30
Fossil Fuel
Pinatubo
1955 1960 1965 1970 1975 1980 1985 1990 1995 2000
-30
CO2 GROWTHRATE
El Nino
La Nina
Mauna Loa(Scripps/NOAA)
(R J Francey, pers. Com)
•“Natural” sink for atmospheric CO2 is highly variable.
•Affected by climatic oscillations such as El Nino.
Definitions
• Gross Primary Production GPP Carbon fixed by plants
• Autotrophic Respiration AR respiration by plants
• Net Primary Production NPP = GPP-AR net carbon fixed by plants
• Soil Respiration SR carbon lost by soil respiration
• Net Ecosystem Production NEP= NPP-SR net carbon fixed by “undisturbed” system
• Net Biome Production NBP = NEP - nonrespiratory factors (fire, harvest) final balance of carbon – “seen” by the atmosphere
Possible causes of the NH mid-latitiude sink
• Land use Change
• Anthropogenic fertilization, chiefly nitrogen deposition
• CO2 fertilization
Land-Use change
• “REVERSE PIONEER” REGROWTH OF FOREST
– In the last century, large areas of forest near population centres in N. America were cleared for crops.
– With the coming of the railways, the centres of crop production moved to the mid-western prairies. Farmland was abandoned and new-growth forest re-established.
– The process is continuing today.
– Similar, less dramatic trend in Europe and Russia.
• FOREST CONSERVATION:
– Suppression of fire
– Suppression of insect infestation
• INCREASED ORGANIC SEDIMENTATION IN RESERVOIRS?
Land use change and the US carbon budget:
estimates from “carbon accounting”
Houghton RA, Hackler JL, Lawrence KTThe US carbon budget: Contributions from land-use changeSCIENCE 285 (5427): 574-578 JUL 23 1999
Sources of anthropogenic nitrogen
• Agricultural fertilizer
• Animal husbandry:– Runoff from farms
– Ammonia emissions
• NOy emissions from transport, other fossil fuels
Current deposition of atmospheric NOy
(mmol N m-2 yr-1)
Cross-section of trunk of Picea abies from the fertilised and irrigated (IL) treatment at the Flakaliden study site -- Boreal forest, Northern Sweden.
Effect of fertilization on tree growth
Effect of beta-factor
0.8
0.9
1
1.1
1.2
1.3
0.5 1 1.5 2 2.5
C / C0
P/P
0
CO2 Fertilization effect.
CO2 is a limiting factor on growth of plants. Higher CO2 may therefore stimulate net growth. CO2 fertilization is usually quantified by the "beta factor";
)/(1 00 CCLnPP
where is usually in the range 0-0.3 P,P0 are the carbon assimilation rates at CO2 concentrations C,C0
0.3
0.2
0.1
0
Uncertainties about CO2 Fertilization
Easily measurable in many plants in “greenhouse” situations, but it is difficult to extrapolate this to the natural world. Questions include:
• How big is the effect in natural ecosystems?
• How is it modified by other limiting nutrient availabilities?
• Does it result in continuous storage of carbon in plants and soils, or is a new equilibrium state rapidly reached?
Whole tree chambers containing Picea abies at the Flakaliden study site, Sweden. (Experiment to study the effects of elevated CO2 and increased temperature
Free-air CO2 Enrichment (FACE) experiments
• Designed to enrich the CO2 in air over a circle of vegetation, with minimal other disturbance.
• A ring of towers able to release CO2, sensors to detect wind speed and direction and measure CO2 concentration.
• Continuous rapid monitoring of the CO2 concentrations. Control system to decide which towers to release from and adjust release rates to keep concentration constant.
Free-air CO2 Enrichment (FACE) experiments
Duke Forest FACE facility
Free-air CO2 Enrichment (FACE) experiments
Results from the Duke Forest experiment (young loblolly pine stand on nutrient poor soil)
• High CO2 results in increased growth.
• But most increased growth goes into short-lived tissues that decompose rapidly (~3 years) suggesting limited potential for long-term carbon storage.
• Plots additionally treated with fertilizer store carbon for longer.
• These FACE results generally confirm earlier experiments using semi-enclosed facilities.
C3 and C4 Photosynthesis
• "C3" plants and "C4" plants have different photochemical pathways
• C4 plants (maize and many subtropical grasses) are capable of photosynthesis at much lower CO2 concentrations than C3 plants (all other higher land plants except some desert-adapted species).
• C3 plants have a CO2 compensation point ~150ppm
• C4 plants have a compensation point ~< 40ppm.
Sources and sinks in the tropics
• Deforestation is a major source• Atmospheric measurements suggest small net
sink for tropical land surfaces during 1980-89. • Deduce therefore that there is substantial net
production in the un-cleared portion of the tropical forests
Courtesy John Grace, U. Edinburgh
Net Tropical balance ~ - 0.5 GtC yr-1
Courtesy John Grace, U. Edinburgh
Courtesy John Grace, U. Edinburgh
Sink saturation?
• Assume that the sink is mostly due to CO2 fertilization.
• Rising CO2 has an immediate effect on photosynthesis
– Leading to net ecosystem uptake of CO2.
• Rising CO2 has a delayed effect on global temperatures.
• Rising temperatures will enhance respiration in the future
– Leading to net ecosystem release of CO2
• Therefore presently observed uptake of CO2 may be a transitory phenomenon only, and the sink will “saturate”.
• The sink may be even more transitory if it is due in whole or in part to land use change, or nitrogen fertilization.
Courtesy John Grace, U. Edinburgh
Sink saturation?
• FACE experiments suggest uptake of CO2 due to CO2 fertilization is itself transitory.
• But: soil warming experiments suggest that the temperature effect on soil respiration may also be transient.
Carbon cycle:change of carbon in vegetation and soils according to the Hadley Centre coupled carbon-climate model.
Conclusions
• We know 3 or 4 possible reasons for the global vegetation sink, but presently we cannot be sure which of these are most important.
• We cannot be sure how long the sink will continue, and whether it will increase or decrease. Many lines of evidence point to a decrease.
Questions
• Should land sequestration of carbon be considered as a serious option for climate change mitigation, given
– our poor understanding of current land sinks
– their possibly transitory nature
– their vulnerability to climate change
• The precautionary principle: if near-catastrophic outcomes of present practices cannot be ruled out, should we be putting maximum effort into emissions reductions?