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?