1. Land sources and sinks of atmospheric CO 2. 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.

2. Historical estimated areas of land use Forest grassland Pasture crops

3. Courtesy John Grace, U. Edinburgh

4. “Pioneer” effect tropical deforestation

5. 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. N. hemisphere Tropics S. hemisphere

6. Variation in the growth rate of atmospheric CO 2, 1957-1999 “Natural” sink for atmospheric CO 2 is highly variable. Affected by climatic oscillations such as El Nino.

7. 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

8. Possible causes of the NH mid-latitiude sink Land use Change Anthropogenic fertilization, chiefly nitrogen deposition CO 2 fertilization

9. 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?

10. Land use change and the US carbon budget: estimates from “carbon accounting” Houghton RA, Hackler JL, Lawrence KT The US carbon budget: Contributions from land-use change SCIENCE 285 (5427): 574-578 JUL 23 1999

11. Sources of anthropogenic nitrogen Agricultural fertilizer Animal husbandry: –Runoff from farms –Ammonia emissions NO y emissions from transport, other fossil fuels

12. Current deposition of atmospheric NO y (mmol N m -2 yr -1 )

13. 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

14. Effect of beta-factor 0.8 0.9 1 1.1 1.2 1.3 0.511.522.5 C / C 0 P/P 0 CO 2 Fertilization effect. CO 2 is a limiting factor on growth of plants. Higher CO 2 may therefore stimulate net growth. CO 2 fertilization is usually quantified by the "beta factor"; where  is usually in the range 0-0.3 P,P 0 are the carbon assimilation rates at CO 2 concentrations C,C 0 0.3 0.2 0.1 0 

15. Uncertainties about CO 2 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?

16. Whole tree chambers containing Picea abies at the Flakaliden study site, Sweden. (Experiment to study the effects of elevated CO2 and increased temperature

17. Free-air CO 2 Enrichment (FACE) experiments Designed to enrich the CO 2 in air over a circle of vegetation, with minimal other disturbance. A ring of towers able to release CO 2, sensors to detect wind speed and direction and measure CO2 concentration. Continuous rapid monitoring of the CO 2 concentrations. Control system to decide which towers to release from and adjust release rates to keep concentration constant.

18. Free-air CO 2 Enrichment (FACE) experiments Duke Forest FACE facility

19. Free-air CO 2 Enrichment (FACE) experiments Results from the Duke Forest experiment (young loblolly pine stand on nutrient poor soil) High CO 2 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.

20. 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 CO 2 concentrations than C3 plants (all other higher land plants except some desert-adapted species). C3 plants have a CO 2 compensation point ~150ppm C4 plants have a compensation point ~< 40ppm.

21. 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

22. Courtesy John Grace, U. Edinburgh

23. Net Tropical balance ~ - 0.5 GtC yr -1 Courtesy John Grace, U. Edinburgh

25. Sink saturation? Assume that the sink is mostly due to CO 2 fertilization. Rising CO 2 has an immediate effect on photosynthesis –Leading to net ecosystem uptake of CO 2. Rising CO 2 has a delayed effect on global temperatures. Rising temperatures will enhance respiration in the future –Leading to net ecosystem release of CO 2 Therefore presently observed uptake of CO 2 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.

26. Courtesy John Grace, U. Edinburgh

27. Sink saturation? FACE experiments suggest uptake of CO 2 due to CO 2 fertilization is itself transitory. But: soil warming experiments suggest that the temperature effect on soil respiration may also be transient.

29. Carbon cycle:change of carbon in vegetation and soils according to the Hadley Centre coupled carbon- climate model.

30. 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.

31. 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?