The past, present and future of carbon on land Bob Scholes CSIR Div of Water, Environment and Forestry Technology South Africa

The global carbon budget, FluxPgC/y* Increase in atmosphere Emissions from burning fossil fuels Ocean to atmosphere Land to atmosphere 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 CO 2 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 CO 2 fertilisation N fertilisation (from atmospheric deposition) Regrowth of forest land cleared 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% change* Carbon+13 Nitrogen+108 Phosphorus+400 Sulphur+113 Water+16 Sediments+200 Falkowski et al 2001 Science 290, *100 x (perturbed-natural)/natural

C,N,P and H 2 O in terrestrial systems Soil CO 2 N2N2 Fire Biological N Fixation P required Decomposition Rubisco Stomata Leaf Wood Denitrification Leaching N 2 O, N 2 Soil water Allocation

Why are African savannas nitrogen-poor? Fires in Africa, May-Oct 1992 Scholes et al JGR 101, 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 Pg/y (Hudson et al 1994 GBC 8, ) 15 N 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,

What message does this signal carry? Ceiling at 270 ppm Floor at 180 ppm Periodicity at years Slow draw-down fine control Rapid rise Petit et al Nature 399,

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 CO 2. 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 CO 2 Falkowski et al 2001 Science 290, (integrating other sources)

The biospheric carbon compensation point [CO2] in atmosphere C assimilation Leaf compensation point ~50 ppm [CO2] in atmosphere C assimilation [CO2] in atmosphere C assimilation Biosphere compensation point ~180 ppm Whole plant compensation point ~120 ppm Increasing scale alters the compensation Point and the saturation level [Hypothesis] respiration water use, nutrient supply fire

Implications of the past Earth System behavior Return to the pre-industrial CO 2 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