Tropical Forests and the Global Carbon Cycle: Quantifying Uncertainties in the Worlds

1. Tropical Forests and the Global Carbon Cycle: Quantifying Uncertainties in the Worlds Most Bio-diverse Ecosystem Simon L. Lewis & Gabriella Lopez-Gonzalez Earth & Biosphere Institute, School of Geography, University of Leeds, UK

2. Tropical Forests and the Global Carbon Cycle: Quantifying Uncertainties in the Worlds Most Bio-diverse Ecosystem • Talk Outline • Overview Tropical Forests & Global Carbon Cycle • Land-use Change Emissions • A Large C sink in Tropical Forests? • Future Scenarios

3. Tropical Forests: why are they important? • Tropical forests cover ~10 % Earth’s land surface • BUT, contain 40-50 % of the globe’s biomass carbon • They are very productive systems, cycling large quantities of carbon with the atmosphere via photosynthesis and respiration • Tropical forests contain 50-70% of Earth’s species. Lewis 2006. Phil Trans. R. Soc. B

4. Therefore, • Relatively small changes across the tropical forest biome may have large effects on the global carbon cycle, and hence the magnitude and rate of climate change. and • Global environmental changesthat alter tropical forests may have large effects on Earth’s biodiversity.

5. Overview: Annual Carbon fluxes, 2000-2005 direct carbon fluxes caused by human activity indirect carbon fluxes caused by human activity net carbon flux from atmosphere to oceans fossil fuel combustion 2.2 ± 0.5 Pg C a-1 7.2 ± 0.3 Pg C a-1 net increase in atmospheric CO2 4.1 ± 0.1 Pg C a-1 land-use change ‘residual’ carbon flux to terrestrial biosphere 0.5 - 3 Pg C a-1 1 - 4 Pg C a-1 IPCC 2007: Annual totals, Pg = 1 x1015g = 1 Giga-tonne = 1 billion metric tonnes

6. Overview: Carbon cycle and the Tropics • Atmospheric CO2 measurements and atmospheric transport models show the tropics, as a whole, to be neutral or a net source of C • Therefore, two options: • Lower deforestation emissions and little/no sink • Higher deforestation emissions and a significant sink Lewis 2006. Phil Trans. R. Soc. B

7. Land-Use Change • FAO 12 million ha yr-1 • Satellite best-estimate, 8.7 million ha yr-1 • With satellite deforestation estimate • 1.1 Pg C yr-1 (Achard et al. 2004) • 1.6 Pg C yr-1 (Houghton 2003*) • 2.1 Pg C yr-1 (Fearnside & Laurance 2004) *scaled to lower deforestation rate

8. Land-Use Change

9. A carbon sink in the tropics? Test with long-term monitoring plots

10. 0.61 ± 0.22 Mg C ha-1 yr-1 0.35 ± 0.35 Mg C ha-1 yr-1 0.57 ± 0.32 Mg C ha-1 yr-1 1.22 ± 0.20 Mg C ha-1 yr-1 Biomass change / Mg C ha-1 yr-1 Distribution of change in total vegetation biomass from 59 plots across South America over late 20th century. Baker et al. 2004. Phil Trans. R. Soc. B (b) (c) (d)

11. Changes within 50 South American plots P<0.001 P<0.05 1980s 1990s Annual rate, % Biomass Growth Biomass Mortality Lewis et al. 2004. Phil Trans. R. Soc. B

12. The most parsimonious explanation is that an increase in plant resource availability (CO2?) is increasing net primary productivity Net C sink Accelerated growth Mortality increasing, but lagging behind the accelerated growth

13. A carbon sink in the tropics? • Plot data suggest 1.2 to 2.1 Pg C yr-1 • Using mean net flux, LUC emissions & outgassing from rivers, from recent studies suggests a sink of 2.6 Pg C yr-1 • ‘Large source/large sink’ option more likely for the contemporary tropics.

14. The Future • Will remaining tropical forest become a C source in the future? • Balance between photosynthesis and respiration • Biodiversity change • Will large-scale tropical forest collapse occur? • The drought route • The fire route Lewis 2006. Phil Trans. R. Soc. B

15. Will remaining forest become a C source? • If higher atmospheric CO2 concentrations are increasing forest growth causing the sink, this cannot continue indefinitely, as other factors will limit growth • Rising temperatures may increase respiration costs and will eventually decrease photosynthesis rates • Most coupled models show reduction in land sink over 21st century and transition to a source, equivalent to 0.1-1.5ºC extra warming (Friedlingstein et al. 2006; Schaphoff et al. 2006). Lewis 2006. Phil Trans. R. Soc. B

16. Lianas in West Amazonia Phillips et al. 2002, Nature

17. Tropical Forest Collapse: the drought route • The Hadley GCM model including dynamic vegetation and a carbon cycle responsive to it, shows a massive die-back of the E. Amazon, due to drought, later this century. • Resulting large flux of carbon further accelerates warming and droughts, a potential dangerous feedback. Cox et al. 2000. Nature

18. Tropical Forest Collapse: the fire route • Tropical forests can burn if they get dry • Rapid carbon losses • As the world warms-up and dries-out more forest can potentially burn. • Potential dangerous feedback: with warmer and more frequent and severe dry periods more forest would likely burn, thus further increasing atmospheric CO2 concentrations, and resulting warmer and periodically drier conditions. Lewis 2006. Phil Trans. R. Soc. B

19. Tropical forests & Climate-Policy • One major new climate-policy threat: • Biofuels • One major new climate-policy opportunity: • Payments for avoided deforestation • Payments for carbon sink services • One ongoing necessity: • interventions to reduce illegal logging

20. Conclusions • Tropical deforestation is a large C source, likely to be ~2 Pg C yr-1. Intact forests are a similarly large C sink. • Biodiversity changes alongside a warming and periodically drying world suggest that it is unlikely that the world’s remaining tropical forest will continue to be a significant carbon sink over the 21st century. • Even remote tropical forests are responding to global environmental changes, affecting Earth’s biodiversity. • Plausible mechanisms exist that show forest-human action-climate feedbacks may greatly accelerate the rate of climate change over the coming decades.