Evaluating the likelihood of a climate-change induced dieback of the Amazon forest Yadvinder Malhi Oxford University Luiz Aragao, Rosie Fisher, David Galbraith, Stevel Sitch, Chris Huntingford, Carol McSweeney, Patrick Meir Oxford Centre for Tropical Forests Malhi et al., Proceedings of the National Academy of Sciences, in press (special issue on “Tipping Points in the Earth System”)

IPCC Fourth Assessment Report

Carbon dioxide Temperature Precipitation

Year 2 Dry Season Water Deficit Wet Season Recharge P ET ~ 100 mm/month Year 1 A simple framework to evaluate climate change in Amazonia

Savanna Seasonal Forest Rainforest Malhi et al., Exploring the likelihood and mechanism of a climate-change induced dieback of the Amazon rainforest, Proceedings of the National Academy of Sciences, submitted A simple rainfall biogeography of Amazonia

Rainforest Savanna Seasonal Forest Malhi et al., Proceedings of the National Academy of Sciences, submitted The Amazonian biogeography of GCMs to , A2 emissions scenario

Why do GCMs underestimate Amazonian rainfall? They tend to simulate the wet season rains as migrating further south during the austral summer In practice this migration is blocked by several fine-scale factors (including “ventilation” of dry air over the Andes, and Rossby-wave induced subsidence over Bolivia); this blocking enhances rainfall over Amazonia It is very challenging for GCMs to adequately capture these fine scale processes. Chou & Neelin (2001) Mechanisms limiting the southward extent of the South American summer monsoon. Geophysical Research Letters

Savanna Seasonal Forest Rainforest Malhi et al., Proceedings of the National Academy of Sciences, submitted

Why would Amazonian rainfall become more seasonal? Under warming temperatures the tropical convection circulation tends to intensify, and the rainfall zone tends to narrow. Increases in tropical north Atlantic sea-surface temperatures will tend to suppress dry season rainfall Chou & Neelin (2004) Mechanisms of global warming impacts on regional tropical precipitation Journal of Climate

How will forest transpiration respond to warming? Carbon dioxideTemperature Precipitation

UK MOSES-TRIFFID Model Low CO 2 = 280 ppm, High CO 2 = 850 ppm Change in T = +4.5 o C

Savanna Savanna ?Rainforest Seasonal Forest

Intact forests seem resilient to substantial seasonal drought, but begin to die back after several successive years of drought Nepstad et al (2007), Ecology Fisher et al. (2007), Global Change Biology Brando et al (2008), Philosophical Transactions of the Royal Society B Sotta et al. (2008), Global Change Biology Drought experiments

Tree Mortality: Large canopy trees are most vulnerable to drought Lianas and trees more vulnerable than palms Nepstad et al. (2007) Mortality of large trees and lianas following experimental drought in an Amazon forest. Ecology.

Savanna Seasonal Forest Rainforest Tapajos drought Caxiuana drought

Fire studies Alencar et al. (2006) Forest understory fire in the Brazilian Amazon in ENSO and non-ENSO years: Area burned and committed carbon emissions. Earth Interactions. Cochrane et al. (1999) Positive feedbacks in the fire dynamic of closed canopy tropical forests. Science 284: Ray et al. (2005) Micrometeorological and canopy controls of fire susceptibility in a forested Amazon landscape. Ecological Applications. Barlow & Peres (2008) Fire-mediated dieback and compositional cascade in an Amazonian forest. Philos Trans R Soc London.

No burn First burn Second or third burn Barlow J & Peres CA (2008) Fire-mediated dieback and compositional cascade in an Amazonian forest. Philos Trans R Soc London

Savanna Seasonal Forest Rainforest Fire-prone forests Tapajos drought Caxiuana drought

Forest fires in eastern Acre State (Brazil) during the 2005 Amazonian drought Aragão, et al, patterns and fire response of recent Amazonian droughts, Geophysical Research Letters. (2007)

Summary Eastern Amazonia is very likely to become more seasonal over this century. There is a significant probability that rainfall may decline sufficiently to no longer support forest, especially of warming temperatures cause transpiration to increase. Fire dynamics will dominate the transition between intact forests and drier, fire adapted biomes. Conversely, maintaining forest area and managing fires may increase the resilience of Amazonia to potential change.