Evaluating the likelihood of a climate-changeinduced dieback of the Amazon forest

Yadvinder MalhiOxford University

Luiz Aragao, Rosie Fisher, David Galbraith, Stevel Sitch, Chris Huntingford, Carol McSweeney, Patrick Meir

Oxford Centre forTropical 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 SeasonWater Deficit

Wet SeasonRecharge

P

ET ~ 100 mm/month

Year 1

A simple framework to evaluate climate change in Amazonia

Savanna

SeasonalForest

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

Intensity of Dry Season

Rainforest

Savanna

Seasonal Forest

Malhi et al., Proceedings of the National Academy of Sciences, submitted

The Amazonian biogeography of GCMs1970-2000 to 2070-2100, 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 precipitationJournal of Climate

How will forest transpiration respond to warming?

Carbon dioxideTemperature

Precipitation

Low T Low CO2 Low T High CO2 High T Low CO2 High T High CO20

1

2

3

4

5

6E

vap

otr

ansp

irat

ion

(m

m d

ay-1

)

UK MOSES-TRIFFID ModelLow CO2 = 280 ppm, High CO2 = 850 ppmChange in T = +4.5 oC

Low T Low CO2 Low T High CO2 High T Low CO2 High T High CO20

1

2

3

4

5

6E

vap

otr

ansp

irat

ion

(m

m d

ay-1

)

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), EcologyFisher et al. (2007), Global Change BiologyBrando et al (2008), Philosophical Transactions of the Royal Society BSotta 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 284:1832-1835.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.