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Amazon Tipping Point

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Thomas Lovejoy at George Mason University
Thomas Lovejoy
Carlos A. Nobre
Carlos A. Nobre
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Lovejoy and Nobre, Sci. Adv. 2018; 4 : eaat2340 21 February 2018
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Amazon Tipping Point
In the 1970s, Brazilian scientist Eneas Salati shattered
the long held dogma that vegetation is simply the
consequence of climate and has no influence on cli-
mate whatsoever (1). Using isotopic ratios of oxygen
in rainwater samples collected from the Atlantic to
the Peruvian border, he was able to demonstrate un-
equivocally that the Amazon generates approximately
half of its own rainfall by recycling moisture 5 to 6 times
as airmasses move from the Atlantic across the basin to
the west.
From the start, the demonstration of the hydrological
cycle of the Amazon raised the question of how much
deforestation would be required to cause the cycle to de-
grade to the point of being unable to support rain forest
ecosystems.
High levels of evaporation and transpiration that forests
produce throughout the year contribute to a wetter at-
mospheric boundary layer than would be the case with
non-forest.This surface-atmosphere coupling is more im-
portant where large-scale factors for rainfall formation
are weaker, such as in central and eastern Amazonia. Near
the Andes, the impact of at least modest deforestation is
less dramatic because the general ascending motion of
airmasses in this area induces high levels of rainfall in
addition to that expected from local evaporation and
transpiration.
Where might the tipping point be for deforestation-
generated degradation of the hydrological cycle? The
very first model to examine this question (2) showed that
at about 40% deforestation, central, southern and eastern
Amazonia would experience diminished rainfall and a
lengthier dry season, predicting a shift to savanna vegeta-
tion to the east.
Moisture from the Amazon is important to rainfall and
human wellbeing because it contributes to winter rainfall
for parts of the La Plata basin, especially southern Paraguay,
southern Brazil, Uruguay and central-eastern Argentina;
in other regions, the moisture passes over the area, but does
not precipitate out. Although the amount contributing
to rainfall in southeastern Brazil is smaller than in other
areas, even small amounts can be a welcome addition to
urban reservoirs.
The importance of Amazon moisture for Brazilian ag-
riculture south of the Amazon is complex but not trivial.
Perhaps most important is the partial contribution of dry
season Amazon evapotranspiration to rainfall in south-
eastern South America. Forests maintain an evapotranspira-
tion rate year-round, whereas evapotranspiration in pastures
is dramatically lower in the dry season. As a consequence,
models suggest a longer dry season after deforestation.
In recent decades, new forcing factors have impinged
on the hydrological cycle: climate change and widespread
use of fire to eliminate felled trees and clear weedy veg-
etation. Many studies show that in the absence of other
contributing factors, 4 degrees Celsius of global warming
would be the tipping point to degraded savannas in most
of the central, southern, and eastern Amazon. Widespread
use of fire leads to drying of surrounding forest and greater
vulnerability to fire in the subsequent year.
We believe that negative synergies between deforesta-
tion, climate change, and widespread use of fire indicate
a tipping point for the Amazon system to flip to non-
forest ecosystems in eastern, southern and central Amazonia
at 20-25% deforestation.
The severity of the droughts of 2005, 2010 and 2015-16
could well represent the first flickers of this ecological
tipping point. These events, together with the severe floods
of 2009, 2012 (and 2014 over SW Amazonia), suggest that
the whole system is oscillating. For the last two decades
the dry season over the southern and eastern Amazon
has been increasing. Large scale factors such as warmer
sea surface temperatures over the tropical North Atlantic
also seem to be associated with the changes on land.
We believe that the sensible course is not only to strictly
curb further deforestation, but also to build back a margin
of safety against the Amazon tipping point, by reducing
the deforested area to less than 20%, for the commonsense
reason that there is no point in discovering the precise
tipping point by tipping it. At the 2015 Paris Conference
of the Parties, Brazil committed to 12 million ha of re-
forestation by 2030. Much or most of this reforestation
should be in southern and eastern Amazonia. The hydro-
logical cycle of the Amazon is fundamental to human well-
being in Brazil and adjacent South America.
– Thomas E. Lovejoy and Carlos Nobre
REFERENCES
1. E. Salati, A. Dall ‘Ollio, E. Matsui, J. R. Gat, Recycling of Water in the Amazon,
Brazil: an isotopic study. Water Resour. Res. 15, 1250–1258 (1979).
2. G. Sampaio,C. A. Nobre, M. H. Costa, P. Satyamurty, B. S. Soares-Filho,
M. Cardoso, Regional climate change over eastern Amazonia caused by pasture
and soybean cropland expansion. Geophys. Res. Lett. 34, L17709 (2007).
10.1126/sciadv.aat2340
Citation: T. E. Lovejoy, C. Nobre, Amazon Tipping Point. Sci. Adv. 4, eaat2340 (2018).
Thomas E. Lovejoy is
University Professor
in the Department of
Environmental Science
and Policy at George
Mason University. Email:
tlovejoy@unfoundation.org
Carlos Nobre is a Member
of the Brazilian Academy
of Sciences and Senior
Fellow of World Resources
Institute Brazil.
on February 22, 2018http://advances.sciencemag.org/Downloaded from
Amazon Tipping Point
Thomas E. Lovejoy and Carlos Nobre
DOI: 10.1126/sciadv.aat2340
(2), eaat2340.4Sci Adv
ARTICLE TOOLS http://advances.sciencemag.org/content/4/2/eaat2340
REFERENCES http://advances.sciencemag.org/content/4/2/eaat2340#BIBL
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  • Jan 2018
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  • T E Lovejoy
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Citation: T. E. Lovejoy, C. Nobre, Amazon Tipping Point. Sci. Adv. 4, eaat2340 (2018).