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Assessing the efficiency of changes in land use for mitigating climate change

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Land-use changes are critical for climate policy because native vegetation and soils store abundant carbon and their losses from agricultural expansion, together with emissions from agricultural production, contribute about 20 to 25 per cent of greenhouse gas emissions1,2. Most climate strategies require maintaining or increasing land-based carbon³ while meeting food demands, which are expected to grow by more than 50 per cent by 20501,2,4. A finite global land area implies that fulfilling these strategies requires increasing global land-use efficiency of both storing carbon and producing food. Yet measuring the efficiency of land-use changes from the perspective of greenhouse gas emissions is challenging, particularly when land outputs change, for example, from one food to another or from food to carbon storage in forests. Intuitively, if a hectare of land produces maize well and forest poorly, maize should be the more efficient use of land, and vice versa. However, quantifying this difference and the yields at which the balance changes requires a common metric that factors in different outputs, emissions from different agricultural inputs (such as fertilizer) and the different productive potentials of land due to physical factors such as rainfall or soils. Here we propose a carbon benefits index that measures how changes in the output types, output quantities and production processes of a hectare of land contribute to the global capacity to store carbon and to reduce total greenhouse gas emissions. This index does not evaluate biodiversity or other ecosystem values, which must be analysed separately. We apply the index to a range of land-use and consumption choices relevant to climate policy, such as reforesting pastures, biofuel production and diet changes. We find that these choices can have much greater implications for the climate than previously understood because standard methods for evaluating the effects of land use4–11 on greenhouse gas emissions systematically underestimate the opportunity of land to store carbon if it is not used for agriculture.
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LETTER https://doi.org/10.1038/s41586-018-0757-z
Assessing the efficiency of changes in land use for
mitigating climate change
Timothy D. Searchinger1,2*, Stefan Wirsenius3, Tim Beringer4 & Patrice Dumas5,6
Land-use changes are critical for climate policy because native
vegetation and soils store abundant carbon and their losses from
agricultural expansion, together with emissions from agricultural
production, contribute about 20 to 25 per cent of greenhouse
gas emissions1,2. Most climate strategies require maintaining or
increasing land-based carbon3 while meeting food demands, which
are expected to grow by more than 50 per cent by 20501,2,4. A finite
global land area implies that fulfilling these strategies requires
increasing global land-use efficiency of both storing carbon and
producing food. Yet measuring the efficiency of land-use changes
from the perspective of greenhouse gas emissions is challenging,
particularly when land outputs change, for example, from one food
to another or from food to carbon storage in forests. Intuitively,
if a hectare of land produces maize well and forest poorly, maize
should be the more efficientuse of land, and vice versa. However,
quantifying this difference and the yields at which the balance
changes requires a common metric that factors in different outputs,
emissions from different agricultural inputs (such as fertilizer) and
the different productive potentials of land due to physical factors
such as rainfall or soils. Here we propose a carbon benefits index
that measures how changes in the output types, output quantities
and production processes of a hectare of land contribute to the
global capacity to store carbon and to reduce total greenhouse
gas emissions. This index does not evaluate biodiversity or other
ecosystem values, which must be analysed separately. We apply
the index to a range of land-use and consumption choices relevant
to climate policy, such as reforesting pastures, biofuel production
and diet changes. We find that these choices can have much
greater implications for the climate than previously understood
because standard methods for evaluating the effects of land use4–11
on greenhouse gas emissions systematically underestimate the
opportunity of land to store carbon if it is not used for agriculture.
We define a more ‘carbon efficient’ use ofland as one that increases
the capacity of global land to store carbon and reduce greenhouse gas
emissions (GHGs) overall, while meeting the same global food demand.
For example, producing more crops, meat or milk on one hectare of
land increases this carbon efficiency by increasing the global capacity
to spare forests and other habitats while producing the same quantity
of food. Gains in efficiency increase capacity to generate valuable out-
puts but do not by themselves guarantee how the added capacity will
be used—for example, for more carbon or more food—or how other
people might react owing to market forces. Yet because land supply is
fixed, only increasing its efficiency can allow the world to meet both
climate and food goals.
Governments, companies and individuals are making land-use
decisions at least partially directed at reducing GHGs. Questions
include whether to encourage conversion of cropland to forest or bio-
energy, what targets to set for national emissions from land use and
how to reduce the carbon footprint of diets or food supply chains. Yet
standard evaluation methods, as discussed below and in more detail
inSupplementary Information, do not properly reflect the land’s oppor-
tunity to store carbon if it is not used for agriculture, which we call its
carbon storage opportunity cost. They can therefore encourage ineffi-
cient results that reduce the global capacity to store carbon.
For example, typical lifecycle assessments (LCAs), which estimate the
GHG costs of a food’s consumption, only estimate land-use demands
in hectares without translating them into carbon costs4,5. Other LCAs
consider land-use carbon costs only if a food is directly produced by
clearing new land6,7, or only for specific crops, meat or milk, where
both that food and agricultural land overall are expanding
8–10
. Such
approaches assign no land-use carbon costs to most of the world’s food
production because previously converted agricultural lands have no
carbon storage opportunity cost12 (Supplementary Information).
Physical optimization models
13,14
can estimate where agricultural
expansion should occur to minimize carbon costs, by assuming likely
crop yields of every hectare in a study area. Such models can count
carbon storage opportunity costs, but they cannot account for the
variability in carbon storage or crop yields in real hectares or estimate
the effects of changes in their yields, output types or production methods
(Supplementary Information).
Economic models provide a common approach to estimating how
conversion of cropland to biofuels or forest affects carbon stored else-
where, called ‘leakage’ or ‘indirect land-use change’ (ILUC). However,
these models do not calculate the true efficiency of the changes to the
hectare analysed (for example, reforesting cropland) because the mod-
els also factor in how resulting increases in food prices cause changes
on other land, by other people and at others’ expense. Such changes
may include lower GHGs through reductions in global food consump-
tion and, although disputed, through simulated increases in the yields
(efficiencies) of other farmland
15
. Such estimated ‘benefits’, paid for by
global consumers, result from the decline in food production on the
hectare whose use was deliberately changed, not from its gain in forest
or bioenergy, and would therefore occur even if that hectare became
supremely inefficient by producing nothing at all.
To appreciate the distinction, we imagine a possible economic anal-
ysis of a strange climate policy banning all cars except petrol-guzzling,
expensive, luxury SUVs (sport utility vehicles). The efficiency of driv-
ing would decline, increasing emissions per kilometre. However, if the
cost of driving rose high enough, an economic model might estimate
overall GHG savings by forcing many people to stay at home and others
to switch to public transit. Even if these outcomes were real, these
switches would not make SUVs more efficient than economy cars.
The actual efficiency of driving matters because governments can reduce
GHGs more generally by using fuel taxes and transit subsidies to encour-
age less travel and higher use of mass transit while also requiring vehicles
that aremore fuel-efficient. Similarly, if governments wished to use higher
prices to reduce food consumption and spur yield gains, they could reduce
GHGs more using taxes and subsidies while encouraging only efficient
land-use changes (LUCs). To implement such policies, however, govern-
ments need to know which LUCs are more efficient in themselves.
1Woodrow Wilson School of Public and International Affairs, Princeton University, Princeton, NJ, USA. 2World Resources Institute, Washington, DC, USA. 3Department of Space, Earth and
Environment, Chalmers University of Technology, Gothenburg, Sweden. 4Integrative Research Institute on Transformations of Human Environment Systems (IRI THESys), Humboldt-Universität
zu Berlin, Berlin, Germany. 5Centre International de Recherche sur l’Environnement et le Développement (CIRED), Montpelier, France. 6Centre de coopération Internationale en Recherche
Agronomique pour le Développement (CIRAD), Montpelier, France. *e-mail: tsearchi@princeton.edu
Corrected: Publisher Correction
13 DECEMBER 2018 | VOL 564 | NATURE | 249
© 2018 Springer Nature Limited. All rights reserved.
... Perseguir la conservación de los bosques y otros ecosistemas con alto contenido de carbono y la restauración de las tierras de cultivo Adoptar dietas saludables que reducen la huella de carbono de los alimentos La producción de carne de vacuno y productos lácteos son los principales impulsores de las emisiones de GEI dentro de la industria de producción de alimentos. Por ejemplo, mientras que el maíz y el trigo generan menos de 30 kgCO e por kg de proteína, el consumo de carne de vacuno emite 1250 kgCO e, la leche de vaca 260 kgCO e, la carne de cerdo 150 kgCO e y las aves de corral 110 kgCO e (Searchinger et al., 2018). La carne de vacuno y los productos lácteos son los principales contribuyentes a la deforestación mundial, ya que utilizan el 77 % de la tierra cultivable a escala mundial (Searchinger et al., 2018). ...
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