A role for tropical forests in stabilizing atmospheric CO2

Article (PDF Available)inNature Climate Change 5(12):1022-1023 · November 2015with 1,603 Reads
DOI: 10.1038/nclimate2869
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Abstract
Tropical forests could offset much of the carbon released from the declining use of fossil fuels, helping to stabilize and then reduce atmospheric CO2 concentrations, thereby providing a bridge to a low-fossil-fuel future.
1022 NATURE CLIMATE CHANGE | VOL 5 | DECEMBER 2015 | www.nature.com/natureclimatechange
opinion & comment
COMMENTARY:
A role for tropical forests in
stabilizing atmospheric CO2
R. A. Houghton, Brett Byers and Alexander A. Nassikas
Tropical forests could oset much of the carbon released from the declining use of fossil fuels,
helping to stabilize and then reduce atmospheric CO2 concentrations, thereby providing a bridge to a
low-fossil-fuel future.
A
35-year transition from fossil to
renewable fuels may be possible1,
but fossil fuel emissions continue to
rise. e world’s largest emitter, China, is
not committing to peak carbon emissions
until 2030, and among developed countries,
very few are aiming to end fossil fuel use
by 2050. us, it is unlikely that fossil fuel
emissions will fall in the next decade, or that
they will fall by more than 80% by 2050.
More likely, emissions from fossil fuels
from 2015 to 2050 will exceed 250PgC,
resulting in cumulative carbon emissions of
over 400PgC between 2000 and 2050, and
a greater-than-50% chance of exceeding a
global warming of 2°C (ref.2). So are we
already committed to a warming of 2°C
or greater? Not necessarily — absorption
of carbon by tropical forests could oset
much of the release of fossil fuel carbon
between now and 2050, thus stabilizing and
then reducing the CO2 concentration in the
atmosphere within just a few decades, and
providing a bridge to a fossil-fuel-free world.
Absorption of carbon by tropical forest
management is not the solution to climate
change. It may be part of the solution3, but
the potential for accumulating carbon in
the world’s forests and soils is small relative
to the amount of carbon in coal, oil and
gas reserves4. For example, the total loss
of carbon from land as a result of human
activity over the past centuries has been
200–300PgC (ref.5), and even if all of
this loss were to be recovered through
reforestation, the uptake of carbon would
be far from sucient to oset unabated
long-term use of fossil fuels. Further, the
current net emissions of carbon from
tropical deforestation and degradation
account for as little as 8–15% of total annual
carbon emissions, and that percentage has
declined as fossil fuel use has continued
to increase6,7,8.
e conclusion that forest conservation
and restoration is largely irrelevant for
mitigation is incorrect, however, for
two reasons. First, the relatively small
net emissions of carbon from forest
management hide a greater potential for
carbon storage. Gross emissions from
forest management are two to three times
greater than net emissions9, suggesting that
enhancing carbon uptake and reducing
emissions could account for as much as 50%
of total carbon emissions. Second, changes
in land management can be implemented
more quickly than the transition from fossil
to renewable fuels.
To achieve a 75% likelihood of avoiding
warming in excess of 2°C through
changes in fossil fuel emissions alone, such
emissions would have to be eliminated over
the next 20years or less (Fig.1). In contrast,
the same likelihood could be achieved if,
rst, tropical forest management removed
Carbon emissions (Pg C yr−1)
Year
15
10
5
0
−5
1850 1900 1950 2000 2050 2100 2150
Figure 1 | The potential role for tropical forest management in stabilizing atmospheric CO2. Annual
emissions of carbon from fossil fuels (solid grey line) and tropical forest management (solid green line),
and total carbon emissions (solid black line), are plotted for 1850–2015. From 2015, the total emissions
that are required for a 75% likelihood of avoiding warming in excess of 2°C (dashed black line) are
shown. The dashed grey line represents the fossil fuel emissions if this likelihood is achieved by fossil
fuel changes alone. The orange line represents the emissions from fossil fuels if simultaneous changes
in forest management are implemented (dashed green line). The hatched area represents the eect of
tropical forest management on carbon emissions mitigation. Negative emissions represent the removal
of carbon from the atmosphere.
© 2015 Macmillan Publishers Limited. All rights reserved
NATURE CLIMATE CHANGE | VOL 5 | DECEMBER 2015 | www.nature.com/natureclimatechange 1023
opinion & comment
5PgCyr−1 from the atmosphere, phased in
linearly over the next 10years, and, second,
fossil fuel emissions were held constant for
the next 10years and then reduced linearly
to a level equal to 20% of 2014 emissions by
2050 before further linear reduction to zero
by 2100. In this latter case, the cumulative
reduction in net emissions over the next
50years from fossil fuel use versus from
forest management would be roughly equal.
is estimate for the potential of forest
management to remove carbon from the
atmosphere is based on the gross sources
and sinks of carbon associated with three
types of forest management in the tropics7,10.
First, deforestation and degradation of
forests in tropical regions currently release
about 1PgCyr−1 (and by some estimates,
twice that amount). If deforestation
and degradation were stopped, those
emissions would cease. Second, carbon
is accumulating in secondary forests
recovering from harvests and from swidden
agriculture at rates as high as 3PgCyr−1.
is gross rate of uptake is taking place
now. If the associated gross emissions
from harvests and re-clearing of fallows
were stopped, the accumulation of up
to 3PgCyr−1 in growing forests would
continue for decades before declining as
the forests mature. A third activity, more
challenging than the rst two because
of higher costs per hectare, would be
the re-establishment of forests on lands
previously forested but not currently
used productively. Estimates of the areas
available for reforestation vary11,12, but re-
forestation of 500million hectares could
sequester at least 1PgCyr−1 for decades.
Together, the three activities would reduce
total emissions by as much as 5PgCyr−1
(a reduced source of 1PgCyr−1 and
increased sinks of 4PgCyr−1). e rate of
carbon accumulation in forests diminishes
as forests mature, however, and thus the
4PgCyr−1 sink would last, conservatively
in our analysis, for approximately 50years
(to 2065) before declining linearly to
zero by 2095 (Fig.1). As the large trees
containing most of the above-ground
forest carbon tend to be absent in degraded
forest (as they are sought aer in selective
logging), and large trees can take well over
a century to mature fully, the absorption is
likely to continue at a high level well beyond
a 50-year period13. Even if the re-forestation
of 500million hectares was not undertaken,
less-conservative assumptions on how
long the remaining 3PgCyr−1 sink would
continue yield similar results.
We recognize that implementation of
these strategies has political and economic
challenges, not least as current management
of tropical forests is responsible for net
emissions of carbon, not a net uptake.
We focus on tropical forest management
because they are currently the dominant
net source of terrestrial carbon to the
atmosphere and hold large stores of carbon
in their biomass. Indeed, estimates of the
sum of above- and below-ground carbon
(that is, peat) within tropical forests are
greater than 500PgC (ref.10), about half
the estimates of the carbon within fossil
fuel reserves2,14. Managing tropical forests
to help stabilize and then reduce CO2
concentrations could be implemented
more quickly than the phasing out of fossil
fuels, not needing as much development
(technical, economic, marketing,
infrastructure, and manufacturing and
installation capacity) as the expansion
of renewables to scale. Furthermore,
many tropical forests are already growing
(accumulating carbon) and will continue
for some decades as long as they are not
selectively logged, harvested, burned, or
cleared for agriculture or other uses. e
biophysical eects of tropical forests (for
example, albedo and evapotranspiration)
do not oset the biogeochemical eects
(carbon sequestration), as they may in
boreal forests15. And, nally, the recent
United Nations Framework Convention
on Climate Change (UNFCCC) eorts
to proceed with REDD+ (‘reduce
emissions from deforestation and forest
degradation in developing countries’;
http://redd.unfccc.int/) demonstrates
political willingness to manage forests for
climate change mitigation16.
Our focus on tropical forests does not
preclude management of forests and other
ecosystems outside the tropics for additional
removal of carbon from the atmosphere.
Eorts in temperate zone forests and in
agricultural lands, grasslands and wetlands
could increase the amount of carbon
withdrawn from the atmosphere each year,
and the huge stores of carbon in arctic and
boreal systems highlight the need to keep
permafrost from thawing17. In this context,
it is worth noting that, in addition to the
sources and sinks of carbon attributable
to the land management suggested above,
other natural processes on land remove
approximately 25% of the carbon emitted
each year6. e calculations here are based
on the assumption that these natural sinks
on land will continue.
Above-ground carbon in forests
represents a vulnerable pool of carbon,
subject to droughts, res, insects and other
disturbances. us, the management of
forests to accumulate carbon must not delay
or dilute the phasing-out fossil fuel use. On
the contrary, the deliberate accumulation of
carbon on land may be of little long-term
benet if climate change proceeds because
of unrestrained use of fossil fuels, and if
forests, as a consequence, return to being
sources of carbon on a warming and
drying Earth18.
Despite reference here to forest
management, we note that the potential for
increasing carbon sinks in tropical forests
is also related to changing traditional
agricultural practices, including eliminating
swidden. ere is potential in modifying
agricultural methods to enhance above- and
below-ground storage of carbon within
agricultural lands.
Used strategically, the removal of
carbon from the atmosphere by tropical
forest conservation and restoration could
help stabilize and then reduce the CO2
concentration in the atmosphere during the
decades needed for an orderly transition
away from fossil fuels, and could play
a role equal to that of timely fossil fuel
elimination in avoiding dangerous climate
change. e Bonn Challenge to re-forest
150million hectares of degraded lands and
the goal of the New York Declaration on
Forests to eliminate deforestation by 2030
are good rst steps. But the timing and
extent of action is critical. Not only should
the restoration of the biosphere happen in
concert with the phasing out of fossil fuels,
but the longer we wait, and the higher the
rate of fossil fuel emissions, the smaller the
potential role of tropical forests in osetting
those emissions.
R.A.Houghton* and Alexander A.Nassikas are at the
Woods Hole Research Center, 149 Woods Hole Road,
Falmouth, Massachusetts 02540-1644, USA.
Brett Byers is at Million Acre Pledge, 310 Hillside
Avenue, Piedmont, California 94611, USA, and
Rainforest Trust, 7078 Airlie Road, Warrenton,
Virginia 20187, USA.
*e-mail: rhoughton@whrc.org
References
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3. Pacala, S. & Socolow, R. Science 305, 968–972 (2004).
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© 2015 Macmillan Publishers Limited. All rights reserved
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    Large quantities of organic carbon are stored in frozen soils (permafrost) within Arctic and sub-Arctic regions. A warming climate can induce environmental changes that accelerate the microbial breakdown of organic carbon and the release of the greenhouse gases carbon dioxide and methane. This feedback can accelerate climate change, but the magnitude and timing of greenhouse gas emission from these regions and their impact on climate change remain uncertain. Here we find that current evidence suggests a gradual and prolonged release of greenhouse gas emissions in a warming climate and present a research strategy with which to target poorly understood aspects of permafrost carbon dynamics.
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    Land use in the tropics, including both deforestation and forest degradation, is estimated to have emitted approximately 1.4 PgC yr-1 to the atmosphere over the interval 1990-2010 (∼15% of anthropogenic carbon emissions). This net emission is composed of gross emissions of at least 2.6 PgC yr-1 and gross sinks of 1.2 PgC yr-1 in forests recovering from wood harvest and in the fallows of shifting cultivation. In contrast to recent management of tropical forests, future management in the region could be used to stabilize the concentration of CO2 in the atmosphere, at least temporarily, with the following three measures: a halt to deforestation and forest degradation, protection of regrowing forests, and the re-establishment of forests on lands not intensively used now that were forests in the past. Together, these three measures have the potential to reduce emissions of carbon and increase uptake by as much as 3-5 PgC yr-1.
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    Commodity crop expansion in the tropics presents the challenge of preserving tropical moist forest (TMF) ecosystems and their role in carbon sequestration. We propose an algorithm, specific to the TMF biome, which identifies 125 Million ha of degraded, low-carbon density land (LCDL) in the Pantropical TMF belt for agricultural expansion. About 65 Million ha of LCDL are in contiguous tracts >5,000 ha and <500 m elevation, meeting the prerequisites for commercial-scale oil palm production, the fastest-expanding industrialized commodity crop in the TMF. These areas could support expansion of commercial agriculture for another 50 years without further conversion of TMF. Confining agricultural expansion to the LCDL can avoid the release of approximately 13 billion tons of CO2 while saving valuable tropical biodiversity. The simplicity and transparency of this easily-monitored metric could prove useful to producers, governments, investors, environmental stewards, and consumers and enhance good governance in tropical regions.This article is protected by copyright. All rights reserved.
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    The carbon budget of the tropics has been perturbed as a result of human influences. Here, we attempt to construct a 'bottom-up' analysis of the biological components of the budget as they are affected by human activities. There are major uncertainties in the extent and carbon content of different vegetation types, the rates of land-use change and forest degradation, but recent developments in satellite remote sensing have gone far towards reducing these uncertainties. Stocks of carbon as biomass in tropical forests and woodlands add up to 271 ± 16 Pg with an even greater quantity of carbon as soil organic matter. Carbon loss from deforestation, degradation, harvesting and peat fires is estimated as 2.01 ± 1.1 Pg annum(-1) ; while carbon gain from forest and woodland growth is 1.85 ± 0.09 Pg annum(-1) . We conclude that tropical lands are on average a small carbon source to the atmosphere, a result that is consistent with the 'top-down' result from measurements in the atmosphere. If they were to be conserved, they would be a substantial carbon sink. Release of carbon as carbon dioxide from fossil fuel burning in the tropics is 0.74 Pg annum(-1) or 0.57 MgC person(-1) annum(-1) , much lower than the corresponding figures from developed regions of the world.
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    Tropical rainforests store enormous amounts of carbon, the protection of which represents a vital component of efforts to mitigate global climate change. Currently, tropical forest conservation, science, policies, and climate mitigation actions focus predominantly on reducing carbon emissions from deforestation alone. However, every year vast areas of the humid tropics are disturbed by selective logging, understory fires, and habitat fragmentation. There is an urgent need to understand the effect of such disturbances on carbon stocks, and how stocks in disturbed forests compare to those found in undisturbed primary forests as well as in regenerating secondary forests. Here, we present the results of the largest field study to date on the impacts of human disturbances on above and belowground carbon stocks in tropical forests. Live vegetation, the largest carbon pool, was extremely sensitive to disturbance: forests that experienced both selective logging and understory fires stored, on average, 40% less aboveground carbon than undisturbed forests and were structurally similar to secondary forests. Edge effects also played an important role in explaining variability in aboveground carbon stocks of disturbed forests. Results indicate a potential rapid recovery of the dead wood and litter carbon pools, while soil stocks (0-30 cm) appeared to be resistant to the effects of logging and fire. Carbon loss and subsequent emissions due to human disturbances remain largely unaccounted for in greenhouse gas inventories, but by comparing our estimates of depleted carbon stocks in disturbed forests with Brazilian government assessments of the total forest area annually disturbed in the Amazon, we show that these emissions could represent up to 40% of the carbon loss from deforestation in the region. We conclude that conservation programs aiming to ensure the long-term permanence of forest carbon stocks, such as REDD+, will remain limited in their success unless they effectively avoid degradation as well as deforestation.
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    Feedbacks between land carbon pools and climate provide one of the largest sources of uncertainty in our predictions of global climate. Estimates of the sensitivity of the terrestrial carbon budget to climate anomalies in the tropics and the identification of the mechanisms responsible for feedback effects remain uncertain. The Amazon basin stores a vast amount of carbon, and has experienced increasingly higher temperatures and more frequent floods and droughts over the past two decades. Here we report seasonal and annual carbon balances across the Amazon basin, based on carbon dioxide and carbon monoxide measurements for the anomalously dry and wet years 2010 and 2011, respectively. We find that the Amazon basin lost 0.48 ± 0.18 petagrams of carbon per year (Pg C yr(-1)) during the dry year but was carbon neutral (0.06 ± 0.1 Pg C yr(-1)) during the wet year. Taking into account carbon losses from fire by using carbon monoxide measurements, we derived the basin net biome exchange (that is, the carbon flux between the non-burned forest and the atmosphere) revealing that during the dry year, vegetation was carbon neutral. During the wet year, vegetation was a net carbon sink of 0.25 ± 0.14 Pg C yr(-1), which is roughly consistent with the mean long-term intact-forest biomass sink of 0.39 ± 0.10 Pg C yr(-1) previously estimated from forest censuses. Observations from Amazonian forest plots suggest the suppression of photosynthesis during drought as the primary cause for the 2010 sink neutralization. Overall, our results suggest that moisture has an important role in determining the Amazonian carbon balance. If the recent trend of increasing precipitation extremes persists, the Amazon may become an increasing carbon source as a result of both emissions from fires and the suppression of net biome exchange by drought.
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    Forests are major components of the global carbon cycle, providing substantial feedback to atmospheric greenhouse gas concentrations. Our ability to understand and predict changes in the forest carbon cycle-particularly net primary productivity and carbon storage-increasingly relies on models that represent biological processes across several scales of biological organization, from tree leaves to forest stands. Yet, despite advances in our understanding of productivity at the scales of leaves and stands, no consensus exists about the nature of productivity at the scale of the individual tree, in part because we lack a broad empirical assessment of whether rates of absolute tree mass growth (and thus carbon accumulation) decrease, remain constant, or increase as trees increase in size and age. Here we present a global analysis of 403 tropical and temperate tree species, showing that for most species mass growth rate increases continuously with tree size. Thus, large, old trees do not act simply as senescent carbon reservoirs but actively fix large amounts of carbon compared to smaller trees; at the extreme, a single big tree can add the same amount of carbon to the forest within a year as is contained in an entire mid-sized tree. The apparent paradoxes of individual tree growth increasing with tree size despite declining leaf-level and stand-level productivity can be explained, respectively, by increases in a tree's total leaf area that outpace declines in productivity per unit of leaf area and, among other factors, age-related reductions in population density. Our results resolve conflicting assumptions about the nature of tree growth, inform efforts to undertand and model forest carbon dynamics, and have additional implications for theories of resource allocation and plant senescence.