Royal Society - David Keith

would need to involve large-scale activities (several GtC/yr)
maintained over decades and more probably centuries. It is
very unlikely that such approaches could be deployed on a
large enough scale to alter the climate quickly, and so they
would help little if there was a need for ‘emergency action’
to cool the planet on that time scale. The time over which
such approaches are effective is also related to the residence
time of the gas in the atmosphere (and the lifetime of a
perturbation to atmospheric CO 2 concentration is much
longer than the residence time of any individual molecule,
of the order of hundreds of years (Archer et al. 2009)).
2.2 Land-based CDR methods
2.2.1 Land use management, afforestation,
reforestation and avoidance of deforestation
Terrestrial ecosystems remove about 3 GtC/yr from the
atmosphere through net growth, absorbing about 30%
of CO 2 emissions from fossil fuel burning and net
deforestation, while the world’s forest ecosystems store
more than twice the carbon in the atmosphere (Canadell
et al. 2007; Canadell & Raupach 2008). Hence simple
strategies based around the protection and management
of key ecosystems could do much to enhance the natural
drawdown of CO 2 from the atmosphere. Yet currently
emissions from land use change, primarily deforestation,
account for about 20% of all anthropogenic greenhouse
emissions and the amount has been continuing to rise
during the early years of the 21st century. Tropical
deforestation alone now accounts for 1.5 GtC/yr (about
16% of global emissions) and is the fastest rising source
of emissions (Canadell et al. 2007).
Interventions to moderate atmospheric CO 2 through
ecosystem management have potential for carbon
sequestration and can take a range of forms including
avoided deforestation, afforestation, reforestation, and
planting of crops or other vegetation types (Royal Society
2001, 2008b; Submission: Reay). Such interventions are
not normally considered to be geoengineering, and have
limited long-term potential (Royal Society 2001). They
are however immediately available, often have significant
co-benefits, may be particularly useful in the immediate
future, and are considered briefly here, since they are
familiar and provide a useful yardstick for comparison
of other methods.
Terrestrial ecosystems store about 2,100 GtC in living
organisms, leaf litter and soil organic matter, which is
almost three times that currently present in the atmosphere.
Among the world’s seven major biomes, tropical and
subtropical forests store the largest amount of carbon,
almost 550 GtC, and tropical deforestation is therefore
contributing substantially to global carbon emissions.
Temperate forests, especially those with the oldest age
classes intact, also have high carbon storage potential
(over 500 tC/ha) and can also show very high positive
annual rates of carbon sequestration (Naidoo et al. 2008).
The boreal forest biome holds the second largest stock of
carbon, most of it stored in the soil and litter. Draining of
boreal forest peatlands, certain forestry practices and
inappropriate fire management may all cause significant
losses of the carbon stored in this ecosystem (UNEP 2009).
About one quarter of the world’s terrestrial land surface is
now classified as agricultural land of some sort and
agricultural systems, at least in temperate areas, tend to
occupy fertile soils that would have formerly supported
temperate grassland or forest. Land clearance for
croplands and pasture has therefore greatly reduced
above-ground carbon stocks and soil carbon stocks are
also often depleted as tillage disrupts the soil, opening it to
decomposer organisms and generating aerobic conditions
that stimulate respiration and release of CO 2 . Land-use
changes over the past 100 years have therefore played a
significant role in altering soil carbon stores and fluxes.
Simply reversing this trend is clearly not an option as there
are continuing demands for land, especially for agriculture.
However, the potential for land-use management should
not be underestimated and may play a small but significant
role in reducing the growth of atmospheric CO 2
concentrations. Reducing emissions from deforestation
and forest degradation is a vital component but
afforestation or replanting can play a significant role too,
especially in the case of degraded agricultural land. The
establishment of new forested areas may however conflict
with other environmental and social priorities, especially
food production and biodiversity conservation.
Afforestation and reforestation should therefore be
approached in an integrated manner considering
competing demands for land.
There are two scales of management that might use
improved ecosystem and land-use management to
reduce atmospheric greenhouse gas concentrations.
At local to regional scales, increased adoption of land
use management that incorporates multiple ecosystem
services, including carbon storage, could deliver
significant benefits. In one spatially explicit watershed
scale study in Oregon, USA, carbon storage could be
doubled through changed land use policies that were
beneficial over a wide range of economic and ecosystem
services (Nelson et al. 2009). Realistic policy changes in
this area could potentially increase carbon sequestration
by 5 million tons in an area of around 30,000 km 2 . At the
global level, mechanisms aimed at both reforestation and
reduced deforestation, underpinned by effective financial
mechanisms and policies, could achieve 0.4 to 0.8 GtC/yr
by 2030 assuming carbon prices of $20 to $100) per ton
of CO 2 (IPCC 2007c; Canadell & Raupach 2008) offsetting
2 to 4% of projected emissions increases over that period.
These mechanisms can be encouraged by well-founded
carbon markets, by effective land-use planning and, in the
case of avoided deforestation, by the new proposals for
‘reducing emissions from deforestation and degradation’
(REDD) under the UNFCCC. Effective implementation will
depend on reliable baseline estimates monitoring and
enforcement. Critically, to achieve worthwhile benefits and
to establish effective incentives, land-use-planning based
solutions will necessitate larger scale planning and
10 I September 2009 I Geoengineering the Climate The Royal Society