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This practice entails restoration of wetlands that formerly were used for agriculture, forestry, or urban/industrial uses through plugging of drain ditches, restoration of prior hydrological conditions, or artificial water diversions.
Use and Potential
Conversion involves inundation of previously drained areas. This inundation
will increase CO2 storage as organic matter because of reduced oxygen in soils,
but it may also increase CH4 emission, depending on whether the new water table
is close to the soil surface. Carbon storage rates will be rapid initially,
then slow over time (few estimates are available in the literature, however).
Overall GHG emissions changes will likely entail small sinks-or even sources,
if CH4 emissions dominate over CO2 sinks in stored organic matter. The area
of wetland that has been converted since 1900 to agriculture, forest, and urban
land (mostly in temperate and tropical regions) is poorly known, with estimates
ranging from 6 percent (Armentano and Menges, 1986) to as much as 50 percent
(Moser et al., 1996) . Most of the loss of wetland area has been associated
with drainage for agriculture in temperate regions, though pressures on tropical
wetlands have increased in recent decades (Moser et al., 1996). The maximum
area available for restoration is in the range of 30-250 Mha (based on the global
wetland area of 570 Mha reported by Matthews et al., 1996).
Current Knowledge and Scientific Uncertainties
Carbon storage results from decreased decomposition rates associated with inundation.
Methane emissions will increase, however. Rates of carbon accumulation in new
wetlands can be high-0.1-1 t C ha-1 yr-1 (Tolonen and Turunen, 1996)-but slow
over time as carbon accumulates and decomposition losses begin to offset additions.
The CO2 sink that is created by reestablishing the wetland may be offset by
increased methane emissions because inundation will cause some of the decomposed
carbon to be released as methane. Completely offsetting carbon storage at rates
of 0.1-1 t C ha-1 yr-1 would require CH4 emissions of ~2-20 g CH4 m-2 yr-1.
These rates are in accord with those measured in natural freshwater wetlands
(which range from ~7 to 40 g CH4 m-1 yr-1; Bergkamp and Orlando, 1999); the
actual amount of emitted methane will depend, however, on the degree of inundation
(methane can be oxidized before emission if the water table is below the surface)
and on whether other oxidants are present. For example, carbon storage effects
will dominate the net GHG budget in coastal wetlands, which do not emit significant
amounts of methane.
There are large uncertainties about the actual areas of wetland converted, some of which result from differing definitions of wetlands and differing interpretations of whether rice agriculture is included as wetland. Uncertainties exist about carbon storage rates and methane emission changes (likely 10-30 percent of reported values) because very few published studies exist for these factors in restored wetlands.
Time Scales
Carbon storage is long term; decomposition rates of organic matter stored under
anaerobic conditions are slow (decades to millennia, depending on the degree
of inundation and the type of vegetation in the new wetland).
Monitoring, Verifiability, and Transparency
Methane fluxes must be monitored. Because methane fluxes are highly variable
in space and time, monitoring of methane emissions involves significant effort
and cost. New carbon storage may be determined by monitoring changes in bulk
density and percentage of carbon. Wetland area may be verified by remote sensing
(regionally) or repeated surveys (locally).
Permanence
Carbon storage depends on continued inundation. Stored organic matter is susceptible
to rapid decomposition if the wetland dries out and exposes it to oxic conditions.
A wetland may dry because of climate change or because of a reversion to drainage.
Associated Impacts
Wetlands have positive impacts on water quality, provide protection against
local flooding, help control soil and coastal erosion, and increase biodiversity.
Several international negotiations pertain to these aspects of wetlands-in particular,
the Convention on Wetlands, the Convention on Biodiversity, and the Marine and
Coastal Work Programme (Bergkamp and Orlando, 1999).
Relationship to IPCC Guidelines
The Guidelines do not explicitly mention wetland restoration, although land
flooding is included in the Reference Manual in relation to methane emissions.
Conceptually, associated changes in soil carbon stocks could be estimated similarly
to the procedures for conversions to wetlands, with the need for appropriate
values for net carbon uptake rates.
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