6.1.2.5 Map of Global CH 4 Emissions in Wetlands
The detailed global methane (CH 4 ) budget was first reported by Saunois et al. (2020)
as follows.
Understanding and quantifying the global methane (CH 4 ) budget is important for
assessing realistic pathways to mitigate climate change. Atmospheric emissions and
concentrations of CH 4 continue to increase, making CH 4 the second most important
anthropogenic greenhouse gas in terms of climate forcing after carbon dioxide
(CO 2 ).
For the 2008–2017 decade, global methane emissions were estimated by atmospheric inversions (a top-down approach) to be 576 Tg CH 4 y
À1 (with a range of
550–594, corresponding to the minimum and maximum estimates of the model
ensemble). Of this total, 359 Tg CH 4 y
À1 or $60% is attributed to anthropogenic
sources, that is, emissions caused by direct human activity (i.e., anthropogenic
emissions; ranging from 336 to 376 Tg CH 4 y
À1 or 50–65%).
Bottom-up methods suggest almost 30% more global emissions (737 Tg CH 4
y
À1 , ranging from 594 to 881) than top-down inversion methods. Indeed, bottom-up
estimates for natural CH 4 sources, such as natural wetlands, other inland water
systems, and geological sources, are higher than top-down estimates. The atmospheric constraints on top-down budgets suggest that at least some of these bottomup emissions are overestimated.
The most important source of uncertainty in the methane budget is natural
emissions, especially those from wetlands and other inland water bodies. In particular, projected wetland emissions can be reduced by approximately 35 Tg CH 4 y
À1
through improved partitioning of wetlands and other inland water bodies.
Wetlands are generally defined as ecosystems in which soils or peats are watersaturated or where surface inundation (permanent or not) dominates the soil biogeochemistry and determines the species composition of the ecosystem (USEPA 2010).
To refine such an overly broad definition for the quantification of methane emissions, we define wetlands as ecosystems with inundated or saturated soils or peats in
which anaerobic conditions lead to methane production (Matthews and Fung 1987;
USEPA 2010).
The largest wetland areas, according to WAD2M (Wetland Area Dynamics for
Methane Modeling), are in Amazonia, the Congo Basin, and the Western Siberian
lowlands. A map of the average emissions from wetlands for 2008–2017, which was
built from 13 models, is shown in Fig. 6.9. The zones with the largest emissions are
the Amazon basin, equatorial Africa and Asia, Canada, western Siberia, eastern
India and Bangladesh. Regions where methane emissions are robustly inferred
(defined as regions where the mean flux is larger than the standard deviation of the
models) represent 61% of the total methane flux due to natural wetlands.
The main primary emission zones are consistent between the models and are
clearly consistent with the projected common wetland extent. However, the different
sensitivities of the models to temperature, vapor pressure, precipitation, and radiation can generate substantially different patterns, such as in India. Some secondary
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211
The detailed global methane (CH 4 ) budget was first reported by Saunois et al. (2020)
as follows.
Understanding and quantifying the global methane (CH 4 ) budget is important for
assessing realistic pathways to mitigate climate change. Atmospheric emissions and
concentrations of CH 4 continue to increase, making CH 4 the second most important
anthropogenic greenhouse gas in terms of climate forcing after carbon dioxide
(CO 2 ).
For the 2008–2017 decade, global methane emissions were estimated by atmospheric inversions (a top-down approach) to be 576 Tg CH 4 y
À1 (with a range of
550–594, corresponding to the minimum and maximum estimates of the model
ensemble). Of this total, 359 Tg CH 4 y
À1 or $60% is attributed to anthropogenic
sources, that is, emissions caused by direct human activity (i.e., anthropogenic
emissions; ranging from 336 to 376 Tg CH 4 y
À1 or 50–65%).
Bottom-up methods suggest almost 30% more global emissions (737 Tg CH 4
y
À1 , ranging from 594 to 881) than top-down inversion methods. Indeed, bottom-up
estimates for natural CH 4 sources, such as natural wetlands, other inland water
systems, and geological sources, are higher than top-down estimates. The atmospheric constraints on top-down budgets suggest that at least some of these bottomup emissions are overestimated.
The most important source of uncertainty in the methane budget is natural
emissions, especially those from wetlands and other inland water bodies. In particular, projected wetland emissions can be reduced by approximately 35 Tg CH 4 y
À1
through improved partitioning of wetlands and other inland water bodies.
Wetlands are generally defined as ecosystems in which soils or peats are watersaturated or where surface inundation (permanent or not) dominates the soil biogeochemistry and determines the species composition of the ecosystem (USEPA 2010).
To refine such an overly broad definition for the quantification of methane emissions, we define wetlands as ecosystems with inundated or saturated soils or peats in
which anaerobic conditions lead to methane production (Matthews and Fung 1987;
USEPA 2010).
The largest wetland areas, according to WAD2M (Wetland Area Dynamics for
Methane Modeling), are in Amazonia, the Congo Basin, and the Western Siberian
lowlands. A map of the average emissions from wetlands for 2008–2017, which was
built from 13 models, is shown in Fig. 6.9. The zones with the largest emissions are
the Amazon basin, equatorial Africa and Asia, Canada, western Siberia, eastern
India and Bangladesh. Regions where methane emissions are robustly inferred
(defined as regions where the mean flux is larger than the standard deviation of the
models) represent 61% of the total methane flux due to natural wetlands.
The main primary emission zones are consistent between the models and are
clearly consistent with the projected common wetland extent. However, the different
sensitivities of the models to temperature, vapor pressure, precipitation, and radiation can generate substantially different patterns, such as in India. Some secondary
6 Natural Capital-Based Societies in the Tropics
211