preservation,” which we use throughout this chapter, is largely synonymous with “carbon sequestration” and “carbon storage.” We use preservation to emphasize the absence of decomposition; this framework has helped us think about the processes and mechanisms in a slightly different way. After discussing carbon preservation, we review (3) the processes leading to emissions of greenhouse gases and other losses of carbon from wetlands, before discussing (4) how wetland management can be used to manipulate those biogeochemical factors that affect wetland carbon preservation and flux. We offer this synthesis in the hopes that it will help guide wise decisions.
3.2. RADIATIVE BALANCES AND RADIATIVE FORCING
The terms “radiative balance” and “radiative forcing” are used when discussing the climatic impacts of an ecosystem or a management action. While these terms are related, they are distinct terms that are often – but mistakenly – used interchangeably. The radiative balance of a wetland or other ecosystem is a static measure of how the ecosystem affects Earth’s energy budget over a defined time period, typically 100 years. In contrast, radiative forcing is a measure of how a perturbation to the ecosystem alters Earth’s energy budget. Thus, a change in radiative balance leads to radiative forcing, which causes the planet to warm or cool. If Earth’s energy budget does not change (that is, if there is no radiative forcing), then there is no climate change.
A wide variety of perturbations can affect the radiative balance of a wetland and, therefore, cause radiative forcing. The radiative balance of an individual wetland can change with changes in biogeochemistry, which may be accidental or purposefully designed into environmental management programs in order to influence climate. For example, rates of wetland carbon sequestration are sensitive to factors including climate, hydrology, and vegetation composition (Chmura et al., 2003; Loisel et al., 2014). The production and emissions of CH4 vary with soil water saturation, salinity, and acid rain inputs of sulfate (SO42–) and nitrate (NO3–), among other factors (Bridgham et al., 2013). Likewise, the rate of nutrient loading to a wetland can alter rates of N2O emissions to the atmosphere (Moseman‐Valtierra et al., 2011). On a broader regional or global basis, the radiative balance of wetlands can change as the area of wetlands changes. Despite some regional increases in the areal extent of wetlands (e.g., Niu et al., 2012), there has been a global loss of wetland area (Millennium Ecosystem Assessment, 2005). The direction of radiative forcing (that is, whether the net loss of wetlands has contributed to warming or cooling of the climate) is dependent on the kinds of wetlands that have been created and lost.
In order to compare the fluxes of different greenhouse gases, it is necessary to normalize them to a common set of units. The global warming potential (GWP), which is the “time‐integrated radiative forcing due to a pulse emission of a given component, relative to a pulse emission of an equal mass of CO2” (Myhre et al., 2013), has long been used by wetland scientists to calculate radiative balances and radiative forcing (e.g., Gorham, 1991; Whiting & Chanton, 2001). For the commonly used 100‐year time scale, the GWP of CH4 is 30 and that of N2O is 265, meaning that a unit mass of CH4 or N2O causes 30 or 265 times more warming, respectively, than the same mass of CO2 when integrated over a century (Myhre et al., 2013). Recently, we argued that the use of GWPs is inappropriate when calculating radiative balances for wetlands and other ecosystems (Neubauer & Megonigal, 2015) because ecosystems exchange greenhouse gases with the atmosphere year after year, not just as a one‐time pulse. To address this issue, we proposed the sustained‐flux global warming potential (SGWP), which is the “time‐integrated radiative forcing due to sustained emissions of a given component, relative to sustained sequestration of an equal mass of CO2” (Neubauer & Megonigal, 2015; Neubauer & Verhoeven, 2019). For a gas like CH4, which has a much shorter lifetime than CO2, the SGWP is very different from the GWP (45 vs. 30 over 100 years). In contrast, because CO2 and N2O have similar average atmospheric lifetimes of roughly 100 years, the 100‐year SGWP and GWP values of N2O are similar (270 vs. 263, respectively; Neubauer & Megonigal, 2015).
The choice of GWP vs. SGWP metrics has large implications for calculating radiative balances and radiative forcing, especially when CH4 fluxes are involved. Using the SGWP instead of GWP would make a wetland appear to be a stronger greenhouse gas source (or a weaker greenhouse gas sink). Although use of the GWP might be tempting here because “the numbers look better,” one should be careful to use the most appropriate metric when calculating how wetland management and restoration activities will influence radiative forcing. Because the SGWP is based on continuous fluxes between ecosystems and the atmosphere, it is the better metric to use when looking at radiative balances in wetlands (Neubauer & Megonigal, 2015).
Table 3.1 Radiative balance and radiative forcing for two hypothetical wetlands at two time periods
| Long‐term carbon preservation rate | CH4 emission rate | Radiative balance | Radiative forcing | |||
|---|---|---|---|---|---|---|
| Wetland | Time | (g CO2 m–2 yr–1) | (g CH4 m–2 yr–1) | (g CO2‐eq m–2 yr–1) | (g CO2‐eq m–2 yr–1) | (g CO2‐eq m–2 yr–1) |
| Wetland 1 | Time 1 | 75 | 10 | 450 | 375 | 0 |
| Time 2 | 75 | 10 | 450 | 375 | ||
| Wetland 2 | Time 1 | 150 | 40 | 1800 | 1650 | –1080 |
| Time 2 | 150 | 16 | 720 | 570 |
For Wetland 1, we assume there is no change in rates of carbon preservation or CH4 emission over time. For Wetland 2, we assume that a management action lowered CH4 emissions but did not affect long‐term carbon preservation. Note that the carbon preservation and CH4 emission rates are mass fluxes (e.g., g CH4 per area per time, not g C or mol C per area per time). The CH4 mass flux is converted to a CO2‐equivalent (CO2‐eq) flux by multiplying the mass flux by the 100‐year SGWP value of 45 (Neubauer & Megonigal, 2015). The radiative balance of a site is the difference between the warming due to CH4 emissions and the cooling due to carbon preservation, with a positive radiative balance indicating that the wetland has a net warming effect over a 100‐year period. Radiative forcing is the difference in the radiative balance between the two time periods, with negative radiative forcing indicating that a wetland is having a smaller warming effect (or a greater cooling effect) in Time 2 vs. Time 1.
We have used the SGWP to calculate the radiative balance and radiative forcing for two hypothetical wetlands (Table