Richmond, Virginia, USA
2 Smithsonian Environmental Research Center, Edgewater, Maryland, USA
ABSTRACT
The recognition that wetlands play an important role in regulating global climate has led to management actions intended to maintain and enhance the globally significant amounts of carbon preserved in wetland soils while minimizing greenhouse gas emissions. Our goal in this chapter is to review the biogeochemical processes that are relevant to wetland climate regulation, which we do by discussing: (1) the concepts of radiative balance and radiative forcing; (2) the mechanisms for wetland carbon preservation; (3) factors influencing greenhouse gas emissions and other carbon losses; and (4) opportunities for wetland management actions to influence carbon preservation and flux. Wetland carbon preservation, which reflects the accumulation of undecomposed organic material, is a function of the redox environment, organic matter characteristics, and physicochemical factors that inhibit decomposition. However, the conditions that favor carbon preservation often result in increased emissions of methane and nitrous oxide such that there is a biogeochemical tradeoff between carbon preservation and greenhouse gas emissions. The losses of carbon via gaseous and dissolved pathways are sensitive to environmental disturbances and raise challenges about fully accounting for the climatic impacts of wetlands. Wetland management and disturbance intentionally or unintentionally affect biogeochemical processes, such that wise environmental management offers opportunities to enhance wetland carbon preservation, prevent the destabilization of accumulated soil carbon, and reduce greenhouse gas emissions, thus maintaining the role of wetlands as regulators of global climate.
3.1. INTRODUCTION
Climate regulation by wetlands is an important ecosystem service that is increasingly a focus of management and restoration efforts (Erwin, 2009; Moomaw et al., 2018). The basis for these efforts is the observation that wetlands have accumulated globally significant amounts of organic carbon in their soils (Mitra et al., 2005), carbon (C) that is no longer in the atmosphere as the greenhouse gas carbon dioxide (CO2). It is becoming apparent that much of the organic carbon preserved in wetlands is not inherently resistant to decay but instead accumulates because its reactivity is reduced under the environmental conditions in wetland soils (e.g., Spivak et al., 2019). An important corollary of this understanding is that changes to the wetland environment (e.g., as initiated through management decisions or disturbances) could alter those conditions, thus destabilizing the large wetland carbon stores and allowing their export to adjacent aquatic systems as dissolved or particulate carbon or their return to the atmosphere as CO2 and methane (CH4). Further, some of the wetland conditions that promote carbon preservation also lead to the production of the greenhouse gases CH4 and nitrous oxide (N2O), the emissions of which can offset some or all of the climatic benefits provided by wetland carbon preservation.
Environmental management and other human actions, whether purposeful or accidental, can affect the pathways of carbon preservation and removal and therefore have the potential to alter the effects of wetlands on the global climate. In this chapter, we briefly summarize (1) the concepts of radiative balance and radiative forcing as ways of describing how ecosystems and management actions influence the climate. We then address (2) the factors that control wetland carbon preservation. The term “carbon 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
