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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 (SO₄^2–) 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 3.1). At Time 1, Wetlands 1 and 2 had a positive radiative balance over a 100‐year period, indicating that the warming due to CH₄ emissions was greater than the cooling due to long‐term carbon preservation in each wetland. For Wetland 1, the radiative balance was exactly the same in the two time periods because carbon sequestration and CH₄ emission rates did not change. Thus, the radiative forcing of Wetland 1 was zero (Table 3.1) and its contribution to Earth’s energy budget had not changed over time. In contrast, the radiative balance in Wetland 2 was lower in Time 2 than in Time 1 due to a management action. This means that radiative forcing was negative, such that the perturbation (that is, the management action) applied to Wetland 2 had offset some of the climatic warming from fossil fuel combustion and land use changes. In this example, Times 1 and 2 correspond to any pair of years. In the context of the attribution of current climate change, the Intergovernmental Panel on Climate Change (IPCC) reports radiative forcing relative to the year 1750 (i.e., the pre‐Industrial era; Myhre et al., 2013). Determining what the radiative balance of a wetland was more than 250 years ago presents considerable challenges.

Finally, please note that the GWP and SGWP are properties of greenhouse gases, not of an ecosystem. We sometimes see them incorrectly used as a synonym for radiative balance, as in the “global warming potential (GWP) was calculated in CO₂ equivalents” or “we observed a significant difference in GWP between aerobic and anaerobic treatments.” We do not wish to single out specific authors, so we have purposely not provided citations for these quotes. Instead, our goal is to illustrate how these terms have been misused in the scientific community.