Restoring tides to reduce methane emissions in impounded wetlands: A new and potent Blue Carbon climate change intervention

Coastal wetlands are sites of rapid carbon (C) sequestration and contain large soil C stocks. Thus, there is increasing interest in those ecosystems as sites for anthropogenic greenhouse gas emission offset projects (sometimes referred to as “Blue Carbon”), through preservation of existing C stocks or creation of new wetlands to increase future sequestration. Here we show that in the globally-widespread occurrence of diked, impounded, drained and tidally-restricted salt marshes, substantial methane (CH4) and CO2 emission reductions can be achieved through restoration of disconnected saline tidal flows. Modeled climatic forcing indicates that tidal restoration to reduce emissions has a much greater impact per unit area than wetland creation or conservation to enhance sequestration. Given that GHG emissions in tidally-restricted, degraded wetlands are caused by human activity, they are anthropogenic emissions, and reducing them will have an effect on climate that is equivalent to reduced emission of an equal quantity of fossil fuel GHG. Thus, as a landuse-based climate change intervention, reducing CH4 emissions is an entirely distinct concept from biological C sequestration projects to enhance C storage in forest or wetland biomass or soil, and will not suffer from the non-permanence risk that stored C will be returned to the atmosphere.


Results
Radiative Forcing per Unit Area. Results indicate that climatic warming, or positive RF, due to tidal restriction (both impoundment and drainage) is large on a per unit area basis relative to the magnitude of cooling from C sequestration in forests and in unaltered coastal wetlands (Fig. 3). For instance, over a period of 20 years following initial alteration, CH 4 emissions from an impounded and freshened salt marsh can result in a cumulative RF that is a factor of ~3 to 20 greater than the magnitude of climatic cooling due to net C stock increase in continental U.S. forests and to soil C sequestration in unaltered salt marsh, and a factor of 1.5 to 10 greater than sequestration in natural mangrove. Over those 20 years, CH 4 emissions from one hectare of impounded wetland are equivalent to RF from 20 years of continuous tailpipe emissions of 1.7 to 6.3 automobiles (Fig. 3).
To quantify the potential for emissions reductions through wetland management, we calculated the time-course, over decades to centuries, of cooling (negative RF) predicted to occur due to CH 4 and CO 2 emissions reductions resulting from tidal restoration in tidally-restricted salt marsh, based on the cumulative difference between the contrasting management action and no action scenarios (Fig. 4). We compared climatic cooling due to tidal restoration to cooling from other wetland carbon management options, including creation of new salt marsh or sea grass beds as biological C sequestration projects, and to rewetting of terrestrial peatland to cease the high rate of CO 2 emissions from drained peatland soils. In general, the tidal restorations in salt marshes, to restore . Conceptual model of carbon cycle processes and greenhouse gas flux changes in response to hydrological management in tidal wetlands. (a) In unaltered, or successfully restored, salt marsh, sulfate ion inhibits methane emission, and high rates of net CO 2 uptake, soil C storage, and soil elevation gain occur. (b) Salt marsh drainage increases exposure of soil carbon stocks to oxygen, and results in a rapid rate of aerobic respiration to CO 2 , loss of carbon stock, and loss of elevation. (c) Impoundment commonly leads to freshening and increased water level, and those conditions are likely to cause an increase in methane emission. Effects of impoundment on soil carbon stocks and rate of soil carbon storage are not well-known, and herein rates are assumed to be similar to natural salt marsh. Diagrams created using Adobe Illustrator CC and arranged using Microsoft PowerPoint vers. 15 natural salinity and water levels, will be dramatically more effective at cooling climate than other wetland-based climate change interventions (Fig. 4). The time course of cumulative RF resulting from reduction in the rate of CH 4 production in previously flooded and freshened soils (Fig. 4a,d), and reduction in CO 2 production in previously drained soils (Fig. 4b,e) indicate sizeable, rapid and sustained climatic cooling. Despite the high rate of C sequestration in salt marshes and mangroves, building and planting of new marsh, mangrove or seagrass bed, provides relatively modest climatic cooling (Fig. 4c,f). Avoided CO 2 emissions from rewetting of drained, terrestrial peatlands (Fig. 4c,f) can be substantial, but are partially offset by resumption of CH 4 emissions in soil rewetted with fresh water 20 ( Table 1).

Geography of Tidally-Restricted Wetlands and Scale of Emissions.
There is substantial potential for GHG reductions based on the intensity of emissions, and on the areal extent of tidally-restricted wetlands in developed landscapes. Tidally-restricted wetlands are widespread throughout the inhabited world (e.g. 7,8,[13][14][15][18][19][20][21][22][23][24][25][26][27][28][29][30][31]. Infrastructure and activities that alter wetland and coastal hydrology have been closely associated with intensity of human development and economic activity in terrestrial and marine portions of the coastal landscape 31,32 , and therefore are expected to increase in the future, particularly in portions of the world that are undergoing rapid development 23,31 . Quantitative geographical data on tidally-restricted wetlands are limited, and indeed the global area of tidal wetlands as a whole is not well-constrained 15 . Thus, it is not possible to estimate global scale GHG emissions due to restrictions. As an indication of scale, we used mapped and areal data along portions of the U.S. Atlantic coast on managed impoundments and on incidental impoundments associated with transportation infrastructure. The analysis resulted in an estimate that those tidal restrictions have caused a reduction in salinity, and enhancement of CH 4 emissions, in approximately 27% of tidal wetlands on the U.S. Atlantic coast ( Table 2). The impoundment and freshening result in enhanced CH 4 emissions in the range of 28,000 to 145,000 tonnes (t) CH 4 y −1 . It is worth emphasizing once again that these are sustained emissions, and thus RF due to those rates of continuous emission over a 20-year period are equivalent to 20 years of continuous emissions from 0.6 to 3.1 million automobiles (see Methods). Note that this is an estimate of CH 4 emissions alone, and does not consider changes in C sequestration rate in soil, nor loss of existing soil C stocks and enhanced soil CO 2 emissions, due to changes in water level and salinity. Further, the area and emissions calculations consider only the effect of freshening and impoundment of tidal wetlands, and do not include area or emissions of tidal wetlands that have been drained, unintentionally nor intentionally, for purposes such as agriculture or land development.

Discussion
Key features of avoided wetland methane emissions. The results of this study indicate that, despite a high rate of C sequestration in coastal wetlands, in many cases tidal restoration in salt marshes will have dramatically greater potential per unit area as a climate change intervention than the other examined ecosystem management actions. Coupled with the common, widespread occurrence of tidal restrictions, there is significant potential for GHG emissions reductions through tidal restoration in salt marshes. Though we did not address tidal restriction and restoration in mangroves, many of the processes and rates presented for salt marshes would be expected to be similar in mangroves. We further note that avoiding tidal restrictions with future coastal development would have similar and perhaps greater benefits in terms of avoided GHG emissions than the restoration scenarios examined in this study. Here we discuss several features of tidal restoration, and particularly avoided methane emissions, that highlight further advantages relative to enhanced CO 2 sequestration in other  Table 1. As described in the text, scenarios were considered based on two emission factors for carbon dioxide in drained marsh and three emission factors for methane in freshened or impounded wetlands. Those EF are shown here as separate bars. For context, emissions in wetlands are compared to terrestrial forest C sequestration and to emissions from an automobile. RF from an automobile was calculated based on twenty years of tailpipe emissions from one U.S. automobile 50 at 4,690 kg CO 2 y −1 .
landuse-based climate change interventions, due to key aspects that result in rapid, substantial, and sustained reduction in RF: First, CH 4 has a radiative efficiency, or instantaneous heat absorption capacity, that is a factor of ~73 greater than that of CO 2 , and a factor of ~94 greater with consideration of indirect effects of CH 4 on atmospheric chemistry 33 . Second, reductions in both CH 4 and CO 2 emissions are soil microbial responses, and thus, though more data are needed, onset following restoration of natural water level and salinity is likely to be rapid 14,21 relative to development of restored C storage capacity following full ecosystem recovery or development in biological sequestration projects 34 . In the present study, ecosystem recovery was represented as a lag of 5 years prior to onset of the full rate of C sequestration in salt marsh soil (Table 1, note k). Thus, climatic cooling due to avoided CH 4 emissions is relatively large on short timescales. In contrast, in projects that promote biological C sequestration in wetland soil, or similarly in forest biomass, accrual of climate change mitigation benefits is a process of gradual accumulation on a timescale of centuries (see "New salt marsh" curve in Fig. 4c and f), due to both the gradual rate of ecosystem development following restoration, and to the modest radiative efficiency of CO 2 , coupled with a long atmospheric perturbation lifetime. As a result, at 15 years post restoration, for instance, negative RF (cooling) due to CH 4 reduction (Fig. 4a) was a factor of 12 to 59 greater than that due to CO 2 sequestration in new salt marsh (Fig. 4c).
Third, in the absence of restoration, enhanced CH 4 emissions in an impounded and freshened wetland can be expected to continue indefinitely, due to a continuous supply of new organic matter substrate for methanogenesis from ongoing primary production 35 . Thus, although CH 4 is considered to be a short-lived climate pollutant, with an atmospheric perturbation lifetime of only 12.4 years, restoration can avoid a long-term, sustained emission, and the cumulative mass of avoided CH 4 and magnitude of climatic cooling will continue to increase indefinitely through time (Fig. 4d). In contrast, although avoidance of CO 2 emissions by tidal restoration in drained salt marshes can have an RF in the first few decades that is as great as avoided CH 4 in freshened marshes, the duration and ultimate cumulative magnitude of enhanced CO 2 emissions in a drained wetland is limited by the mass of the C stock contained in soil above the level of drainage. Therefore, where wetlands have been drained for decades to centuries, the benefit of restoration may be relatively minor (Fig. 4b emission factor 1). Where pre-restoration CO 2 respiration is rapid, re-wetting will be of substantial benefit (Fig. 4b emission factor 2), but the anticipated duration of that benefit will be uncertain (Fig. 4e). Based on IPCC Tier 1 marsh soil C stocks and respiration rates in drained marshes, the stock in the top meter of soil would be consumed within approximately 3 decades following drainage 18 .
A fourth critical feature of wetland tidal restoration as a climate change intervention is that the CH 4 emissions thus avoided will have what can be referred to as "inherent permanence": following tidal restoration and onset of reduced methane emissions, the emissions thus avoided cannot be cancelled, even if emissions were to resume in the future due to a change in ecosystem status. For instance, if a tidal restriction were re-established 30 Restoration Scenario GHG Emission Factors Prior to, and Post-Restoration CH 4 prior (g C m −2 y −1 ) CH 4 post (g C m −2 y −1 ) CO 2 prior (g C m −2 y −1 ) CO 2 post k (g C m −2 y −1 ) years after restoration, emissions would resume, but the biosphere and atmosphere would maintain for extended time a deficit of heat and of GHG equivalent to 30 years of reduced emissions. Further, since GHG emissions from tidally-restricted wetlands are caused by human activity, they are anthropogenic emissions, and reducing anthropogenic wetland emissions will have an effect on climate that is equivalent to reduced emission of an equal quantity of fossil fuel GHG. Therefore, avoiding CH 4 emissions as a landuse-based climate change intervention is a distinct concept from biological C sequestration projects to enhance C storage in forest or wetland biomass or soil. Biological C sequestration accumulates climate benefit at a relatively slow rate, and the C sink must persist for a century or more to have significant impact on climate (see Fig. 4f, new salt marsh example). During that time, there is a risk that the stored C will be rapidly returned to the atmosphere, through processes such as fire or ecosystem degradation and organic matter decomposition. As a result, the monetary and climate change mitigation value of biological C sequestration projects must be discounted in C markets and in trading programs to account for the "non-permanence risk" that stored C will be returned to the atmosphere 36,37 . To be clear, this discussion is not intended to suggest that temporary reductions in CH 4 emissions will achieve effective management of climate change, but rather to highlight a distinction between avoided emissions and C sequestration.

Implications for GHG emission inventories and reduction programs.
Tidal restoration to avoid CH 4 emissions is a relatively new concept for Blue Carbon management, and warrants consideration within GHG markets and emission offset programs, national and local-level efforts to reduce emissions, and national GHG inventories based on IPCC guidance. Within international climate change treaty negotiations there has been increasing emphasis on co-benefits accruing from GHG mitigation actions that involve improved landscape management, including forests, soils and potentially coastal and marine ecosystems. In 2013, the International Panel on Climate Change released the 2013 Supplement to the 2006 IPCC Guidelines for National GHG Inventories: Wetlands 18 , to guide accounting of emissions and management of wetland soils. The Supplement includes guidance on CO 2 emissions for drained wetlands, but does not consider GHG emissions from impoundments in general, nor freshened or impounded wetlands due to tidal restriction. The U.S. has recently completed the first national inventory worldwide to include coastal wetlands in a National GHG Inventory, and the UN Framework Convention on Climate Change has asked countries to provide feedback on experiences with application of the Supplement, to inform future expansion of the accounting guidance where science is available. The analysis presented here indicates that tidally restricted wetlands meet the primary criteria for inventoried ecosystems in that they are managed landscapes, with emissions and sinks of substantial magnitude and rate of change. If other countries ultimately follow suit, then inclusion of those emissions in the U.S. Inventory will promote widespread recognition and management of the issue, and justify development of CH 4 EF for tidal restrictions and impoundments in IPCC guidance for GHG inventories. Note that since ecological restoration through tidal reconnection has been a widespread practice in some parts of the world, restorations that were conducted since the baseline year, typically 1990, may represent considerable, but as yet unrecognized, emissions reductions. Wetland C and GHG management can also play a role as an additional method for reducing regional, state and local GHG emissions, including through regulatory or voluntary carbon markets or cap & trade programs. Verified Carbon Standard (VCS) has developed a Methodology for crediting tidal wetland carbon offset projects, designed for both voluntary and regulatory C markets worldwide, and within that methodology there is accommodation for avoided CH 4 projects as a creditable activity 37 . The implications of the results presented in the present study may support continued development of concepts related to crediting of climate change mitigation through avoided CH 4 emissions in wetlands, to reflect the distinctions between avoided CH 4 and C sequestration projects: First, at present, in the VCS methodology, there must be reasonable expectation that a wetlands-based, emissions offset project will have a lifetime of at least 100 years. The mandated lifetime is in response to the concept that in projects that promote C sequestration in soil or biomass, the sequestered C must be kept out of the atmosphere for an extended time to have significant climate change mitigation value. However, in the context of avoided CH 4 emissions, the 100-year time frame is arbitrary, since such offset projects do not rely on storage of C in a soil or biomass reservoir, and they accrue considerable, long-term and quantifiable mitigation benefit on timeframes of decades. Given that the persistence of any project for 100 years is difficult to assure, a shortened minimum lifetime for avoided CH 4 projects could increase utilization of tidal wetlands as offset projects.
Second, the application of a 100-year CH 4 global warming potential of 21 to calculate carbon dioxide equivalents (CO 2 e) of avoided CH 4 emissions underestimates the climate change mitigation value of the avoided CH 4 to a substantial degree. As an example, if we were to apply that GWP value in a CO 2 e approach to scenarios 1 and 3 in Table 1 (tidal restoration in salt marsh to avoid CH 4 emissions vs. creation and planting of new salt marsh to enhance C sequestration), the range of CH 4 emission factors would result in a calculated GHG reduction benefit for avoided CH 4 (scenario 1) that is similar to the C sequestration benefit in scenario 3 (scenario 1 = 0.7 to 3.5 times the value of scenario 3; not shown). The ratio would remain constant for the 100-year lifetime of the projects. That result is in contrast to the more nuanced calculation of change in radiative forcing that we noted previously in the manuscript, in which reduction in RF (cooling) due to CH 4 reduction (Fig. 4a) was a factor of 12 to 59 greater than RF reduction due to CO 2 sequestration in new salt marsh (Fig. 4c), at 15 years post restoration. Even at 100 years post restoration, the benefit of avoided CH 4 emissions (scenario 1) remains a factor of 3 to 11 greater (Fig. 4d and f). As noted previously, a standard GWP approach is inappropriate for sustained emissions or  Table 2. Geography of tidally restricted wetlands on the Atlantic coast of the United States. Available published data were compiled on surface areas of managed wetland impoundments and incidental, full or partial impoundment resulting from transportation infrastructure. Tidally restricted wetland areas were used as a sample to estimate restricted wetland area as a percentage of total tidal wetland area. Percentages were then extrapolated to the areas without data. Based on salinity data 9 and vegetation as an indicator of salinity 49 , it was estimated that 70% of the impounded wetlands were freshened as a result. (see Methods).
sinks that are typical of ecosystems 4 . A 100-year GWP that is calculated based on simulation of a single pulse of CH 4 , with an atmospheric lifetime of 12.4 years, integrates 12 years of heat retention across 100 years of impact, and ignores the fact that with sustained emissions a pool of CH 4 would be maintained in the atmosphere indefinitely, rather than for just 12 years. There is ongoing debate regarding the value, in terms of addressing climate change, of reducing CH 4 emissions vs CO 2 emissions, and the debate is relevant to selection of a GWP value for CH 4 , and crediting timeframe, in C markets and offsets. Though there is not a simple answer, it is clear that aggressive reduction in emissions of CH 4 and other short-lived climate pollutants (SLCP) can reduce the rate of increase in global temperature during the current century, and can reduce peak temperature if coupled with rapid elimination of CO 2 emissions 38 . Further, it has been shown that, although CH 4 is a short-lived climate pollutant, during the time that it resides in the atmosphere, a portion of the heat that it traps within the biosphere will be retained in oceans and contribute to sea level rise for several centuries 39 . However, if actions to reduce emissions of SLCP result in delayed reductions in CO 2 emissions, then higher peak temperatures will result 38 . Therefore, society is likely to benefit most from simultaneous, but separate, efforts to reduce both CO 2 and SLCP emissions, to manage both short-term and long-term climate change. Utilization of offsets and emissions trading programs therefore requires relative evaluation of pollutants with distinct interactions with climate. Solutions might be to limit maximum utilization of offsets in C markets, as is done in California's Cap and Trade Program 40 , or to develop separate reduction and trading programs for CO 2 and SLCP. For instance, the state of Massachusetts has specific emissions reduction goals for natural gas transmission 41 , New Zealand has separate targets for methane reductions in their Intended Nationally Determined Contribution 42 , and many national and subnational governments have programs to reduce methane emissions from agricultural sources 43 . As described in the present study, degraded tidal wetlands can be significant sources of both CO 2 and CH 4 , while intact wetlands are generally a strong CO 2 sink. Thus, wetland management can contribute to management of both short and long-lived climate pollutants.
Finally, beyond C markets and national GHG inventories, awareness of anthropogenic emissions in tidally-restricted wetlands, and the potential for mitigation through ecosystem restoration, may be of significant value to land management agencies with wetlands, tidal impoundments, and coastal infrastructure under their purview, such as the U.S. Fish and Wildlife Service, the National Park Service, local and state government entities, and private land owners. In wetlands that are intentionally managed, in many cases opening tidal restrictions would be prohibited by ongoing important land uses, or where restoration would put developed land at risk of flooding. But, increasingly, decisions are being forced, by accelerating sea level rise and increasing storm intensity, regarding whether to spend resources to preserve and upgrade aging dikes and tide gates that maintain wetlands in drained or impounded condition (for example see Fig. 1c, Prime Hook NWR). Management decisions and objectives are based in part on anticipated changes in the value of ecosystem services to society, including coastal protection, quality and type of habitat, recreational utility, and others. Reduction in GHG emissions has significant value to society, and can be highlighted as a benefit, regardless of whether the benefit is monetized. Even where opening tidal restrictions is contraindicated, there still may be potential for management of water levels and salinity at targeted times of the year, specifically for the purpose of reducing GHG emissions and promoting C sequestration, while maintaining utility of the landscape for those other uses or protecting infrastructure from flooding.
In other circumstances, the decisions may be less complicated in terms of tradeoffs of services, such as where tidal restriction is due to inadvertent, transportation-related obstruction of flow, and thus there is no current beneficial use of the impaired wetland (for example, see Fig. 1b). In some cases, ecosystem restoration through opening of tidal restrictions can be consistent with coastal resilience planning, since exposure to tidal flow will tend to promote resumption of natural accretion of wetland elevation in response to sea level rise, resulting in enhanced protection of the infrastructure landward of the wetland.
There is significant potential for C and GHG management in coastal wetlands, but continued advances are needed on the scientific basis for policy frameworks and for quantification of emission factors in response to management actions. There has been particularly little study of changes in soil and biomass C stocks, and of GHG fluxes, in response to tidal restriction and tidal restoration in salt marshes and mangroves. Further, in all areas of the world the state of knowledge is poor regarding the geographic distribution of tidal wetlands, of salinity within those wetlands, of occurrence of tidally-restricted salt marshes and mangroves, and of the potential for tidal restoration. The findings presented here therefore suggest a research agenda to study GHG emissions and the fate of carbon stocks in natural, restricted and restored wetlands, across geographic settings, as well as mapping occurrence and characteristics of tidal wetlands and tidally-restricted wetlands.

Methods
Radiative Forcing-To calculate radiative forcing, we modeled the time-course of change in the atmospheric inv entory of each GHG per square meter of wetland, based on emission rate and on rate of destruction or removal from the atmosphere. Methane was modeled as a single reservoir with atmospheric perturbation lifetime of 12.4 years, and first order removal due to oxidation to CO 2 . RF from management of methane emissions includes forcing from the resulting changes in both the pool of atmospheric CH 4 and CO 2 produced through CH 4 oxidation. We calculated the mass of CH 4 in the atmosphere, per square meter of wetland at time t, according to Neubauer and Megonigal 4 eq. 1: is the mass of atmospheric CH 4 (g C m −2 ), is the emission factor (g C m −2 y −1 ), dt is the time step (0.2 y), and T CH4 is the atmospheric perturbation lifetime of CH 4 .
Mass of atmospheric CO 2 was modeled based on a synthesis of models, utilizing four non-interacting reservoirs with distribution of emissions among the reservoirs and atmospheric perturbation lifetimes in the reservoirs as in Joos et al. 44 , Table 5 and IRF model within that publication. The mass of CO 2 in the atmosphere was calculated as: where f i is the fraction of CO 2 emissions distributed to the i th reservoir, and other terms are as defined for the CH 4 model. In both models, the instantaneous RF was calculated at each time step as the product of the mass of each gas in the atmosphere and its radiative efficiency 33 (1.75 × 10 −15 W m −2 (kg CO 2 ) −1 and 1.28 × 10 −13 W m −2 (kg CH 4 ) −1 with CH 4 radiative efficiency adjusted by a factor of 1.65 to account for indirect effects 45 . Cumulative RF is the sum of instantaneous RF over a given time period. N 2 O emissions were not considered due to insufficient data.
Geography-Large scale, geographic data on tidal restrictions are rare, but a large number of examples and case studies of restriction and restoration occur within the literature. As an indication of scale, approximately 50% of pre-development tidal saltmarsh area in the U.S. has been destroyed by human activities, with impoundment and drainage as prominent causes 23 , and a quarter of remaining coastal wetlands in the state of Massachusetts, for example, is landward of nearly 600 transportation-related tidal restrictions [46][47][48][49] . To estimate the scale of tidally restricted wetlands area and the proportion of existing wetlands that are restricted, we compiled published information on the occurrence along the U.S. Atlantic coast of managed wetland impoundments and incidental, full or partial impoundments caused by transportation infrastructure ( Table 2). In the northeast U.S., wetland and impoundment areas landward of primarily transportation-related restrictions comprise 29% of total tidal wetland. In the southeast U.S. 10% of total tidal wetland area is diked and impounded for waterfowl and mosquito management. The tidal wetlands in the regions and states represented in the analysis comprise 55% of the total tidal wetland on the U.S. Atlantic coast, and thus are expected to be approximations of the entire coast and may be similar to many developed coastal regions elsewhere. Based on the assumption that similar levels of transportation-related and diked restrictions occur throughout the U.S. east coast, we estimate that 39% of total tidal wetland area on the U.S. Atlantic coast, ~3,800 km 2 , is above tidal restrictions or blockages. Montague et al. 23 , reported that 87% of the southeast U.S. mosquito and waterfowl impoundments included in Table 2 had salinity less than 16 psu. Thus, they are expected to be significant methane sources. It is not known what proportions of restricted wetlands related to transportation infrastructure are in freshened, drained, or relatively unaltered condition. In Massachusetts 70% of tidally restricted wetland area is described as freshwater marsh or fresh shrub, with 52% of the vegetated area colonized by invasive Phragmites australis, while the remainder remains brackish to saline marsh 48,49 . In general, where purposeful actions have not been taken to drain tidally-restricted wetlands, such as is the case with incidental restrictions caused by transportation infrastructure, flooding and freshening are the most likely result. Thus, we can approximate that 70% of restricted wetland area compiled here, or 2,650 km 2 , is in a flooded/freshened condition. To calculate minimum and maximum estimates of CH 4 emission from those wetlands, we multiplied the area by the minimum and maximum annual emissions rates of CH 4 from impounded wetlands (Table 1, Scenario 1, EF1 and EF3 pre-restoration minus post-restoration). For context, the annual emission rates from 2,650 km 2 of freshened wetlands were compared to average annual tailpipe emissions from a U.S. automobile 41 , at 4,690 kg CO 2 y −1 , with the comparison calculated as cumulative RF over a 20-y period of continuous emissions at those rates. The resulting anthropogenic wetland emission estimate was 28,000 to 145,000 tonnes CH 4 y −1 , with RF equivalent to 20 years of continuous emissions from 0.6 to 3.1 million automobiles (not shown).