When left undisturbed, wetlands can store a significant amount of carbon for centuries, if not millennia. However, wetlands are also a major source of emissions, particularly methane, a potent greenhouse gas. Whether a wetland is a net source of cooling or warming primarily depends on the balance between its greenhouse gas emissions and avoided carbon loss. This balance depends on local circumstances, and therefore, different wetlands require different approaches for mitigating climate change. Ultimately, we found that wetland protection strategies that focus on (1) restoring and conserving coastal wetlands or (2) conserving inland freshwater wetlands could be impactful in preventing carbon loss.

Blue carbon credits, which focus on ocean and coastal ecosystems, are an increasingly popular method of restoring and protecting coastal wetlands such as mangroves and saltmarshes. However, blue carbon credits run into the same issues of additionality, permanence, and leakage that other types of nature-based carbon offsets face. Overall, we suggest a cautious approach to evaluating projects that support coastal wetland restoration and protection through blue carbon credits. In order to get our recommendation, a project would need especially good data on its actual increase in carbon storage, a plan to maintain permanence, and information on how it has addressed potential issues with additionality and leakage. Additionally, policy could have high leverage in restoring and protecting wetlands, but we have not found any organizations that are trying to maximize the climate impact of wetland restoration. We therefore do not believe that policies focused on wetlands is an especially promising area for climate philanthropy.

This report last updated April 21, 2022. Questions and comments are welcome.

Download the full report to access the full list of works cited.

Image: Mohmed Nazeeh

Background

What are wetlands?

Definitions for wetlands tend to vary between different agencies, but common elements include the following (Cherry, 2011):

• Presence of water at the surface or root zone,

• Soil that is dominated by anaerobic processes, and

• Vegetation adapted to flooded, anaerobic conditions.

Wetlands can be roughly categorized as either coastal/tidal wetlands or inland/non-tidal wetlands. Coastal wetlands are found along coastlines, while inland wetlands are found alongside inland bodies of water, such as rivers and lakes, and in low-lying areas that collect groundwater and/or precipitation. Specific types of wetlands include:

• Peatlands (bogs and fens),

• Mineral wetlands (marshes and tundra),

• Seasonal or permanent flood plains,

• Mangrove swamps, and

• Saltmarshes.

Wetlands can also be described as freshwater or saltwater wetlands.

Why are wetlands important for climate change?

Overview

Organic matter such as dead plant debris decays slowly in wetlands, enabling long-term carbon storage in wetland soils. At the same time, wetlands release greenhouse gases (GHGs) such as methane (CH4), carbon dioxide (CO2), and nitrous oxide (N2O) into the atmosphere. Whether a wetland has a net cooling or warming effect largely depends on its balance between carbon storage rate and GHG emissions. Human activities have led to a loss in the global surface area of wetlands and reduced their total carbon storage.

Carbon storage in wetlands depends on the balance between carbon inputs and outputs.

Relative to their area, wetlands store a disproportionate amount of Earth’s total soil carbon content. It is estimated that wetlands hold “between 20 and 30% of the estimated 1,500 Pg of global soil carbon despite occupying 5–8% of its land surface” (Nahlik & Fennessy, 2016).

Carbon storage in wetlands equals the amount of carbon that has entered minus the amount that has left the system (Were et al., 2019). For example, carbon enters wetlands when wetland plants take in atmospheric CO2, which is then assimilated into the wetland plants’ roots and tissues. When these plants die, the dead plant debris becomes part of the soil’s organic matter. Carbon outputs include CH4 and CO2, which are emitted from wetlands when microorganisms in the soil decompose organic matter. Additional carbon inputs and outputs include dissolved and particulate forms of inorganic and organic carbon.

Wetlands release greenhouse gases into the atmosphere.

Wetlands emit the following GHGs:

• Methane – Wetlands are the largest natural source of methane, which has a global warming potential (GWP) at least 28 times greater than CO2 over a 100-year time frame (Zhang et al., 2017). CH4 is produced in wetland soils by microorganisms, specifically Archaea, as they break down organic matter in the absence of oxygen. Therefore, restoring a wetland can reestablish CH4 emissions.

  • Coastal wetlands release relatively little CH4 because they typically contain brackish or salty water; sulfate in saltwater suppresses methane production (Oremland & Polcin, 1982).

• Carbon dioxide – When wetland soils are exposed to oxygen, such as when wetlands are drained or disturbed, the carbon in the soil’s organic matter is oxidized, and CO2 is released into the atmosphere.

• Nitrous oxide – Microorganisms in wetland soils can reduce nitrate and nitrite to N2O via denitrification. N2O has a GWP at least 265 times greater than that of CO2 over a 100-year time frame. High nitrate loads can come from agricultural runoff or wastewater treatment discharge. Drained peatlands also release N2O.

Wetlands can switch from having a net warming effect to a net cooling effect.

Wetlands can help mitigate climate change, but this depends on the balance between their carbon storage and GHG emissions. A wetland’s age at which it changes from having a net warming effect to a net cooling effect is its “switchover time.” At that point, the amount of cooling due to sequestered carbon (e.g., avoided carbon loss) exceeds the amount of heating from its GHG emissions (Moomaw et al., 2018). In general, saltwater wetlands have shorter switchover times than freshwater wetlands because CH4 production tends to be inhibited in saltwater wetlands. According to a meta-analysis on the climate change mitigation potential of wetlands and their switchover times,

The shortest time periods calculated were for mangroves (0 year, meaning that mangroves never have a net warming effect) and saltmarshes (17 years). Peatlands (boreal and temperate) and freshwater marshes had important switchover time variability between study sites with a median value of 298.2 ± 100.6 and 2184 ± 1029 years, respectively (Taillardat et al., 2020).

Although peatlands have a relatively long switchover time, it has been argued that they should still be immediately rewetted (restored) because they store especially large amounts of carbon and rewetting would curb CO2 and N2O emissions. Namely, rewetting would come with the temporarily high cost of CH4 emissions but this could be outweighed by the reduction in CO2 and N2O emissions (Günther et al., 2020). However, this effect depends on local circumstances (Ojanen & Minkkinen, 2020).

Human activity is affecting the extent and health of global wetlands.

The world has lost between 54 and 57 percent of its wetlands since 1900 AD (Davidson & Davidson, 2014). Stored carbon in wetlands is lost and emitted into the atmosphere as CO2 when wetlands are converted and degraded. The rate of carbon loss depends on how the wetland was disturbed. For example, peat extraction abruptly removes carbon from a wetland, whereas drainage leads to gradual carbon loss as the carbon in the soil is exposed to air and oxidized to CO2. It is estimated that conversion and degradation of vegetated coastal ecosystems (e.g., marshes, mangroves, and seagrasses) has led to emissions of 0.15 to 1.02 billion tons of CO2 annually; this is equivalent to 3 to 19 percent of emissions from global deforestation (Pendleton et al., 2012). In contrast, wetlands can store carbon for millennia when left undisturbed (Ezcurra et al., 2016).

Importance

The Intergovernmental Panel on Climate Change (IPCC) reported that protecting and restoring peatlands and coastal wetlands could potentially mitigate between 1.02 and 9.56 billion tons of CO2-equivalent per year (Nabuurs et al., 2022). The IPCC reported their mitigation potential in terms of technical potential (how much can be mitigated without considering financial and other constraints) and economic potential (how much can be mitigated at a cost up to $100 per ton of CO2-equivalent). These values can be found in the table below:

Table 1: Mitigation potential of various wetland interventions

Tractability

How can wetlands be used for mitigating climate change?

Inland and coastal wetlands demand different strategies.

It may take restored inland freshwater wetlands decades, if not centuries, to have a net cooling effect because of their long switchover times (Taillardat et al., 2020). Therefore, climate change mitigation efforts should focus on preserving existing inland freshwater wetlands that are centuries to thousands of years old because they already have a long-term cooling effect. In contrast, coastal wetlands tend to have shorter switchover times. For example, mangroves are immediately net sinks of carbon, while it takes about a couple of decades for salt marshes to become net sinks. Therefore, it would be appropriate to both restore and conserve coastal wetlands.

Rewetting peatlands may also be an effective strategy for avoiding CO2 and N2O emissions. However, compared to mangroves, the dynamics of their GHG emissions are more complex because rewetting peatlands would reestablish CH4 emissions.

Wetland manipulation can modify carbon inputs and outputs.

Researchers are currently investigating methods for modifying carbon inputs and outputs in wetlands. Examples of how wetlands can be manipulated include the following (Were et al., 2019):

• Biotechnology – Managing wetland vegetation and soil microbes can potentially reduce GHG emissions and enhance carbon storage.

• Biochar – Biochar is a charcoal-like material made by burning biomass in the absence of oxygen. Studies have found that adding biochar to rice paddies, an artificial wetland, can reduce CH4 emissions (Pratiwi & Shinogi, 2016; Qin et al., 2016). Farmers would probably resist adding biochar to their rice paddies if it compromises the quality and quantity of their rice yields.

• Fertilization – Fertilization can enhance plant growth and the amount of biomass available for carbon storage. However, nitrate from fertilizers can contaminate drinking water surfaces and lead to eutrophication. Additionally, fertilizers can increase the release of N2O.

• Humic acids – Humic acids can help prevent organic matter from decomposing as rapidly. Currently, there is limited understanding of how humic acids affect nutrient availability and uptake in wetland soils.

What are some obstacles to restoring or conserving wetlands?

Wetlands are often converted for economic reasons, such as resource extraction, agricultural, agricultural development, infrastructure construction, and settlement expansion (Asselen et al., 2013). Efforts to restore or conserve wetlands may be unsuccessful if the underlying reasons for wetland conversion are left unaddressed.

What types of wetland are most promising for mitigating climate change?

Mangrove restoration may be especially promising for mitigating climate change because they have a switchover time of zero years. As a result, restoring mangroves never leads to net warming, and they have higher cost-effectiveness than other wetlands in reducing GHG emissions (Taillardat et al., 2020). Notably, mangrove restoration is cheaper in developing countries than in developed countries by two orders of magnitude (Taillardat et al., 2020). Currently, about 8,120 square kilometers of former mangrove area are considered restorable (Worthington & Spalding, 2018).

We are unsure what type of wetland would be most promising to conserve if we wanted to optimize for reduced emissions and cost.

Neglectedness

Most wetland organizations do not focus on climate change mitigation.

Global organizations that work on conserving and restoring coastal and inland wetlands include Wetlands International. At a national level, the Audubon Society and Ducks Unlimited have both worked to protect US wetlands for the sake of birds. In addition, regional and local organizations also work on wetland restoration and conservation, such as the Wetlands Initiative, which focuses on wetlands in the US Midwest. However, relatively few wetlands organizations have a keen focus on climate change and typically focus on conserving biodiversity instead. This likely explains why many organizations that work on wetlands tend to restore and conserve both inland and coastal wetlands instead of optimizing for climate change mitigation.

Blue carbon credits are becoming more popular but are not a perfect solution.

Blue carbon, or carbon captured by the ocean and coastal ecosystems, has received greater attention over the past year because of its potential for carbon sequestration and avoided emissions (Cerf, 2021; Jones, 2021; McVeigh, 2021). For example, blue carbon credits help pay for mangrove restoration and conservation projects. However, blue carbon credits most likely face the same challenges with additionality, permanence, and leakage as forestry carbon credits. In addition, there are measurement uncertainties in determining how much carbon is being stored in these ecosystems. For example, one study found that estimates of carbon stored in mangrove roots led to root biomass values that were 40 ± 12 percent larger than those obtained from field measurements (Adame et al., 2017). In order for Giving Green to recommend a particular blue carbon credit, it would need to have exceptional evidence that addresses concerns related to additionality, permanence, and leakage.

Wetlands other than mangroves are more challenging to incorporate into existing emission reduction frameworks because they can be both a sink and source of GHGs.

There are policy efforts to restore and protect wetlands.

Policy efforts that could help protect wetlands include the following:

• Incorporating wetland actions into climate mitigation and resiliency plans – According to the Convention on Wetlands, policymakers should include goals for restoring and protecting wetlands in national policies, including plans for climate action plans, adaptation, and disaster risk reduction (Convention on Wetlands, 2021). Wetlands also provide benefits such as shoreline protection and flood storage and can therefore be incorporated into land and water use management solutions.

• Establishing protected areas or species – Establishing a wetland as a protected area would prevent development that would lead to degradation. Protecting a species, such as a specific type of plant or animal, could further limit environmental degradation.

• Regulating to protect wetlands – Regulations against upstream pollution and restrictions on water allocation can indirectly safeguard wetlands.

• Addressing development concerns – Wetland conversion is typically driven by economic factors, such as land demand for farming. Policies that address the needs of local communities in ecologically sound ways could help reduce wetland destruction and degradation.

• Addressing knowledge gaps – Research can help improve plans to protect wetlands, reduce emissions, and increase carbon storage. Building additional knowledge on wetlands’ co-benefits could also make a stronger case for restoring and conserving them.

We have not found promising policy initiatives for mitigating climate change via wetlands.

Cost-effectiveness

The IPCC reports that there is insufficient data on the cost of peatland and coastal restoration and blue carbon in coastal wetlands. However, according to an article by S&P Global, blue carbon credits are being traded at a cost between $13 and $35 per metric ton of CO2 (Favasuli, 2021). Using CarbonPlan’s permanence calculator, when we assume a discount rate of 3% and risk of project failure of 10% per year, the cost of permanent CO2 removal ranges from $65 to $232 per metric ton. We are unsure how much it would cost to conserve wetlands due to a lack of available data.

Conclusion

Different types of wetlands require different approaches for climate change mitigation. For example, restoring inland freshwater wetlands is generally not a suitable short-term solution for climate change mitigation because they tend to be net sources of heating for a long time before they become net sinks. From a climate perspective, it would be better to conserve those types of wetlands. In contrast, coastal wetlands tend to have shorter switchover times and are better suited for both restoration and protection. Mangroves may be an especially suitable target for restoration because they have a switchover time of zero years, meaning that they never have net warming effects. It is unclear how wetland restoration costs compare to conservation because the two are often lumped together.

Donors can support wetland restoration and mitigation through the purchase of carbon credits, or through supporting policy organizations. Blue carbon credits are of particular interest because they focus on coastal ecosystems, and coastal wetlands tend to have shorter switchover times than inland freshwater wetlands. However, blue carbon credits suffer from issues with additionality, permanence, leakage, and carbon measurement. While there are a number of policy organizations working on wetlands, we have not found any that focus their activities explicitly on climate mitigation, and therefore do not believe that there are high-leverage opportunities in wetland policy at the moment.