Flooding wetlands in an effort to combat climate change could backfire.

Despite making up a fairly small portion of the planet's surface area, wetlands store a disproportionate amount of carbon on land. Wetland soil disturbance can release massive volumes of greenhouse gases, which is why they are at the centre of climate debates.

A significant climate initiative in Denmark under the Green Tripartite Agreement seeks to re-wet around 140,000 hectares of low-lying land, including meadows, bogs, and other peat-rich areas, primarily through flooding.

The reasoning is straightforward: prevent carbon from leaving as CO2, limit decomposition, and keep soils moist. However, a recent University of Copenhagen study contends that "flood it and forget it" can backfire. Methane, a greenhouse gas that readily develops in completely wet, oxygen-free environments, is the cause.

Why flooding seems appealing on paper

Draining peat soils allows oxygen to enter, bacteria to start working, and stored organic matter decomposes more quickly. CO2 is released into the atmosphere as a result of that breakdown.

Re-wetting slows the "burning" of old carbon by reducing the amount of oxygen in the soil. Keeping the water high, keep the carbon in the earth has long been the main justification for flooding low-lying, carbon-rich terrain.

The issue with methane

According to the Copenhagen team, pushing water all the way to the surface alters the climate narrative. Soils that are completely inundated turn into perfect methane factories.

To put it another way, you might reduce CO2 emissions but unintentionally increase methane emissions, which are much more harmful in the short run.

The majority of people currently anticipate widespread flooding of converted Danish low-lying soils. However, our research indicates that this is not a good idea," said Copenhagen-based study main author Bo Elberling.

"Methane emissions can be reduced by partially converting produced methane to the less dangerous greenhouse gas CO2 before it is released by keeping the water level slightly below ground level." Re-wetting without completely flooding may therefore produce the optimal climate result.

The actual work is being done by microbes.

Microorganisms that reside in the top soil layers are the important participants in this situation. Methane-oxidizing microorganisms that can "eat" methane before it escapes into the atmosphere are among the vast array of microbial life in soils that Danish researchers have studied.

However, this is only possible in the presence of oxygen. This natural methane "filter" is shut down if the soil surface is inundated because the oxygen in the higher layers quickly vanishes.

The suggested sweet spot is essentially just wet enough to slow down the loss of CO2 without removing oxygen from the upper layer, which is where methane conversion can occur.

Identifying the ideal water level

Maglemosen, a peat swamp roughly 20 kilometres north of Copenhagen that has remained substantially intact for more than a century, provides the team with its evidence. This is significant because it exhibits characteristics of a "baseline" wetland system.

Elberling and associates modelled a 16-year period, from 2007 to 2023, after measuring CO2 and methane emissions over a number of years.

In order to create a large dataset and determine which water levels result in the optimum overall climate balance, they also monitored water level, vegetation, soil, and air temperatures.

The most climate-friendly water level at Maglemosen, according to our data from 2007 to 2023, is about 10 centimetres below ground level. Overall, this level offers the optimal balance between CO2 and methane emissions, according to Elberling.

They emphasise that it won't be the same everywhere because different wetlands have different plants and soils, but they anticipate that the optimal range will frequently land between 5 and 20 cm below ground.

The most important factor is the direction of travel: keep the water near the surface, but typically not above it.

The difficulty of controlling water levels

Naturally, saying 'hold the water table steady' is one thing, but putting it into practice in a real landscape that alternates between summer droughts and autumn downpours is quite another.

Maintaining a steady water level in the new Danish wetlands is obviously difficult. Elberling clarified that ideal circumstances call for somewhat damp conditions but not water to the surface. "So, what do you do, for instance, during the dry summer months or during periods of heavy rain in the autumn?"

The study suggests active management, which essentially treats water level control like infrastructure and includes monitoring, drainage when necessary, and water addition when necessary. And the Netherlands serves as a paradigm for that.

If the Netherlands did not consistently maintain a rather stable water table, they would be under water. We should thus look in that direction," Elberling stated.

Flooding low-lying areas and then letting the water table fluctuate freely is not an option. Pumps that can maintain a steady water level will need to be powered by renewable energy, such solar electricity.

The methane story can be altered by plants.

There are more variables than water level. Because some species function as methane "chimneys," transferring gases from soil to air through their tissues, plant communities are important.

Canary grass predominates in Maglemosen. It has the ability to move methane upward and oxygen downward into the earth. According to the team, this can influence how much methane escapes and how much is converted initially.

About 80% of the methane in Maglemosen is expelled by the plants, and Canary grass in particular is predicted to grow in importance in the future in lowland regions that have been converted, according to Elberling.

"Therefore, a smaller percentage of methane will be converted before being released because this plant species is likely to increase the transport of methane from the soil to the atmosphere." Therefore, the dominant plants can still raise or drop emissions even when the water level is at its ideal level.

Wetlands that flood could cause new problems.

Lastly, nitrous oxide is another climate concern that the researchers have identified. Although it receives less attention than CO2 or methane, it is a very potent greenhouse gas. Additionally, as marsh water levels fluctuate, it may spike.

"Nitrous oxide emissions could significantly reduce the climate benefits if the water level in flooded lowlands is allowed to fluctuate at the whim of the weather gods in the future," Elberling said.

"Don't re-wet wetlands" is not the study's recommendation. It's saying that regulated re-wetting is the most effective. Large-scale flooding could exchange one greenhouse gas issue for another.

While allowing oxygen-loving bacteria to catch some methane before it escapes, a slightly lower, stable water table could preserve carbon. Wetlands can therefore help the climate, but only if we stop thinking that "more water" is always better.