Flooding bogs and meadows has been seen as a climate fix. New research from Denmark suggests a smarter strategy: keep wetlands wet, but not underwater.
Wetlands have long been cast as climate villains when drained and climate saviors when flooded. New research from Denmark suggests the reality is more nuanced — and that the best climate strategy is to keep wetlands wet, but not underwater.
A team led by Bo Elberling of the University of Copenhagen has found that fully flooding low-lying peatlands can unintentionally boost emissions of methane, a greenhouse gas far more potent than carbon dioxide. Their work, published in Communications Earth & Environment, points to a different solution: carefully managing water levels so they sit just below the soil surface.
The findings come as Denmark plans to rewet about 140,000 hectares of low-lying land, including bogs and meadows, under a national climate agreement. Similar rewetting projects are underway or being considered across Europe and beyond as countries look for nature-based ways to cut greenhouse gas emissions.
Wetlands cover only a small fraction of Earth’s land surface but store a huge share of its soil carbon. When these peat-rich soils are drained for farming or development, oxygen rushes in, microbes break down centuries of stored organic matter, and carbon dioxide escapes into the air. Rewetting slows that decay and has been promoted as a way to lock carbon back into the ground.
The new study shows that simply flooding these landscapes is not enough — and can even backfire.
“Most people currently expect that converted Danish low-lying soils will be flooded on a large scale. But our research shows that this is not a good idea. By keeping the water level slightly below ground level, methane produced can be partly converted to the less harmful greenhouse gas CO2 before it is released, thereby limiting methane emissions,” Elberling, a professor in the Department of Geosciences and Natural Resource Management at the University of Copenhagen, said in a news release.
Methane forms in waterlogged, oxygen-poor soils as microbes break down organic matter. In many wetlands, other microbes living closer to the surface can consume some of that methane and convert it to carbon dioxide, but they need oxygen to do it. If the upper soil layers are also flooded, oxygen disappears and this natural methane filter shuts down.
To understand how water levels shape this balance, Elberling and colleagues turned to Maglemosen, a peatland about 20 kilometers north of Copenhagen. The site has been largely undisturbed for more than a century and represents a typical Danish wetland with peat soils.
At Maglemosen, the team continuously measured carbon dioxide and methane emissions over several years and compiled a detailed record of water levels, plant communities, and soil and air temperatures. They then used this large data set to build and test a model that simulates greenhouse gas emissions under different water table conditions over a 16-year period from 2007 to 2023.
“Based on our data from 2007 to 2023, we can see that the most climate-friendly water level in Maglemosen is around 10 centimeters below ground level. This is the level that overall provides the best balance between methane and CO2 emissions,” Elberling added.
The exact optimal depth will vary from wetland to wetland, the researchers note, likely falling somewhere between 5 and 20 centimeters below the surface. But the broader message is consistent.
“A stable water level below ground level will almost always provide the greatest climate benefit,” added Elberling.
Hitting that climate “sweet spot” is not just a scientific challenge but an engineering one. Water tables naturally rise and fall with rain, drought and seasons. Climate change is expected to make those swings more extreme in many regions, complicating efforts to keep wetlands in the narrow range that minimizes total greenhouse gas emissions.
“It is clearly a challenge to ensure a stable water level in the new Danish wetlands. Optimum conditions require quite wet conditions but not water to the surface. So, what do you do, for example, in the dry summer months or in the autumn with heavy rain events?” Elberling added.
Managing water at that level of precision will require infrastructure and energy. Elberling points to the Netherlands, where pumps, ditches and dikes are used to maintain a steady water table in a country that would otherwise be largely underwater.
“The Netherlands would be under water if they did not constantly maintain a fairly stable water table. That is why we should look in that direction. We cannot just flood low-lying areas and then leave the water table to fluctuate freely. It will be a matter of using green energy, such as solar energy, to power pumps that can keep the water level stable,” he said.
The study also highlights how plants and other greenhouse gases complicate the picture.
In Maglemosen, a grass species known as Canary grass dominates. Like rice, it can move gases through its tissues, acting as a kind of pipeline between soil and air. That matters for methane.
“In Maglemosen, around 80 percent of the methane is released via the plants, and in particular Canary grass is expected to become more dominant in converted lowland areas in the future. Therefore, this plant species is likely to increase the transport of methane from the soil to the atmosphere, meaning that a smaller proportion of methane will be converted before being released,” Elberling added.
As rewetting projects proceed, shifts in plant communities could therefore change how much methane escapes and how much is consumed in the soil, even if water levels are managed carefully.
On top of carbon dioxide and methane, the researchers warn that nitrous oxide — another greenhouse gas — must be considered. Nitrous oxide is far more powerful than carbon dioxide over a 100-year period and tends to spike when soils repeatedly switch between wet and dry conditions.
“If the water level in flooded lowlands in the future is allowed to fluctuate at the whim of the weather gods, nitrous oxide emissions could significantly reduce the climate benefits,” Elberling added.
Taken together, the findings suggest that rewetting peatlands is still a crucial climate strategy, but it has to be done with precision. Rather than a simple choice between drained fields and permanent ponds, the most effective approach may be to design and manage wetlands as dynamic but controlled systems.
That means monitoring water tables, planning for future plant changes, and investing in infrastructure — ideally powered by renewable energy — to keep conditions in the range that maximizes climate gains.
For students and policymakers looking at nature-based climate solutions, the Danish work underscores a broader lesson: ecosystems are powerful allies in the fight against global warming, but they respond to details. Getting those details right could determine whether restored wetlands become long-term carbon sinks or unexpected sources of potent greenhouse gases.
Source: University of Copenhagen