Panorama of a wetland
Credit: herbert2512/Pixabay

Wetlands—now known for their biodiversity, flood protection, and water purification—were once thought to be unproductive lands brimming with disease.

“If you believe those estimates, then that means that there used to be 3 or 4 or 5 or 6 times more [wetlands] in the past.”

They’ve long been drained to pave the way for cities, croplands, and pasture, but researchers haven’t had a good estimate of how much of the world’s wet and wild places have disappeared since preindustrial times. Now, an analysis has suggested that some 3.4 million square kilometers—an area roughly the size of India—have been lost since 1700.

Previous studies had suggested that something like 70% or 80% of total wetlands had vanished, said Etienne Fluet-Chouinard, a limnologist at the Swiss Federal Institute of Technology (ETH) Zurich in Switzerland. Some of these figures were based on extrapolating local losses to a global scale, whereas others had been plucked out of thin air, he said. “If you believe those estimates, then that means that there used to be 3 or 4 or 5 or 6 times more in the past.”

To get a better estimate, Fluet-Chouinard and his colleagues dug through a slew of data sources. First, they pulled together 121 regional reports on percentage of wetland area lost. Then they gathered thousands of records from countries and smaller areas on wetlands drained or altered for four key reasons: forestry; harvesting peat; the cultivation of upland or dry crops, which require good drainage and less irrigation; and the cultivation of wet crops, which may retain some of a wetland’s qualities. The researchers used these data to create a timeline of wetland losses for these four drivers. But they aren’t the only factors driving wetland loss, so the team used the regional reports to model wetland areas that had been converted for pasture, for urban areas, or specifically to grow rice.

The team looked to current maps of Earth’s wetland coverage and previous studies that simulated past coverage to figure out where wetland loss had occurred and surmise patterns in areas drained or degraded across time. This sort of research, which connects the big picture to local changes, is “extremely valuable,” said Nandita Basu, an environmental engineer at the University of Waterloo in Canada who was not part of this work. Understanding where wetlands have been lost can help guide restoration efforts, she said.

Hot Spots for Losses

Overall, the team’s analysis suggested that around 21% of Earth’s wetlands have been lost since preindustrial times, but that number could be as high as 35%, Fluet-Chouinard said. The conversion of wetlands accelerated around 1900, the researchers reported in Nature.

A world map illustrating where wetlands have been lost, drained, converted, and degraded since 1700.
The new research documents where wetlands have been lost, drained, converted, and degraded since 1700. Credit: Etienne Fluet-Chouinard

“Sometimes wetlands are disproportionately important in terms of some of those Earth system processes.”

“I think that the rates are much higher,” said Irena Creed, an ecosystem scientist at the University of Toronto Scarborough who wasn’t part of the work. Fluet-Chouinard’s team may be missing wetlands because of gaps in the data they used, she suggested. There were few regional estimates from Africa, for instance, and in Canada, where Creed works, there isn’t a robust inventory of wetlands to include with the analysis. Also, the team’s definition of what constitutes a wetland excludes small areas, she said. “Sometimes those [small] wetlands are disproportionately important in terms of some…Earth system processes,” such as mitigating greenhouse gas emissions, floods, and droughts, she explained.

“This [study] has put a spotlight on a very important need to be able to map wetlands,” Creed said. And mapping wetlands is necessary for conservation and climate change mitigation. “Governments urgently need knowledge about wetlands and their role in greenhouse gas sequestration as well as release,” she explained. Some countries are spending billions on nature-based climate solutions, including wetland restoration, for instance, but they need better data to benchmark improvements.

Even though the total percentage of wetlands lost may not be as high as previously thought, people shouldn’t be complacent in protecting them, Fluet-Chouinard said. The study identified many regions with very high losses. Parts of Illinois and Indiana, for instance, may have lost more than 75% of their wetlands, as have places in the Indus River basin in India and Pakistan and the Yangtze River basin in China. Losses are still accelerating in places such as Indonesia and Malaysia, where wetlands are converted for oil palm cultivation. Regions where large areas of wetlands appeared to be relatively untouched include parts of northern Canada, Russia, and the Congo Basin.

“If you’ve lost wetlands, you’ve lost a number of services,” Fluet-Chouinard said. Some wetland conservation policies push the creation of wetlands elsewhere to offset those that are destroyed, but one wetland isn’t a substitution for another in terms of benefits, such as supporting endemic species and biodiversity or regulating nutrient flow.

This sort of analysis could help point to an appropriate baseline for wetland restoration efforts, Fluet-Chouinard said. The COP15 Convention on Biological Diversity set a goal of protecting 30% of Earth’s ecosystems, but it uses the 1970s as a baseline. For wetlands, “by 1970, the show [was] over,” he said. “It [was] already an impoverished world in 1970.”

—Carolyn Wilke (@CarolynMWilke), Science Writer

Citation: Wilke, C. (2023), Mapping wetland loss across three centuries, Eos, 104, Published on 10 March 2023.
Text © 2023. The authors. CC BY-NC-ND 3.0
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