Historical reconstruction provides new insights
Now understood to be vital sources of water purification, groundwater recharge, and carbon storage, wetlands were historically seen as unproductive areas teeming with disease-bearing insects and good only for draining to grow crops or harvest peat for fuel or fertilizer. Over time, unrelenting drainage for conversion to farmland and urban areas along with alteration caused by fires and groundwater extraction have made wetlands among the world’s most threatened ecosystems.
Until now, a lack of historical data has hindered efforts to understand the full global impact of wetland loss, forcing scientists to make estimates based on incomplete collections of regional data. In a first of its kind historical reconstruction, the team, bringing together researchers from Stanford, Cornell, and McGill universities, combed through thousands of records of wetland drainage and land-use changes in 154 countries, mapping the distribution of drained and converted wetlands onto maps of present-day wetlands to get a picture of what the original wetland areas might have looked like in 1700.
Decline in wetlands – less than previously thought
The researchers found that the area of wetland ecosystems has declined by between 21-35 per cent since 1700 due to human intervention. That’s far less than the 50-87 per centlosses estimated by some previous studies. The lower estimate likely results from the study’s expanded focus beyond regions with historically high wetland losses, and its avoidance of large and possibly misleading extrapolations. Still, the authors estimate that at least 3.4 million square kilometres of wetlands have been lost globally over the past 300 years—an area about the size of India. Five countries with the highest losses, USA, China, India, Russia, and Indonesia, alone account for over 40 per centof global losses.
The map shows both the differences in the geographic distribution of anthropogenically-impacted wetlands versus non impacted wetlands and the estimated cumulative percentage of wetland loss between 1700 to 2020. Regions where there have been high rates of loss are shown in yellow to red and regions with dense present-day wetland and with low rates of loss are shown in blue.
“Many regions of the world have sustained dramatically high wetland losses, but our results suggest that losses are lower than previously thought once aggregated globally. Yet, it remains urgent to halt and reverse the conversion and degradation of wetlands, particularly in high-loss regions. The geographic disparities in losses are critical because the disappearance of ecosystem services caused by wetland drainage in one location cannot be replaced by the existence of wetlands elsewhere,” said lead author Etienne Fluet-Chouinard, a postdoctoral associate in Stanford’s Department of Earth System Science at the time of the research, who conceived of this study during his master’s degree in McGill’s Department of Geography.
Canada is estimated to have lost 3.8 per cent (83,000 km2) of its wetland cover since 1700 but given the enormity of the approximatively 2 million square kilometres of its wetland cover, Canada contributes ~2 per cent of global losses. However, these estimates from the new study do not include losses and degradation from mining and fossil fuel extraction. Much of Canada’s untouched wetlands are peatlands, bogs and fens located in boreal and arctic regions. Most of these wetlands may not be directly threatened by land use conversion but might be impacted by climate change, through permafrost thaw, droughts, and fires.
Another chance to act on wetland loss
“Wetlands, in their natural state, are among the most important ecosystems to regulate our water resources, which benefits both humans and the environment,” adds coauthor Bernhard Lehner, a global hydrologist at McGill University. “Discovering that fewer wetlands have been historically lost than we previously thought gives us a second chance to take action to ensure wetland cover does not decline further. As part of that, we need to improve our capacity to map their past and current extents and monitor their status using satellites. This will allow us to establish meaningful conservation goals and restoration targets.
“Wetlands, and in particular peatlands, can take up carbon dioxide from the atmosphere and store it as soil carbon for millennia, but wetlands are also emitters of methane, another greenhouse gas. Drainage of these unique water-logged ecosystems alters their greenhouse gas budgets and their impact on climate. Our new results will help quantify how drained wetlands influenced Earth’s climate in the past,” said study coauthor Avni Malhotra, a senior scientist at the University of Zurich and PhD graduate in geography from McGill.
Accurately estimating the extent, distribution and timing of wetland loss is key to understanding their role in natural processes and the impact of wetland drainage on the water and carbon cycles. These new results will help evaluate the cumulative impacts of historical wetland loss on greenhouse gases and can help plans to protect or restore these fragile ecosystems that are crucial to human health and livelihood.