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On a warm day in August, Anthony Stewart hikes through a forest on Washington State’s Olympic Peninsula, making his way through a tangle of ferns and grasses. Wispy, lichen-coated branches hang overhead, providing shade as he sets down his backpack and shovel, and he and his team prepare to dig.

This is one of Stewart’s favorite study sites, he says. It’s relatively dry on the surface, but just underneath it, a layer of reddish soil, full of organic matter, gives way to gray-blue, claylike soil. These layers, formed over time as water flooded the area, are signs of a wetland. But like many forested wetlands in the Pacific Northwest, this area doesn’t appear on any state maps.

In a recent study published in Nature Communications, Stewart and his team report the surprising abundance of unmapped, carbon-rich wetlands in the Pacific Northwest’s forests. The scientists studied the Hoh River watershed, which snakes westward across the Olympic Peninsula, documenting wetlands that were invisible to satellite imaging, the standard technique for measuring wetlands, owing to the thick forest canopy. Including them in estimates of the watershed’s carbon-storage capacity increases them fivefold.

Conserving forested wetlands not only protects valuable habitat; it could help stabilize the climate. But first, the wetlands must be put on the map—and that is no easy task.

Wetland ecosystems are stunningly effective at soaking up carbon from the atmosphere. Despite covering only roughly six to eight percent of the world’s land surface, they contain roughly 20 to 30 percent of the carbon stored in the soil. And because the plant matter in the waterlogged soil decays slowly, the carbon of wetland ecosystems tends to stay put.

Wetlands provide other benefits, too: More than 10 percent of all species, including 50 percent of all endangered ones, rely on wetlands. The gnarled roots of wetland trees and plants purify water, and the wetland soils absorb it, providing flood protection to nearby areas.

Since the 1920s, the US Department of Agriculture (USDA) has surveyed the types and quality of soil throughout the nation. But until recently, these soil maps focused primarily on agricultural land, leaving out most forests and leaving huge gaps in knowledge about the Pacific Northwest and Alaska. On top of that, “wetlands were not at all a focus in forested landscapes,” says David D’Amore, a soil scientist with the USDA Forest Service and a coauthor of the study.

To identify these hidden forested wetlands and estimate their carbon content, the researchers used the Wetland Intrinsic Potential tool, a wetland mapping tool that uses lidar, an aerial remote sensing technique that can resolve details underneath the tree canopy. The researchers then randomly selected 36 sampling sites across the entire Hoh River watershed, many of which were far from any sort of trail. Armed with shovels, hoses, and pumps, the researchers drove along bumpy backcountry roads and bushwhacked their way through thick woods. Once they arrived at their sampling locations, they used shovels to dig holes roughly a meter deep in the ground. “It’s really intensive to get a carbon measurement,” Stewart says. “It’s not an easy path.”

The team scooped the soil into four-liter plastic bags and carried it back to the University of Washington. In the laboratory, Stewart ground the samples to a fine powder and heated them to 1,000 °C. At that temperature, the carbon-containing substances in the soil were completely decomposed and transformed into carbon dioxide, allowing the scientists to measure the carbon content. Finally, the researchers combined the soil carbon data with remote sensing topography information to create a model of the amount of soil carbon that is stored across the watershed. “We just rediscovered these really carbon-rich forested wetland areas that weren’t being mapped by the currently available land data sets,” Stewart says.

It was a labor-intensive endeavor to come up with these maps of wetlands in the Hoh River watershed on Washington State’s Olympic Peninsula. The work required the low-tech method of digging holes and collecting dirt. Graphic by Stewart et al./Nature Communications

In the 2016 National Wetland Condition Assessment, a federal survey of the United States’ wetlands, Amanda Nahlik, an ecologist and biogeochemist in the Office of Research and Development at the Environmental Protection Agency, concluded that wetlands in the West held about six percent of the total carbon stored by wetlands in the lower 48. “We recognized we were probably underestimating the amount of carbon stored in the West,” she says. Stewart’s study confirms this hunch. “There is this landscape that’s uncharacterized that we need to start to target,” Stewart says.

In general, wetlands aren’t faring well. More than 50 percent of the wetlands in the lower 48 have disappeared since European settlement, and over the past decade, the rate of wetland loss has doubled, according to the most recent US Fish and Wildlife Service National Wetlands Inventory. Roughly half of the wetlands in the Mountain West region are in poor condition. Though there are thought to be fewer wetlands in the arid West than on the East Coast, “that does not mean that those wetlands are less important,” says Megan Lang, the inventory’s chief scientist. “In fact, it might mean that those wetlands are more important, because there are fewer of them.”

The two main drivers of wetland loss in the West, says Lang, are drought and cattle grazing. Climate change, which is expected to increase aridity in the West, could dry up half the region’s remaining wetlands by 2050. And when wetlands are destroyed, their carbon is often released into the atmosphere, further worsening global warming.

The 2023 decision by the Supreme Court of the United States in Sackett v. Environmental Protection Agency changed the federal definition of a wetland, disqualifying thousands of kilometers of ephemeral streams and millions of hectares of wetlands in the West—including those along the Hoh River—from protection under the Clean Water Act. Some federal and state initiatives are attempting to compensate: late last month, US president Joe Biden announced a goal to restore more than 32,000 square kilometers of wetlands over the next six years. And early in May 2024, Colorado became the first state to pass legislation protecting the wetlands excluded by the previous year’s Supreme Court decision.

Lang emphasizes that it’s crucial to map, measure, and conserve the wetlands we still have. “If we’re going to maintain resilience to climate change, if we are going to have clean water for the future, if we’re going to keep feeding our families, if we are going to be safe from flooding, we are going to need to do better in terms of wetland conservation,” Lang says.