Millions of Lakes, Shallower Than Expected
The Earth is home to more than 100 million lakes of at least half an acre in size. Thanks to satellite data, the surface areas of these lakes are fairly well known. But recent research suggests that they’re much shallower than previously thought, raising concerns about their role in the carbon cycle and their ability to handle pollution.
The research provides one of the most statistically rigorous estimates of the total volume and average depth of Earth’s lakes. And it adds to findings in the past decade suggesting that lakes are an often overlooked contributor to the carbon cycle — in some ways, even more significant than oceans.
B.B. Cael, a physical oceanography doctoral student at MIT and the Woods Hole Oceanographic Institution, and his colleagues developed and tested the mathematical approach to estimating the global volume and average depth of Earth’s lakes. The model is less expensive and less time consuming than some direct-measurement approaches, which can include boating around lakes and repeatedly making SONAR soundings. Not to mention that many lakes are located in hard-to-reach places like the Arctic and remote forests.
The new approach starts with math that describes Earth’s topography. Cael used that tool to derive a volume estimate for Earth’s lakes, based on established surface area data for 28 million of them. The volume figure easily led to a calculation of the average depth of all lakes.
His team then tested the volume and depth predictions against the most up-to-date surveys of lakes in regions including the Adirondack Mountains of New York, Wisconsin, Sweden, and Quebec, as well as data in the EPA’s Eastern and Western Lakes Survey. The researchers found a strikingly strong match between their estimates and the survey data. The approach also yields figures that match up well with recent spacecraft measurements of dry lake beds on Mars, Cael says.
If you enclosed Earth’s overall lake water volume into a sphere, it would have a diameter close to the distance between Boston and Providence, Rhode Island, says Cael’s co-author Adam Heathcote, an ecologist at the Science Museum of Minnesota. The findings thus underscore the relative scarcity of lake water, the scientists wrote in January in Geophysical Research Letters.
The team’s global average lake-depth prediction — 41.8 meters, or 137 feet — was particularly surprising. It came in about 30 percent lower than previous estimates, suggesting that there are more small- and medium-sized lakes on the planet than previously thought.
Shallower lakes present problems.They have less capacity to dilute pollution that runs off fertilized farmland and other land, which can result in less clean freshwater for supporting ecosystems. And shallower lakes hold implications for Earth’s carbon cycle, including the storage and emissions of methane. Bacteria that live in dark, oxygen-deprived lake bottoms emit the gas, meaning that shallower waters might allow an unexpectedly large amount of it to bubble to the surface and escape into the atmosphere. That’s a particular concern because methane contributes more powerfully to global warming than does the same amount of carbon dioxide.
“If this bubbling component is more important than previously thought and also is relatively under-measured,” Heathcote says, “then we could be missing a big piece of the overall lake greenhouse gas emission puzzle.”
The research was funded in part by the National Science Foundation, which was not designated for significant cuts in President Trump’s proposed 2018 budget released last month. But basic research on a broad array of natural and physical science topics including earth science, ecology, oceanography, limnology (the study of lakes), and climate science would suffer under the cuts to other agencies. In addition to the planned EPA cuts, the budget details a 16 percent cut to the budget for the National Oceanic and Atmospheric Administration, a 10 percent cut in funds for the U.S. Geological Survey, a 5 percent cut in funds for NASA’s earth sciences division, and a 1 percent cut to NASA’s budget. If scientists are to continue to figure out even some of the most fundamental features of the planet, such shallow thinking won’t do.
This post has been updated to include the correct link to the January paper in Geophysical Research Letters.