Using satellite photos and computerized mapping technologies, an international research team counted all of the lakes on Earth. They found about 117 million lakes, covering almost four percent of the world’s land surface, not counting the glaciers on Greenland and Antarctica, according to a new study.
It is the first time the world’s lakes have been counted using a reliable method, the study claims.
The research team from Sweden, Estonia, France and United States undertook the study to help them understand more about the role of lakes in the global carbon cycle and other natural processes.
“If we are to be able to make realistic estimates of the collected effects of the different processes in lakes, for example their contribution to global warming, we first need a good map. We now have that. And it differs significantly from the assumptions previously made regarding the number and size distribution of lakes,” said study leader Lars Tranvik, professor of limnology at Uppsala University in Sweden, in an article published on the American Geophysical Union website.
When microorganisms break down organic substances in nature, carbon dioxide and methane are released into the atmosphere. Research teams such as Tranvik’s are finding that lakes contribute significantly to these and other natural processes.
The actual counting was done by Charles Verpoorter, now a researcher at the University of Lille Nord in France and lead author of an article about the lake inventory, which has been accepted for publication in Geophysical Research Letters. He and another co-author, David Seekell of the University of Virginia, are also members of the Global Lake Ecological Observatory Network (GLEON).
Mapping the shapes and sizes of lakes allowed the research team to determine the length of shoreline, another key finding of the study.
“The border area between land and water is an important zone for many [ecological] processes,” said Verpoorter.
If you were to add up the length of shoreline on all of the world’s lakes, the total is roughly 250 times the length of the equator, according to Verpoorter. Because the shoreline measurement is constrained by the scale of the study, it would be an even higher number at a higher resolution, Tranvik explains.
Technological Advances and Tremendous Effort Make World Lakes Count Possible
Previous estimates were thought to be either too high or too low, so the research team developed a method that was reliable in counting lakes down to a size of 0.2 hectares, or one-half acre, in size — roughly the size of one and a half Olympic-sized swimming pools or half a football field. This size threshold would include what are commonly referred to as lakes, reservoirs and ponds.
Tiit Kutser, a member of the research team and remote sensing expert at the Estonian Marine Institute of the University of Tartu, started working on a more reliable method for counting lakes when he returned home from working at Uppsala University ten years ago.
The breakthrough came with financing from the Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning “Color of Water” program.
As seen in the above images from the Advanced Land Imager (ALI), a demonstration mission of NASA’s Landsat 8 technology, it is quite easy to find waterbodies, which appear black from space, whereas land appears much brighter. But turbid or shallow lakes can appear bright as well. Shadows from clouds and mountains, and dark, coniferous forests can appear as dark as lakes, making the analysis quite difficult, Kutser explains.
The ALI images were not used in the current study, but the research team is now using Landsat 8 images for the next phase of their work. It will be a few years before cloud-free data are available for most of the Earth. With this newer technology, Kutser is hoping to be able to estimate global lake carbon content directly from space, something that was not possible with earlier technologies.
How Do the Numbers Compare to Previous Counts?
The latest number is smaller than the previous statistical estimate of 304 million lakes, but the total surface area of the world’s lakes is larger than previously thought, according to Kutser.
The world’s lakes have a combined surface area of about 5 million square kilometers (about 2 million square miles), which is 3.7 percent of the Earth’s non-glaciated land area, the study says. (The Caspian Sea is excluded from the count.)
Another key finding of the study is that large and intermediate lakes dominate the total surface area of lakes on Earth, contrary to what was found using statistical methods.
In terms of distribution, the study found that “the highest concentration, area, and perimeter of water bodies appear at boreal and arctic latitudes” between 45 degrees to 75 degrees north latitude.
The study also found that “size-distribution of water bodies decrease drastically across altitude, where 85 percent of lakes, and 50 percent of lake area, and 50 percent of total lake perimeter are located at altitudes lower than 500 meters (about 1,600 feet) above sea level.”
Of the total, about 90 million lakes are in the smallest size category, between 0.2 and 1 hectare (0.5 to 2.5 acres) in size, according to the study. This means that about 27 million of the world’s lakes are what I would refer to as “bigger than a farm pond.”
Size Matters in Estimating the Role of Lakes in the Global Carbon Cycle
Why is it important to know how big the lakes are? In general, small, shallow lakes tend to be rich in nutrients, and light penetrates through a majority of the volume, Kutser wrote in an email. It means that these lakes are where lots of carbon-related processes take place. “In large, deep lakes, these processes occur much slower, as water is usually more dilute, light is available only in upper layer of the water column, and the surface-to-volume ratio is quite small. The latter is important because degassing of carbon dioxide and methane takes place through the lake’s surface,” he said.
The 2007 report of the Intergovernmental Panel on Climate Change (IPCC) essentially ignored rivers and lakes in terms of global carbon models. “It treated lakes and rivers as pipes transporting carbon from land to the oceans,” said Kutser. The 2013 IPCC report acknowledged the important role lakes play in the global carbon cycle, but future reports will need to be refined based on the latest count, as well as other new research findings.
With the results of this latest study, researchers will be able to more accurately estimate the amount of water contained in lakes throughout the world. Over the next few years, the study team also will be able to reassess the amounts of methane and carbon dioxide that are released to the atmosphere from lakes.
About a decade ago, I was awed by the first comprehensive map of the world’s lakes, which included 250,000 lakes over 10 hectares (about 25 acres) in size. It’s amazing to see how advances in technology and international collaboration are allowing us to gain a greater appreciation of the importance of lakes on this planet.
Lisa Borre is a lake conservationist, freelance writer and avid sailor. With her husband, she co-founded LakeNet, a world lakes network, and co-wrote a sailing guide called “The Black Sea” based on their voyage around the sea in 2010. A native of the Great Lakes region, she served as coordinator of the Lake Champlain Basin Program in the 1990s. She is now an active member of the Global Lake Ecological Observatory Network.