TOULOUSE, France — A lot of the water on Earth’s surface has shifted locales in the past few decades. That’s the finding of a surprising new study. Some lakes once thought to be permanent now exist only part of the year. Others have disappeared altogether. In still other places, new lakes and wetlands have been emerging.
Scientists sometimes refer to the depiction of Earth’s surface waters as its water “map.” At first glance, the reasons for changes in the map might seem unrelated. In fact, some sort of human activity is the root cause for many.
When people build a dam, it almost immediately begins to collect water. That change can be seen right away. But in most places, the landscape changes too slowly to notice. So it can be hard to spot a change in the map of surface waters unless you compare two time periods that are far apart, explains Luca De Felice. He’s an analyst who focuses on geospatial (GEE-oh-spay-shul) data — the types that map what’s happening across wide swaths of the globe. He works at the European Commission’s Joint Research Center in Brussels, Belgium.
De Felice looks at a lot of satellite images. For the past couple of years, he has been part of a team analyzing images taken by Landsat satellites between 1984 and 2015. The researchers focused on Earth’s lakes, rivers and other surface waters.
The task was not easy, De Felice notes. The group had more than 3 million images to review. Plus, team members had to develop software that would highlight where the water is. That’s not as simple as finding the blue stuff in an image. Microbes in lake water, for instance, might turn the water greenish. And some rivers, especially muddy ones, appear brown. That can make water difficult to distinguish from land.
To tackle the job, the team’s computers worked overtime. There were 10,000 of them working together to analyze all of those satellite photos, De Felice says. If the researchers had been running their study on just one laptop, their analysis might have taken more than 1,200 years, he estimates.
De Felice described his team’s work here, in France, on July 13. His presentation was part of a big conference known as the EuroScience Open Forum.
Mapping the changes
Over a 32-year period, the global map of Earth’s surface waters changed dramatically, the team found. Some areas where water had once been present year-round now hosts water for only part of the year. Such areas where the water cover is now temporary total some 72,000 square kilometers (27,800 square miles), De Felice reports. That’s an area a little bigger than the nation of Ireland. An even bigger share of the globe that used to be permanently wet is now dry all year long. It’s not in one spot. But all of the contributing spots together span roughly 90,000 square kilometers (34,000 square miles). That’s an area about the size of Portugal.
Consider, for example, what has happened to Central Asia’s Aral Sea. At one time, it was the world’s fourth-largest freshwater lake. Today, De Felice notes, it has largely disappeared. People have diverted a lot of water from the rivers that used to feed the lake. Much of that water is used to irrigate thirsty crops, such as cotton.
Most of the water that’s gone missing in the last three decades has disappeared from five Asian nations: Afghanistan, Kazakhstan, Uzbekistan, Iran and Iraq. (What’s left of the Aral Sea lies on the border between Kazakhstan and Uzbekistan.)
The missing water isn’t really missing, of course. It had to go somewhere. Evaporation might have sent some of it into the sky. That would have later come down elsewhere as rain and snow. Some moisture might have ended up in the oceans.
More likely, De Felice says, a large amount of the “missing” water ended up elsewhere on land. His team found that during the 32-year period it analyzed, some 184,000 square kilometers (71,000 square miles) of land that had previously been dry is now under water year-round. That’s a combined region the size of Syria.
Some of this newly drowned land sits on China’s Tibetan Plateau. There, says De Felice, climate change has melted snows to create lakes and wetlands. Those bodies of water now cover about 8,000 square kilometers (3,100 square miles) that herders had once used to graze their livestock. In other parts of the world, the formerly dry land now sits under reservoirs behind new dams.
These shifts in water have blessed some regions. Other regions have lost a life-sustaining resource.
All of these changes in Earth’s water map need to be considered in computer models of weather and climate, says Gianpaolo Balsamo. He’s a meteorologist with the European Center for Medium-Range Weather Forecasts. It’s in Reading, England. Knowing the locations of lakes helps to make simulations of weather better, he notes. For one thing, bodies of water absorb more heat during the daytime than will the nearby land. Then, at night, the water gives up that heat.
Lakes and other bodies of water also help moisturize the air. In the right conditions, that water vapor can end up as rain.
Even small lakes can affect local weather, notes Balsamo. His team’s models suggest that bodies of water as small as a couple of kilometers across can influence the weather nearby.
climate The weather conditions that typically exist in one area, in general, or over a long period.
climate change Long-term, significant change in the climate of Earth. It can happen naturally or in response to human activities, including the burning of fossil fuels and clearing of forests.
computer model A program that runs on a computer that creates a model, or simulation, of a real-world feature, phenomenon or event.
crop (in agriculture) A type of plant grown intentionally grown and nurtured by farmers, such as corn, coffee or tomatoes. Or the term could apply to the part of the plant harvested and sold by farmers.
data Facts and/or statistics collected together for analysis but not necessarily organized in a way that gives them meaning. For digital information (the type stored by computers), those data typically are numbers stored in a binary code, portrayed as strings of zeros and ones.
develop To emerge or come into being, either naturally or through human intervention, such as by manufacturing., trees and grasslands are cut down and replaced with structures or landscaped yards and parks.
freshwater A noun or adjective that describes bodies of water with very low concentrations of salt. It’s the type of water used for drinking and making up most inland lakes, ponds, rivers and streams, as well as groundwater.
irrigation (v. irrigate) The supply of water to land or crops to help growth.
livestock Animals raised for meat or dairy products, including cattle, sheep, goats, pigs, chickens and geese.
meteorologist Someone who studies weather and climate events.
microbe Short for microorganism. A living thing that is too small to see with the unaided eye, including bacteria, some fungi and many other organisms such as amoebas. Most consist of a single cell.
model A simulation of a real-world event (usually using a computer) that has been developed to predict one or more likely outcomes. Or an individual that is meant to display how something would work in or look on others.
plateau A flat area of land, high above sea level. It’s sometimes referred to as “tableland.” Several of its edges tend to be steeply sloped (cliffs).
range The full extent or distribution of something. For instance, a plant or animal’s range is the area over which it naturally exists. (in math or for measurements) The extent to which variation in values is possible. Also, the distance within which something can be reached or perceived.
reservoir A large store of something. Lakes are reservoirs that hold water. People who study infections refer to the environment in which germs can survive safely (such as the bodies of birds or pigs) as living reservoirs.
satellite A moon orbiting a planet or a vehicle or other manufactured object that orbits some celestial body in space.
sea An ocean (or region that is part of an ocean). Unlike lakes and streams, seawater — or ocean water — is salty.
simulation (v. simulate) An analysis, often made using a computer, of some conditions, functions or appearance of a physical system. A computer program would do this by using mathematical operations that can describe the system and how it might change over time or in response to different anticipated situations.
software The mathematical instructions that direct a computer’s hardware, including its processor, to perform certain operations.
water vapor Water in its gaseous state, capable of being suspended in the air.
weather Conditions in the atmosphere at a localized place and a particular time. It is usually described in terms of particular features, such as air pressure, humidity, moisture, any precipitation (rain, snow or ice), temperature and wind speed. Weather constitutes the actual conditions that occur at any time and place. It’s different from climate, which is a description of the conditions that tend to occur in some general region during a particular month or season.
wetland As the name implies, this is a low-lying area of land either soaked or covered with water much of the year. It hosts plants and animals adapted to live in, on or near water.