Renewable energy might be able to green a desert | Science News for Students

Renewable energy might be able to green a desert

Wind turbines and solar panels appear able to boost nearby rains — and plant growth
Nov 26, 2018 — 6:45 am EST
a bank of solar panels in front of a row of wind turbines on a sunny day

Large wind and solar farms could change the amount of rain in nearby areas.

Wind turbines and solar panels that create electricity are examples of environmentally friendly — or “green” — technology. A new study finds that these forms of renewable energy might be green in another sense, too. Large collections of those turbines or so-called farms of solar panels appear capable of bringing rains to the desert. And that would allow more plants to grow.

Eugenia Kalnay is an expert on weather and climate. She works at the University of Maryland in College Park. She also has worked for the National Weather Service and NASA. In each place, she has used computers to model weather and climate. Such models help scientists understand how temperatures and rain might change over time. Day-to-day changes are known as weather. Longer-term patterns, such as seasonal trends that persist for years, describe a region’s climate.

Wind turbines and solar panels can change how air moves. As winds move through the spinning blades of a turbine, some of their power is converted to electricity. This weakens those winds. Turbines may also change the path of the winds, directing some share of them around the outside of the wind farm.

Both technologies also can affect nearby temperatures. Solar panels can raise the adjacent temperature by 3 to 4 degrees Celsius (5 to 7 degrees Fahrenheit). Turbines also boost temperatures, largely by keeping the nights warmer. Warm air rises. If it rises high enough, and holds much water vapor, it could eventually condense into clouds that produce rain.

In these ways, wind and solar farms could affect climate. But would the changes be large enough to matter? That’s what Kalnay and others wanted to know. Their new computer models show that a mix of these energy technologies might boost rainfall and eventually transform deserts into plant-rich areas.

Putting it to the test

Kalnay teamed up with Safa Motesharrei, a systems scientist at Maryland. Systems scientists study how complex systems, such as climate, function. The Maryland pair recruited Yan Li, a geoscientist at Beijing Normal University in China, to join them. These three brought in other scientists from Maryland, Italy and China to join in their study. Building large wind or solar farms just to study their question was not an option. It would be too costly. It might also create unexpected climate issues. So the team instead used computer models to probe how wind turbines and solar farms might alter a region’s climate.

Weather and climate models work from data collected over decades. They include data on the weather that developed when certain conditions were in place. These conditions included temperature and rains or snowfall. They also included the air pressure, winds, sunlight and the movement of heat into and out of the ground and large bodies of water.

For their new study, the researchers developed a model of North Africa’s Sahara Desert. The world’s largest desert, the Sahara supports little life. Although few people live here, many reside in the areas around it. So putting wind and solar farms in this area could help meet their electricity needs.

a map of north Africa showing both the Sahara and the Sahel
The Sahara is the world’s largest desert. Immediately south of its border is a not-quite-so-dry region known as the Sahel.
Rainer Lesniewski/iStockphoto

The southern edge of the desert is an area called the Sahel. In this transition zone, the desert becomes a grassy savanna dotted with trees. There isn’t much rainfall in the Sahel, and climate change has reduced those rains in recent years. Because growing crops helps to feed the local people, rains are important here.

The team ran its model several times. In one run, it assumed that people would erect wind farms only. Another run assumed people would install just solar farms. A third assumed people would build both. All three of these scenarios would affect the desert’s weather — but differently, the model showed.

Wind farms raised the temperature by an average of 2.16 degrees Celsius (3.89 degrees Fahrenheit). Most of that increase occurred at night, when the land cooled down less than normal. Wind turbines also doubled the Sahara’s rainfall, which doubled in the Sahara. But doubling a small number still ends up being a small effect. The average daily rain increased by only 0.25 millimeter (0.01 inch). The Sahel would see a slightly bigger daily increase, the model projected — 1.12 millimeters (0.04 inch) more rain.

Solar farms would increase temperatures less, by 1.12 degrees Celsius (2.02 degrees Fahrenheit), the models showed. And that change occurred mainly during the day. They’d also increase average rain per day, but less than the wind farms would. The increase could add up to an extra 47.5 millimeters (1.9 inches) per year in the desert and by 208 millimeters (8.2 inches) in the Sahel.

an aerial photo of a village in Mali
In the Sahel, like this village in Mali, the land is dry but not too dry to prevent some plant growth. This area outside of the nearby Sahara is also where many people live.
lonelytravel/iStockphoto

Installing a mix of solar farms and wind turbines brought about a bigger change — not in temperature but in rainfall. In the Sahara, having both wind and solar farms more than doubled the amount of rain that fell, up to 215.4 millimeters (8.5 inches) per day. The nearby Sahel would see an even more dramatic boost — up to 500 millimeters (20 inches) of additional rain each year.

The models predicted that the average rainfall would increase over time, as more plants grew. Plants move water from the ground to the air. The process they use has a long name: evapotranspiration (Ee-VAP-oh-tran-spur-AY-shun). Warmer air can carry more water. So as this air rises, it can ferry more water up to become clouds. Those clouds can release rain, slaking the thirst of plants below. Over time, this process repeats itself over and over. This so-called positive feedback can greatly boost rains. That’s especially true in the Sahel, the models showed, where people rely on that water.

Eventually, the models showed, rainfall would level off at some new, higher amount. That’s because plants would now be a consistent part of the landscape and the climate more stable.

Kalnay and her colleagues described this trend September 7 in Science.

Such rainfall changes could lead to better agriculture, says Motesharrei. More rain also would help wild plants grow, he adds, giving livestock more to graze on.

“We believe that the countries in the Sahara and nearby Sahel region should seriously consider investing in wind and solar power,” say Motesharrei and Li. This would produce large amounts of clean, renewable electricity, they argue. As an added benefit, it would bring rain to a region that has been getting drier.

Not a simple solution

“The impact of solar [farms] on climate and ecosystems is complex,” notes Ibrahima Diédhiou. He’s an ecologist in West Africa at the Université de Thiès in Senegal. Solar farms could have boost rains in the Sahel, he says. But they also might cut how much carbon dioxide (a greenhouse gas) is taken up by crops, pastures and forests. So the overall impact on climate change could be mixed, he says.

Unintended effects might go beyond climate issues, warns Rebecca Hernandez. She is an ecologist at the University of California, Davis who specializes in dry landscapes, such as deserts. Wind turbines and solar farms often disturb ecosystems in ways that lets invasive plant species take over, she says. As the edge out native plants, animals that depend on those native plants could also suffer.

People clearly need to think carefully about how and where they site renewable energy farms, Hernandez says. Putting wind and solar farms in places where people have already built cities and towns would be particularly helpful, she argues. Their environment has already been disrupted, she notes, so adding the energy systems might not cause all that much additional harm. What’s more, these are where people use the most energy.

Power Words

(for more about Power Words, click here)

agriculture     The growth of plants, animals or fungi for human needs, including food, fuel, chemicals and medicine.

air pressure     The force exerted by the weight of air molecules.

average     (in science) A term for the arithmetic mean, which is the sum of a group of numbers that is then divided by the size of the group.

carbon dioxide     (or CO2) A colorless, odorless gas produced by all animals when the oxygen they inhale reacts with the carbon-rich foods that they’ve eaten. Carbon dioxide also is released when organic matter burns (including fossil fuels like oil or gas). Carbon dioxide acts as a greenhouse gas, trapping heat in Earth’s atmosphere. Plants convert carbon dioxide into oxygen during photosynthesis, the process they use to make their own food.

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.

cloud     A plume of molecules or particles, such as water droplets, that move under the action of an outside force, such as wind, radiation or water currents. (in atmospheric science) A mass of airborne water droplets and ice crystals that travel as a plume, usually high in Earth’s atmosphere. Its movement is driven by winds. 

colleague     Someone who works with another; a co-worker or team member.

computer model     A program that runs on a computer that creates a model, or simulation, of a real-world feature, phenomenon or event.

condense     To become thicker and more dense. This could occur, for instance, when moisture evaporates out of a liquid. Condense can also mean to change from a gas or a vapor into a liquid. This could occur, for instance, when water molecules in the air join together to become droplets of water.

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. 

ecology      A branch of biology that deals with the relations of organisms to one another and to their physical surroundings. A scientist who works in this field is called an ecologist.

ecosystem     A group of interacting living organisms — including microorganisms, plants and animals — and their physical environment within a particular climate. Examples include tropical reefs, rainforests, alpine meadows and polar tundra. The term can also be applied to elements that make up some an artificial environment, such as a company, classroom or the internet.

environment     The sum of all of the things that exist around some organism or the process and the condition those things create. Environment may refer to the weather and ecosystem in which some animal lives, or, perhaps, the temperature and humidity (or even the placement of things in the vicinity of an item of interest).

evapotranspiration     The process by which plants release excess moisture into the air as water vapor. The vapor passes through tiny pores in the plants’ leaves.

feedback     A response or assessment that follows some a particular act or decision. Or a process or combination of processes that propel or exaggerate a change in some direction. For instance, as the cover of Arctic ice disappears with global warming, less of the sun’s warming energy will be reflected back into space. This will serve to increase the rate of Earth’s warming. That warming might trigger some feedback (like sea-ice melting) that fosters additional warming.

function     A relationship between two or more variables in which one variable (the dependent one) is exactly determined by the value of the other variables.

green     (in chemistry and environmental science) An adjective to describe products and processes that will pose little or no harm to living things or the environment.

greenhouse gas     A gas that contributes to the greenhouse effect by absorbing heat. Carbon dioxide is one example of a greenhouse gas.

livestock     Animals raised for meat or dairy products, including cattle, sheep, goats, pigs, chickens and geese.

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.

NASA     Short for the National Aeronautics and Space Administration. Created in 1958, this U.S. agency has become a leader in space research and in stimulating public interest in space exploration. It was through NASA that the United States sent people into orbit and ultimately to the moon. It also has sent research craft to study planets and other celestial objects in our solar system.

National Weather Service     An agency of the National Oceanic and Atmospheric Administration. Created in 1870, its current role is to collect weather, precipitation and climate data. It also issues forecasts and warnings 24 hours a day for the entire United States, focusing on signs of possible conditions that could threaten lives and structures.

native     Associated with a particular location; native plants and animals have been found in a particular location since recorded history began. These species also tend to have developed within a region, occurring there naturally (not because they were planted or moved there by people). Most are particularly well adapted to their environment.

renewable energy     Energy from a source that is not depleted by use, such as hydropower (water), wind power or solar power.

savanna     A grassland sometimes also populated with trees. Most are fairly dry for part or much of the year.

scenario     A possible (or likely) sequence of events and how they might play out.

species     A group of similar organisms capable of producing offspring that can survive and reproduce.

technology     The application of scientific knowledge for practical purposes, especially in industry — or the devices, processes and systems that result from those efforts.

transition     The boundary where one thing (paragraphs, ecosystems, life stage, state of matter) changes or converts into another. Some transitions are sharp or abrupt. Others slowly or gradually morph from one condition or environment to another. 

turbine     A device with extended arm-like blades (often curved) to catch a moving fluid — anything from a gas or steam to water — and then convert the energy in that movement into rotary motion. Often that rotary motion will drive a system to generate electricity.

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.

wind turbine     A wind-powered device — similar to the type used to mill grain (windmills) long ago — used to generate electricity.

Citation

Journal:​ ​​Y. Li et al. Climate model shows large-scale wind and solar farms in the Sahara increase rain and vegetation. Science.​ ​Vol. 361, September 7, 2018, p. 1019. doi:  10.1016/j.jaip.2018.07.014.