Published: November 2015

Climate Change How do we know it's happening?

How Do We Know It's Happening?

To Take Earth’s Pulse, You Have to Fly High

Satellite and airborne sensors won't cure the Earth. But they promise the clearest picture yet of its various ailments.

Ecologist Greg Asner and his team at the Carnegie Institution for Science can measure the biomass of a forest from the air. In this false-color video of Amazon rain forest in Peru, the biggest, heaviest trees are red, while yellow, green, and blue trees are progressively lighter. Carnegie's research aircraft is equipped with a scanning lidar—a laser ranging device that works like radar—and imaging spectrometers.Video by GREGORY ASNER, CARNEGIE INSTITUTION FOR SCIENCE

Story by Peter Miller
Published October 15, 2015

The view out the window was bad enough. As his research plane flew over groves of California’s giant sequoias, some of the world’s tallest trees, Greg Asner could see the toll the state’s four-year drought had taken. “It looked wicked dry down there,” he said. But when he turned from the window to the video display in his flying lab, the view was even more alarming. In places, the forest was bright red. “It was showing shocking levels of stress,” he said.

The digital images were coming from a new 3-D scanning system that Asner, an ecologist with the Carnegie Institution for Science, had just installed in his turboprop aircraft. The scanner’s twin lasers pinged the trees, picking out individual branches from 7,000 feet up. Its twin imaging spectrometers, one built by NASA’s Jet Propulsion Laboratory (JPL), recorded hundreds of wavelengths of reflected sunlight, from the visible to the infrared, revealing detailed chemical signatures that identified each tree by species and even showed how much water it had absorbed—a key indicator of health. “It was like getting a blood test of the whole forest,” Asner said. The way he had chosen the display colors that day, trees starved of water were bright red.


As California's historic drought continues, scientists are turning to remote sensing from the skies. Orbiting satellites measure groundwater depletion, and aircraft monitor the snowpack and the tree canopy's chemical composition, bringing crucial information to those working to alleviate the drought—and to the people who depend on them.

Disturbing as the images were, they represented a powerful new way of looking at the planet. “The system produces maps that tell us more about an ecosystem in a single airborne overpass,” Asner wrote later, “than what might be achieved in a lifetime of work on the ground.” And his Carnegie Airborne Observatory is just the leading edge of a broader trend.

At a time when human impacts on the planet are unprecedented, technology offers a chance to truly understand them.

A half century after the first weather satellite sent back fuzzy pictures of clouds swirling over the North Atlantic, advanced sensors are doing for scientists what medical scanners have done for doctors—giving them ever improving tools to track Earth’s vital signs. In 2014 and early 2015 NASA launched five major Earth-observing missions (including two new instruments on the space station), bringing its total to 19. Space agencies from Brazil, China, Europe, and elsewhere have joined in. “There’s no question we’re in a golden age for remote sensing,” said Michael Freilich, NASA’s earth science director.

Picture of Carnegie Institution for Science's plane documenting the California drought

Four years of drought have taken a harsh toll on California's farms and forests. Last spring Greg Asner and his team flew over the Sierra Nevada, home to sequoias and other giant trees. With the new instruments on their airplane, the researchers completed in days a damage survey that would have taken a lifetime from the ground.Photograph by GREGORY ASNER, CARNEGIE INSTITUTION FOR SCIENCE

Picture of the earth from space

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The news from all these eyes in the sky, it has to be said, is mostly not good. They bear witness to a world in the midst of rapid changes, from melting glaciers and shrinking rain forests to rising seas and more. But at a time when human impacts on Earth are unprecedented, the latest sensors offer an unprecedented possibility to monitor and understand the impacts—not a cure for what ails the planet, but at least a better diagnosis. That in itself is a hopeful thing.

Map showing atmospheric carbon dioxide in the world

WHAT THIS IS It’s a map of atmospheric carbon dioxide over land last summer, made by NASA’s OCO-2 satellite. Red areas have a bit more CO₂, green areas a bit less, than the global average of 400 parts per million.

WHAT THIS TELLS US Forests and oceans have slowed global warming by soaking up some of the CO₂ we emit. OCO-2 will shed light on where exactly it’s going—and on how fast the planet could warm in the future. Map by NGM Staff: SOURCE NASA/JPL

Water is Earth’s lifeblood, and for the first time, high-flying sensors are giving scientists a way to follow it as it moves through every stage of its natural cycle: falling as rain or snow, running into rivers, being pumped from aquifers, or evaporating back into the atmosphere. Researchers are using what they’ve learned to predict droughts, warn of floods, protect drinking water, and improve crops.


Picture of a rainforest in Panama using a lidar device

WHAT THIS IS The Carnegie Airborne Observatory made this image of rain forest in Panama with its scanning lidar, which probes the trees’ size and shape, and a spectrometer that charts their chemical composition.

WHAT THIS TELLS US The technique allows Asner's team, flying at 7,000 feet, to identify individual trees from their chemical signatures—and even to say how healthy they are. The reddish trees here (the colors are arbitrary) are growing the fastest and absorbing the most CO₂. Photograph by GREGORY ASNER, CARNEGIE INSTITUTION FOR SCIENCE

In California the water crisis has turned the state into something of a laboratory for remote-sensing projects. For the past three years a NASA team led by Tom Painter has been flying an instrument-packed aircraft over Yosemite National Park to measure the snowpack that feeds the Hetch Hetchy Reservoir, the primary source of water for San Francisco.

Until now, reservoir managers have estimated the amount of snow on surrounding peaks the old-fashioned way, using a few gauges and taking surveys on foot. They fed these data into a statistical model that forecast spring runoff based on historical experience. But lately, so little snow had fallen in the Sierra Nevada that history could offer no analogues. So Chris Graham, a water operations analyst at Hetch Hetchy, accepted the NASA scientists’ offer to measure the snowpack from the sky.


Picture of a rainforest in Peru using a lidar device

WHAT THIS IS It’s an image of the Tambopata River in eastern Peru made by the scanning lidar aboard the Carnegie observatory.

WHAT THIS TELLS US The area in this image is actually covered with rain forest. Some lidar pulses penetrate the forest and reflect off the ground, revealing the subtle topography—red is a few feet higher than blue—and faint, abandoned river channels that have shaped the forest and helped create its rich biodiversity. Photograph by GREGORY ASNER, CARNEGIE INSTITUTION FOR SCIENCE

Painter’s Twin Otter aircraft, called the Airborne Snow Observatory, was equipped with a package of sensors similar to those in Greg Asner’s plane: a scanning lidar to measure the snow’s depth and an imaging spectrometer to analyze its properties. Lidar works like radar but with laser light, determining the plane’s distance to the snow from the time it takes the light to bounce back. By comparing snow-covered terrain with the same topography scanned on a snow-free summer day, Painter and his team could repeatedly measure exactly how much snow there was in the entire 460-square-mile watershed. Meanwhile the imaging spectrometer was revealing how big the snow grains were and how much dust was on the surface—both of which affect how quickly the snow will melt in the spring sun and produce runoff. “That’s data we’ve never had before,” Graham said.


Picture of California and Nevada taken from space

WHAT THIS IS NASA’s Aqua satellite captured these visible-light images of California and Nevada on March 27, 2010 (left), the most recent year with normal snowfall, and on March 29, 2015 (right).

WHAT THIS TELLS US After four years of drought, the snowpack in the Sierra Nevada—a crucial water reservoir for California—is just 5 percent of the historical average. Snow has virtually vanished from Nevada. And west of the Sierra, in the Central Valley, much of the fertile farmland is fallow and brown. Photographs courtesy NASA

Painter also has been tracking shrinking snowpacks in the Rocky Mountains, which supply water to millions of people across the Southwest. Soon he plans to bring his technology to other mountainous regions around the world where snow-fed water supplies are at risk, such as the Himalayan watersheds of the Indus and Ganges Rivers. “By the end of the decade, nearly two billion people will be affected by changes in snowpacks,” he said. “It’s one of the biggest stories of climate change.”

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With less water flowing into California’s rivers and reservoirs, officials have cut back on the amount of water supplied to the state’s farmers, who typically produce about half the fruits, nuts, and vegetables grown in the U.S. In response, growers have been pumping more water from wells to irrigate fields, causing water tables to fall. State officials normally monitor underground water supplies by lowering sensors into wells. But a team of scientists led by Jay Famiglietti, a hydrologist at the University of California, Irvine, and at JPL, has been working with a pair of satellites called GRACE (for Gravity Recovery and Climate Experiment) to “weigh” California’s groundwater from space.

Planet Probes
Earth's vital signs are monitored by NASA's 19 Earth-observing missions. Ten of the most critical, shown here, circle the globe up to 16 times a day, collecting data on climate, weather, and natural disasters.
Monica serrano, NGM STAFF; tony sChick
SOURCE: Steven E. Platnick and Claire L. Parkinson, NASA Goddard Space Flight Center

The satellites do this by detecting how changes in the pull of Earth’s gravity alter the height of the satellites and the distance between them. “Say we’re flying over the Central Valley,” Famiglietti said, holding a cell phone in each hand and moving them overhead like one satellite trailing the other. “There’s a certain amount of water down there, which is heavy, and it pulls the first satellite away from the other.”

The GRACE satellites can measure that to within 1/25,000 of an inch. And a year later, after farmers have pumped more water out of the ground, and the pull on the first satellite has been ever so slightly diminished, the GRACE satellites will be able to detect that change too.

Depletion of the world’s aquifers, which supply at least one-third of humanity’s water, has become a serious danger, Famiglietti said. GRACE data show that more than half the world’s largest aquifers are being drained faster than they can refill, especially in the Arabian Peninsula, India, Pakistan, and North Africa.

Since California’s drought began in 2011, the state has been losing about four trillion gallons a year (more than three and a half cubic miles) from the Sacramento and San Joaquin River Basins, Famiglietti said. That’s more than the annual consumption of the state’s cities and towns. About two-thirds of the lost water has come from aquifers in the Central Valley, where pumping has caused another problem: Parts of the valley are sinking.

Picture of a concrete wellhead in California

This concrete wellhead on Allan Clark’s almond farm at Chowchilla, east of Los Banos in California’s Central Valley, used to be flush with the ground. But groundwater pumping accelerated by drought has caused the land to sink—in some places, according to satellite measurements, by around a foot a year. Two of Clark’s irrigation wells have run dry; he’s on a waiting list to have one deepened. Photograph by MARK THIESSEN, NGM STAFF

The spectrometer view would be like “Star Trek technology”: We’d be able to see and name individual trees from space.

Tom Farr, a geologist at JPL, has been mapping this subsidence with radar data from a Canadian satellite orbiting some 500 miles up. The technique he used, originally developed to study earthquakes, can detect land deformations as small as an inch or two. Farr’s maps have shown that in places, the Central Valley has been sinking by around a foot a year.

One of those places was a small dam near the city of Los Banos that diverts water to farms in the area. “We knew there was a problem with the dam, because water was starting to flow up over its sides,” said Cannon Michael, president of Bowles Farming Company. “It wasn’t until we got the satellite data that we saw how huge the problem was.” Two sunken bowls had formed across a total of 3,600 square miles of farmland, threatening dams, bridges, canals, pipelines, and floodways—millions of dollars’ worth of infrastructure. In late 2014 California governor Jerry Brown signed the state’s first law phasing in restrictions on groundwater removal.

As evidence has mounted about Earth’s maladies—from rising temperatures and ocean acidification to deforestation and extreme weather—NASA has given priority to missions aimed at coping with the impacts. One of its newest satellites, a $916 million observatory called SMAP (for Soil Moisture Active Passive), was launched in January. It was designed to measure soil moisture both by bouncing a radar beam off the surface and by recording radiation emitted by the soil itself. In July the active radar stopped transmitting, but the passive radiometer is still doing its job. Its maps will help scientists forecast droughts, floods, crop yields, and famines.

Picture of Portugal and Spain from space

No one gets a better look at how we’ve transformed Earth—and conquered night—than astronauts on the space station. The view here is to the north over Portugal and Spain. The green band is the aurora. Photograph courtesy NASA

“If we’d had SMAP data in 2012, we easily could have forecast the big Midwest drought that took so many people by surprise,” said Narendra N. Das, a research scientist at JPL. Few people expected the region to lose about $30 billion worth of crops that summer from a “flash drought”—a sudden heat wave combined with unusually low humidity. “SMAP data could have shown early on that the region’s soil moisture was already depleted and that if rains didn’t come, then crops were going to fail,” Das said. Farmers might not have bet so heavily on a bumper crop.

Climate change also is increasing the incidence of extreme rains—and SMAP helps with that risk too. It can tell officials when the ground has become so saturated that a landslide or a downstream flood is imminent. But too little water is a more pervasive and lasting threat. Without moisture in the soil, a healthy environment breaks down, as it has in California, leading to heat waves, drought, and wildfires. “Soil moisture is like human sweat,” Das said. “When it evaporates, it has a cooling effect. But when the soil is devoid of moisture, Earth’s surface heats up, like us getting heatstroke.”

Despite all the challenges to Earth’s well-being, the planet so far has proved remarkably resilient. Of the 37 billion metric tons or so of carbon dioxide dumped into the atmosphere each year by human activities, oceans, forests, and grasslands continue to soak up about half. No one knows yet, however, at what point such sinks might become saturated. Until recently, researchers didn’t have a good way to measure the flow of carbon in and out of them.

That changed in July 2014, when NASA launched a spacecraft called the Orbiting Carbon Observatory-2. Designed to “watch the Earth breathe,” as managers put it, OCO-2 can measure with precision—down to one molecule per million—the amount of CO₂ being released or absorbed by any region of the world. The first global maps using OCO-2 data showed plumes of CO₂ coming from northern Australia, southern Africa, and eastern Brazil, where forests were being burned for agriculture. Future maps will seek to identify regions doing the opposite—removing CO₂ from the atmosphere.

Greg Asner and his team also have tackled the mystery of where all the carbon goes. Prior to flying over California’s woodlands, they spent years scanning 278,000 square miles of tropical forests in Peru to calculate the forests’ carbon content.

At the time, Peru was in discussions with international partners about ways to protect its rain forests. Asner was able to show that forest areas under the most pressure from logging, farming, or oil and gas development also were holding the most carbon—roughly seven billion tons. Preserving those areas would keep that carbon locked up, Asner said, and protect countless species. In late 2014 the government of Norway pledged up to $300 million to prevent deforestation in Peru.

Within the next few years NASA plans to launch five new missions to study the water cycle, hurricanes, and climate change, including a follow-up to GRACE. Smaller Earth-observing instruments, called CubeSats—some tiny enough to fit into the palm of a hand—will hitch rides into space on other missions. For scientists like Asner, the urgency is clear. “The world is in a state of rapid change,” he said. “Things are shifting in ways we don’t yet have the science for.”

Within the next decade or so the first imaging spectrometer, similar to the ones used by Asner and Painter, could be put into Earth orbit. It would be like “Star Trek technology” compared with what’s up there now, Painter said. “We’ve orbited Jupiter, Saturn, and Mars with imaging spectrometers, but we haven’t had a committed program yet for our own planet,” he said. The view from such a device would be amazing: We’d be able to see and name individual trees from space. And we’d be reminded of the larger forest: We humans and our technology are the only hope for curing what we’ve caused.

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