Mapping the World's Forests in 3D: How Scientists Weigh the Carbon in Trees

Trees cool and moisten the air, shelter countless species, anchor soil and supply food, fuel, medicine and timber. They also help balance the Earth's carbon budget. Of the roughly nine billion tons of carbon that people release each year by burning fossil fuels and changing the landscape, about four billion stay in the atmosphere and two billion dissolve in the ocean. The other three billion go into ecosystems on land — but exactly where remains an open question.

Forests are a prime suspect. They cover about 30 percent of the land but account for 50 percent of plant productivity, and hold as much as 45 percent of the carbon stored on land. Clearing them releases carbon that tall trees could have held for centuries, and the crops, pastures or pavement that replace them store less, or nothing. "The biggest natural sink of terrestrial carbon lies in our forests and trees," says forest ecologist Steve Running of the University of Montana, "and the biggest natural source of carbon on land is also the forest."

The key measure is biomass: as a rule of thumb, half a tree's dry weight is carbon. Estimate the biomass of every forest, and you know how much carbon the land stores; repeat it over years, and you can see it moving.

Seeing forests in three dimensions

Counting trees on the ground and flying over forests works on small scales but is far too slow and costly for the whole planet, and satellites that measure the land's "greenness" cannot tell how tall trees are. Yet height is what matters most. "We need to see Earth's vegetation in three dimensions," says NASA ecologist Jon Ranson.

In 2010 Michael Lefsky of Colorado State University produced the first global map of forest height, combining broad views from NASA's MODIS instruments with height measurements from the laser on the ICESat satellite, which fired more than 250 million pulses at the Earth between 2003 and 2009. The tallest forests clustered in the Pacific Northwest and parts of Southeast Asia: conifers such as Douglas fir, redwood and sequoia, often over 40 metres. Boreal forests of spruce, fir, pine and larch usually stay under 20 metres, and temperate broadleaf and undisturbed tropical forests average about 25. Lefsky called it "a first draft".

The first global map of forest height, built from wide views of vegetation and narrow tracks of laser measurements (black lines). NASA map by Jesse Allen and Robert Simmon.

The tropics

Tropical forests grow all year and are thought to be the most productive on Earth, yet their carbon was known only roughly. Their clearing and degradation cause 10 to 20 percent of all human carbon dioxide emissions, driven by palm oil, soybeans, beef and leather as well as by small farmers.

Fires clearing forest in Brazil, photographed from the International Space Station on August 14, 2010. NASA astronaut photograph ISS024-E-11941.

NASA's Sassan Saatchi and 14 colleagues from 10 institutions combined four space instruments — ICESat's laser, MODIS, the QuikSCAT scatterometer and the Shuttle Radar Topography Mission — with 4,079 ground plots and more than three million tree-height measurements. Their map, released in May 2011, covered 2.5 billion hectares of forest in 75 countries: the first systematic map of carbon across the whole tropics. It found nearly 247 billion tons of carbon in tropical forests, 193 above ground and 54 in roots; 49 percent in Central and South America, 26 in Southeast Asia and 25 in sub-Saharan Africa.

Just as important, the map showed how certain each estimate was — within 1 to 5 percent at national and regional levels, against 10 to 20 percent errors in ground surveys of large plots — so that managers and economists could trust it and scientists knew where to look harder.

Carbon stored in the forests of New Guinea. NASA map by Robert Simmon, from Saatchi et al., 2011.

The United States, tree farm by tree farm

In April 2011, Josef Kellndorfer's team at the Woods Hole Research Center released the National Biomass and Carbon Dataset, mapping U.S. forests and their carbon down to 30 metres. Over six years, working with the U.S. Forest Service and the U.S. Geological Survey, they combined radar from the Shuttle Radar Topography Mission, flown in 2000, with Landsat land-cover maps and the Forest Service's inventory, which keeps a survey plot for every 6,000 acres of woodland. They divided the country into 66 zones and mapped 265 million segments.

The dataset’s 66 zones; the coastal Pacific Northwest has the densest biomass in the country. Map by Robert Simmon, from Woods Hole Research Center data.

The map shows the checkerboard of logging in the Pacific Northwest's old growth, the managed tree farms of the Southeast, trees tracing rivers and field edges in the Midwest, and forests returning to once-cleared land in the Mid-Atlantic and New England.

What next

Three teams produced three maps within fifteen months, with slightly different answers — a healthy competition, Ranson says, like laboratories attacking cancer from different angles. The goal is a single, regularly updated global map of forest height and carbon. But the tools were shrinking: ICESat ended in 2009, the space shuttle retired in July 2011, and a planned radar-and-lidar mission, DESDynI, was put on indefinite hold in spring 2011 by budget cuts. NASA scientists turned to aircraft, flying radar, lidar and a radiometer over forests in the eastern U.S. and Canada in the summer of 2011.

Lidar measures tree height by bouncing laser light off the canopy. NASA image by Robert Simmon.

The maps are in demand. Countries measuring their forest carbon for the UN's REDD+ programme, and future carbon markets, need them. "If there is going to be billions of dollars in carbon trading," Running says, "then knowing where the carbon is and how much of it there is takes on huge political and economic importance."

Sources

• NASA Earth Observatory, "Seeing Forests for the Trees and the Carbon: Mapping the World's Forests in Three Dimensions"; rewritten in hubnx's own words, as the situation stood in 2011. Maps and photographs from NASA.