The Missing Carbon: How NASA's BOREAS Study Searched the Northern Forests

Instruments at different heights on towers like this one, in Manitoba, measured how carbon dioxide, heat and moisture moved between the forest and the air. NASA, BOREAS project.

When scientists added up the world's carbon in the 1990s, the books did not balance. Of the carbon released each year by burning fossil fuels, between 15 and 30 percent — one to two billion metric tons — could not be accounted for. Some stays in the air and some goes into the ocean; the rest, it was assumed, was taken up by plants on land. But nobody knew where, or how much, or whether it would stay there.

The prime suspect

To find out, NASA led the Boreal Ecosystem-Atmosphere Study (BOREAS) from 1994 to 1997, with 85 science teams from five nations working across two Canadian provinces. Their suspect was the boreal forest, named after Boreas, the Greek god of the north wind: the mostly evergreen forest that rings the Earth between about 43°N and 65°N, covering 16 to 20 million square kilometres.

There were clues. Measurements of carbon dioxide from ships at different latitudes, fed into models that track air masses, suggested that most of the carbon was being absorbed somewhere north of 40°N, about the latitude of New York City. And the boreal forest, though only about 21 percent of the world's forested land, holds about 43 percent of all the carbon stored in the world's soils. For some 7,000 years its floor has piled up carbon at about 30 grams per square metre each year, roughly the carbon in a paper napkin, but over so vast an area that it adds up. "When you walk through the Canadian boreal forest, you can literally go from ankle deep to in over your head in carbon litter," said NASA physicist Forrest Hall, the BOREAS project scientist.

Boardwalks kept scientists from sinking into the soft, waterlogged soil, where cold and water stop fallen needles and branches from rotting for most of the year. NASA, BOREAS project.

How they measured

The teams chose two large study regions about 500 km apart in Manitoba and Saskatchewan. For four years, in every season, they measured the exchange of sunlight, heat, water and carbon at the scale of single leaves, from towers standing above the canopy, and over whole regions from aircraft and satellites, comparing each scale with the others. Piers Sellers and Forrest Hall of NASA's Goddard Space Flight Center designed and oversaw the effort.

One old spruce stand

Steven Wofsy's team built a tower in a stand of 120-year-old black spruce in Manitoba, typical of much of North America's boreal forest, and made 22,000 hours of measurements, from the soil to high above the trees, between March 1994 and October 1997.

The forest's year turned abruptly with the seasons. From late May through July the spruce took in 1 to 1.5 grams of carbon per square metre each day. In the hot, dry weeks of August and September the uptake fell almost to zero, and from October the forest breathed carbon back out, all winter long.

Carbon gained (positive) or lost (negative) by the black spruce site, totalled from each January 1. Hot, dry 1995 lost far more than usual; 1998, with a long growing season and a cool, wet summer, gained carbon. Chart by Robert Simmon, NASA, from data by Wofsy.

The surprise was in the soil. Most of the carbon lies 40 to 80 cm down, deeper than even big fires reach, where it stays frozen much of the year. When it thaws in midsummer it is waterlogged, too short of oxygen for bacteria to break it down. But as it dries in late summer, the bacteria wake up and decompose it fast, releasing carbon dioxide. "What's going on in the soils is more important than what is going on in the trees," Wofsy said.

The balance was fine. The site took up about 800 grams of carbon per square metre each year, but in 1995 it gave off 890, a net loss of 90, and in 1997 790, a net loss of 10. In 1998, when spring came early and the soil stayed cool and moist for longer, the site gained 40 grams per square metre. Small shifts in climate decided whether this forest stored carbon or released it.

Probes like this one measured the carbon released from the forest floor, along with soil temperature and moisture. The moss insulates the soil and keeps it from drying out. NASA, BOREAS project.

What it could mean

The researchers could not scale up one old, nutrient-poor stand near the forest's northern edge to the whole boreal zone, and Hall pointed out that elsewhere a longer growing season might help trees store more. Wofsy's team found no sign of high carbon uptake driven by extra carbon dioxide or warming.

The worry was this. The interiors of high-latitude continents, where the boreal forest grows, were warming faster than anywhere else. Wofsy and colleagues wrote that a rise of about 2 °C in global temperature by 2100 would probably thaw the deep frozen soil at their site completely and speed its decay. If that happened across the boreal forest, a store of carbon could become a source, and carbon dioxide would build up in the atmosphere faster still. Answering the question would take more data, satellite maps of the whole northern forest, and better models.

Sources

• David Herring and Robert Kannenberg, NASA Earth Observatory, "The Mystery of the Missing Carbon," 1999; rewritten in hubnx's own words. Photographs from the BOREAS project; a map adapted from a 1972 publication left out.
• M. L. Goulden, S. C. Wofsy and others, "Sensitivity of Boreal Forest Carbon Balance to Soil Thaw," Science 279 (1998); P. J. Sellers, F. G. Hall and others, "BOREAS in 1997," Journal of Geophysical Research 102 (1997).