Picture This: A Living, Breathing Earth

HAVE you ever seen the Earth breathing? That's what the map shown here is all about.

It charts the exchange of the heat-trapping gas carbon dioxide (CO2) between the atmosphere and the planet's land plants and soils. This is a key process in determining to what extent the release of CO2 through burning fossil fuels and land-use changes may bring on global climate warming.

The map represents the first time that satellite data have been combined with computer simulation of soil processes to show the geography of the CO2 exchanges, according to ecologist Christopher Potter of the National Aeronautics and Space Administration (NASA) Ames Research Center in Mountain View, Calif.

Growing plants take CO2 out of the air to use in photosynthesis. Soil microbes, on the other hand, release CO2 as they break down organic matter.

Green areas on the map show where Earth is ``breathing in'' as photosynthesis absorbs more CO2 than the soil releases. Red areas show where Earth is ``breathing out'' as the soil releases more CO2 than the process of photosynthesis absorbs.

This map, which Dr. Potter and his colleague Steven Klooster have constructed, represents a typical June. The two scientists are working to map other months and to extend this kind of analysis for a series of years. Potter says they now are processing data taken over the past 10 years.

Dr. Klooster explains that ``remote-sensing satellites give us a new, unique view of the Earth as a living, breathing system.'' This continuous satellite monitoring shows seasonal changes that ``help us understand the carbon distribution between the tropics and high latitudes and how that distribution changes month to month,'' he says.

The natural carbon-dioxide recycling system involves complex interactions of air, land, and sea. Volcanoes, soil-decay processes, and the sea release CO2. On the other hand, some of the gas dissolves in sea water and is stored deep within the ocean for long periods of time.

Marine biological processes also remove the carbon from CO2, some of which then is locked away in sedimentary rock. On land, green plants incorporate the carbon from CO2 into their structure, while soil microbes convert the carbon in organic matter back into CO2.

Humans perturb this system by releasing the carbon locked away in fossil fuels and by changing the balance of photosynthesis and soil-decay processes through such land-use changes as cutting down forests.

The big climate question is what happens to this extra human-released CO2. Some of it goes into the sea. Some of it stays in the atmosphere. But scientists have been unable to account for a significant amount of it.

In 1990, the Intergovernmental Panel on Climate Change gave the following estimates. They are stated in terms of billions of tons per year of carbon.

Fossil fuels release around 5.4 units of carbon in the form of CO2. Land-use changes release about 1.6 units. About 3.2 units remain in the air, while 2 units go into the sea. But 1.8 units are unaccounted for. Geochemists now think this ``missing'' carbon is absorbed on land, especially by northern forests (see related story, left).

Geochemist Jorge L. Sarmiento at Princeton University in Princeton, N.J., says he now believes that ``the most likely thing is that this `missing' [carbon] sink is terrestrial.'' He adds that ``our ignorance of the terrestrial biosphere is, in my opinion, the greatest obstacle to understanding the carbon cycle.''

Dr. Sarmiento notes that, as skepticism about human-induced global warming gives way to concern, governments need to know what to do. Here, he says, the critical uncertainty is the CO2 uptake of the terrestrial biosphere. Will Earth's breathing retard the buildup of CO2 in the atmosphere or, as the climate warms, will it add to that buildup?

That's where the Ames mapping technique may help. Potter says his group is ``essentially trying to account for every gram of carbon going into or coming out of the [terrestrial] biosphere.'' But he also notes that this now means making ``educated guesses'' about such key factors as the efficiency of photosynthesis and soil-decay processes. He explains that ``the satellite sees where green leafy areas of our planet are distributed and where they are most dense.'' This provides the precise geographical information his computer simulation needs. But Potter says the simulation also needs detailed studies of what's going on in various plant-soil systems.

Steven Wofsy, a geochemist at Harvard University in Cambridge, Mass., agrees. Studies that his group has made in the Harvard forest at Petersham, Mass., are one way to provide that detail. His instrumentation tracks CO2 flow throughout the year. Dr. Wofsy says he and his colleagues are learning ``interesting things.''

For example, their findings suggest that computer simulations of forest ecological processes don't realistically mirror moisture flows. They also suggest that vigorously growing forests like the one they are studying may be absorbing more CO2 than computer modeling shows.

Wofsy's group is expanding its work to include other types of forest in other parts of the world. Essentially, they have demonstrated a research technique that Wofsy says can contribute the kind of detailed understanding that will make computer simulations such as the Ames mapping more realistic. He adds that researchers studying the details of Earth's ``breathing'' at specific sites need to learn how to link that knowledge to the global picture. Potter says, ``I think [our mapping] can help them do that.''

It may also give new insight into how Earth responds to climate warming. Potter notes that the past 10-year period his group is analyzing includes two El Nino events. These involve a warming of surface waters in the equatorial Pacific Ocean that has planetwide weather effects. Potter explains that this may serve as a short-term natural ``experiment,'' which may indicate how Earth's ``breathing'' responds to a climate perturbation.