NASA Satellite Measures Earth's Carbon Metabolism

Combining space-based measurements of a range of plant properties collected by the Moderate Resolution Imaging Spectroradiometer (MODIS) with a suite of other satellite and surface-based measurements, NASA scientists produce composite maps of our world's "net primary production" every 8 days. This new measurement is called net production because it indicates how much carbon dioxide is taken in by vegetation during photosynthesis minus how much is given off during respiration. Scientists expect this global measure of the biological productivity of plants to yield new insights into how the Earth's carbon cycle works, a critical step toward solving the climate change puzzle.

The rate of carbon fixation through photosynthesis is a basic property of life on planet Earth. It is the basis for capturing and storing the energy that fuels our world's living systems and forms the foundation of the food webs. The oxygen we breathe is a byproduct of this photosynthesis. According to its creators, these new net primary productivity maps provide a fascinating new insight into the intimate connection between the living world and the physical world.

"We are literally watching the global garden grow," says Steve Running, MODIS Science Team member and director of the Numerical Terradynamic Simulation Group at the University of Montana. "We now have a regular, consistent, calibrated and near-real-time measure of a major component of the global carbon cycle for the first time. This measure can also be the basis for monitoring the expansion of deserts, the effects of droughts, and any impacts climate change may have on vegetation growth, health, and seasonality."

On land, notes Running, photosynthesis is the foundation for agricultural crop production, rangeland grazing capacity and forest growth. "We also anticipate that our new productivity maps should help to significantly improve analysis of global crop commodities."

The new maps show that the highest mid-summer productivity rates are found at temperate latitudes with mild climates and not at tropical latitudes, as some might have expected. However, tropical forests are more productive over a full year because of their longer growing season. Viewing the global maps sequentially in a 2-year movie reveals some fantastic seasonal cycles of plant growth, especially at high latitudes across North America, Europe, and Asia. The movie also reveals the almost immediate response of land plants to changing daily weather patterns.

However, plant life in the ocean is somewhat more buffered and therefore not as directly driven by weather patterns, states Wayne Esaias, biological oceanographer at NASA's Goddard Space Flight Center. The growth of microscopic marine plants (phytoplankton) in the ocean responds more to seasonal changes-currents, temperature, and sunlight. So, whereas certain areas on land will swing abruptly from very low to very high rates of photosynthetic activity, biological productivity in the ocean is ongoing steadily and is spread over much wider areas.

"It doesn't surprise Earth scientists, but the public might be surprised to learn that there is so much photosynthesis in the oceans," observes Esaias. "When you average the productivity rates over the whole world, the ocean is roughly equal to the land."

Esaias is examining how plant productivity rates in the ocean vary in response to changes in the ocean's current patterns. In particular, he says, these new primary productivity maps will help fisheries scientists understand why there are good catches some years and poor catches in others.

For the last two decades, using data from earlier satellite sensors, scientists have been able to map global concentrations of chlorophyll, the green pigment marine and land plants use for photosynthesis. But it was still a leap for scientists to estimate how much carbon was converted to organic material by plants-a measure now routinely provided by the net primary productivity maps.

The new MODIS maps mark a major milestone in the careers of both Running and Esaias-a milestone they have been working toward for more than 20 years. "As Earth systems science began in the 1980s, ecology was way behind the atmosphere and oceans disciplines in achieving a global perspective because our training was on single organisms (i.e., dissecting frogs and counting dandelions), so we had no global-scale theory or measurements," states Running. "But this new measurement attests that ecology is now catching up in global science."

Esaias adds that this is just the first cut and there is much work left to do to refine their maps. "The world is a big place and we are only just beginning to fully understand and validate what we see in our data around the globe and over time. We know we can make improvements in some areas, but it is good to now have the global context to pull together research that is being done locally in various regions around the world."

Launched in December 1999 and May 2002, Terra and Aqua are the flagships of the Earth Observing System series of satellites and a central part of NASA's Earth Science Enterprise. The mission of the Earth Science Enterprise is to help us understand and protect our home planet.