Greening the Earth

Leaving aside for a moment the deforestation and other land cover change that continue to accompany an ever-growing human population, the last two decades of the twentieth century were a good time to be a plant on planet Earth. In many parts of the global garden, the climate grew warmer, wetter, and sunnier, and despite a few El Niño-related setbacks, plants flourished for the most part

Numerous small-scale studies over the past twenty years suggested that patches of the garden were getting greener, but that trying to paint a global picture would be a monumental project. A team of eight scientists from across the country worked for almost a year and half to pull together satellite data on vegetation and ground- and satellite-based climate observations. Their results show us not only how vegetation productivity has changed during two of the warmest decades in the record books, but they also reveal which of the many factors that influence plant productivity have been most important in those changes.

When scientists talk about productivity they are specifically talking about how much carbon ends up stored in the living biomass--roots, trunks, and leaves of plants--after they tally up carbon gains through photosynthesis and carbon losses through respiration. This tally of gains minus losses is called "net primary production." Scientists estimate net primary production by observing how leafy vegetation is and how much sunlight it is absorbing, which can both be measured by satellite. Combined with climate data on rainfall, temperature, and available radiation, the satellite observations reveal where carbon intake increased--and biomass grew--across the globe.

"Between 1982 and 1999, 25 percent of the Earth's vegetated area experienced increasing plant productivity--a total increase of about 6 percent," says Ramakrishna Nemani, the study's lead scientist. "That increase occurred mainly in the tropics, and secondarily in high northern latitudes. What's interesting about our results is that they show how the increase in each of these regions is due to a different climate factor. "In the tropics, Nemani and his colleagues discovered that the increase in productivity was caused by lack of clouds and increased Sun exposure, while in the northern latitudes, it was mainly due to increased temperatures and to a lesser extent, water availability.

Increases in productivity are important in a practical sense, since plant biomass is the food and fuel for all animals--including humans--on the planet. It's also important in the way that everything related to carbon has become important in recent years. Scientists and environmental policy decision makers across the world want to know what is happening to all the carbon in the carbon dioxide pumped into the atmosphere through fossil fuel and biomass burning, such as forest fires or firewood used as fuel. If carbon dioxide is "food" for plants, maybe more of it in the atmosphere caused plants to grow better.

"Experiments conducted in places like the U.S and Europe, where scientists pumped extra amounts of carbon dioxide gas into forests, did seem to show that such 'carbon dioxide fertilization,' caused plants to grow better--up to a point," says Nemani. "But this didn't go on year after year. Most people agree that a doubling of carbon dioxide could increase plant growth between 0 and 25 percent depending on resource limitations such as soil nutrition. With the 9 percent increase in carbon dioxide that occurred between 1980 and 2000, even the upper limit cannot explain the productivity increases in Amazon." Clearly, carbon dioxide fertilization couldn't be solely responsible for the change; climate change must be playing a role as well.

To reach these conclusions, Nemani, and colleagues from Scripps Institute of Oceanography, University of Montana, NASA's Goddard Space Flight Center, and Boston University used global climate data from the National Center for Environmental Prediction to determine the relative importance for various locations of the three key variables that influence plant growth: temperature, water availability, and sunlight. They indexed areas based on which of those factors most limited plant growth across the Earth. Lack of sufficient water limits the growth of 40 percent of Earth's vegetation, temperature limits 33 percent, and lack of sufficient sunlight limits the remaining 27 percent. Of course, these factors overlap in some cases; for example, both cold winters and dry summers limit plant growth in the western U.S.

After identifying key regulators of plant growth across the globe, Nemani and his colleagues then looked at how those climate conditions changed over the past two decades. They compared these changes to satellite-based maps of vegetation collected by the National Oceanic and Atmospheric Administration's