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.
Over the past 20 years, scientists have measured an increase in the average growth rate of plants on a global scale. More sunshine may be benefitting this tree and others like it in the Costa Rican rainforest. (Photograph Copyright Phillip Greenspun)
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
series of AVHRR (Advanced Very High Resolution Radiometer) sensors. The digital
satellite observations were processed and refined into maps by NASA’s Global
Inventory Modeling and Mapping Studies project headed by Compton Tucker at
Goddard Space Flight Center. It was the comparison of satellite-based vegetation
and climate data that allowed them to pinpoint decreased cloudiness in the
tropics as the main driver of increased productivity, something that hadn’t
been seriously considered before.
|The growth of bristlecone pines in the Inyo Mountains of eastern California is limited by the extreme aridity of their climate, and by cold temperatures in the (relatively) damp spring. Increased rainfall or warmer winters would likely lead to a higher growth rate in these ancient forests. (Photograph Copyright Phillip Greenspun)|
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 series of AVHRR (Advanced Very High Resolution Radiometer) sensors. The digital satellite observations were processed and refined into maps by NASA’s Global Inventory Modeling and Mapping Studies project headed by Compton Tucker at Goddard Space Flight Center. It was the comparison of satellite-based vegetation and climate data that allowed them to pinpoint decreased cloudiness in the tropics as the main driver of increased productivity, something that hadn’t been seriously considered before.
The group of scientists who did this study have been working together off and on for many years. Nemani worked with University of Montana colleague Steve Running in 2001 on a study of whether good vintages could be scientifically tied to climate variation, and found that they could—even suggesting that good vintages might be predicted by observing sea surface temperatures off California and winter climate. Since the late 1990s, Ranga Myneni of Boston University has published a series of papers in collaboration with some of the same researchers on the impact of the last two decade’s climate changes on Northern Hemisphere vegetation, describing how lack of snow cover and warmer temperatures are lengthening the Northern Hemisphere growing season by almost two weeks and increasing productivity. After all these years of working together, it was natural for them to pool their expertise and interests to “go global” with the study of climate and vegetation.
Nemani says it would be nice if the next decade were as favorable for plants as the past two seem to have been. “Unfortunately, we have no way of knowing yet whether climate changes will continue to have a positive effect on vegetation productivity,” he cautions. “India, for example, got a blessing from nature during the 1990s. For 100 years, there has been a strong relationship between El NiNiñoo and the monsoon season that brings rain to India and Southeast Asia; El NiNiñoo events interrupt the monsoon and create drought. In the 1990s, that relationship broke down, and the monsoon rains came despite a severe and persistent El Niño.” As a result, while much of the globe saw a decrease in productivity during El Niño events, India was one of the places where productivity increased. Whether the region can count on such a lucky break this decade can’t be predicted.
That unpredictability means that in all likelihood, we shouldn’t be dismissing our worries about carbon dioxide in the atmosphere or congratulating ourselves on our green thumb just yet. “Humans claim about half of all the net primary production on Earth,” says Myneni. “Productivity may have increased 6 percent in the last 18 years, but human population has increased by over 35 percent over that same time. One half of a 6 percent increase in the net productivity compared to a 35 percent increase in population means that these net primary productivity changes have not improved global habitability in any significant way.”
“This global study is a good foundation,” concludes Nemani. “It helps us decide where on the globe we should look more closely at what is happening with Earth’s vegetation. But as far as the whole carbon cycle goes, this is only part of the picture. We didn’t look at how climate changes might have influenced other ecosystem processes that release carbon dioxide back into the global system.” Warmer temperatures could increase the rate at which soil microbes decompose organic matter and release carbon dioxide. In the tropics, lack of cloud cover might temporarily increase productivity until the increased evaporation caused by all that extra sunlight makes water availability more important. Productivity could drop just as easily as it has risen. What this study does tell is that, so far, climate change is making the Earth’s vegetation more productive, but the impact is small compared to how quickly the human population is growing. This knowledge could be a key piece of information for societies around the world as they cultivate the global garden through agriculture, natural resource management and environmental policy.
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