According to a new study published in Science, global plant growth has decreased in the past decade, reversing trends observed over the past 20 years. The authors of the study, Maosheng Zhao and Steven Running, found a significant reduction in the global terrestrial net primary production (NPP), a measure of global plant growth that is calculated by a measure of estimated photosynthesis activity. Ultimately, the study reveals that plant productivity is decreasing, which means plants are taking less carbon dioxide out of the atmosphere as biomass, and there is, therefore, more CO2 in the air to reinforce current warming trends.
The Methods
By analyzing digital photographs of the earth in the visible and near-infrared spectrum accumulated from the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA's Terra satellite, along with global meteorological data, Zhao and Running were able to take daily measurements of plant productivity for every square mile of the earth, which amounts to about 68,350,830 square miles daily. They used equations based on solar radiation, day length, temperatures, water-stress levels, and drought to infer photosynthesis activity within each area of the earth. Though Zhao and Running found that the high temperatures of the past decade resulted in longer growing seasons in the northern hemisphere and recorded an increase in plant growth there, the droughts caused by globally rising temperatures in the southern hemisphere resulted in a decrease in growth, which overwhelmed the increase in the northern hemisphere. The end result: an estimated 1 percent decrease in global plant productivity over the past decade. The authors' findings are only made more significant by the fact that in the 20 years before the beginning of their study, global plant productivity was still increasing despite the global warming trends and corresponding droughts that have continued to the current day.
The Conclusion
Currently, terrestrial plants absorb 4.5 percent of fossil-fuel emissions every year, and, Running says, "if our biospheric uptake decreases, the carbon-dioxide concentration in the atmosphere will increase even faster than it already is." Though further decreases of global plant productivity are not inevitable, if the observed trend continues, it will hold enormous significance for future food security, our ability to turn to biofuel as an alternative energy source and the strength of terrestrial carbon sinks. As for further ecosystem disturbances, we can expect more accelerated wildfires, like those we are seeing in Russia, as well as large-scale insect epidemics, like those observed in the western U.S., which kill enormous numbers of trees, Running says.
The Implications
Aside from a continuation of global monitoring of plant productivity, there seems to be little that scientists and individuals can do to reverse the trend. "Studies of global irrigation patterns show that we are irrigating dry areas to capacity," Running says, "and the amount of land we are able to irrigate might even go down in the next decade because we are running out of water with which to irrigate; rivers are currently being drained to capacity." The researchers do not expect massive food shortages or famines in the near future, but the study does indicate that biofuels, such as ethanol or biodiesel, could be a more risky alternative to fossil fuels than previously thought, given the downward trend in available biomass.
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