El Nino events or volcanic eruptions can boost or depress global temperatures within months, but their strongest impacts on the earth's biosphere may not occur until one to three years later, according to a paper published in the October 31st issue of Science.
Regional analyses show that a global warm spell's initial boost in plant activity is clustered in polar and temperate areas. On the other hand, heat-stressed tropical and semiarid regions may show an initial drop in plant production.
The results, reported by scientists affiliated with the National Science Foundation (NSF)-funded National Center for Atmospheric Research (NCAR) in Boulder, Colorado, lend credence to the notion that biological effects of global change can vary substantially across the globe.
According to NCAR's David Schimel, one of the paper's authors, the results highlight the power of new data sets on global change, as well as the usefulness of computer models that connect the atmosphere and biosphere.
"We were looking specifically for delayed ecosystem responses in this study because they had been predicted by the models," Schimel notes.
The global temperature record revealed several multiyear patterns, including warming associated with El Nino events in the 1980s. These patterns were correlated globally with carbon dioxide levels and regionally with vegetation growth. Global carbon dioxide levels, which are steadily rising due to human activities, tended to rise more quickly over the first few months after a global temperature peak. The carbon dioxide levels rose at a slower pace during the one-to-three-year period after the temperature peak, followed by another gradual acceleration.
The authors studied the temperature- vegetation relationship by region at data points separated by one degree latitude and longitude (roughly 85 by 110 kilometers, or 50 by 70 miles). At the peak of a warm period, plant growth tended to increase in polar and temperate regions and decrease at lower latitudes, including tropical rainforests and drier savanna/grassland regimes. "This contrast suggests that . . . temperature may have direct negative impacts on plant growth, or may increase water stress in semiarid ecosystems," the authors note.
However, in the one-to-three-year period after a temperature peak, the patterns appear to reverse: plant growth is enhanced in the warmer and drier regions and limited at higher latitudes. Thus, low-latitude plant growth appears to be driving the enhanced uptake of carbon dioxide during this period.
The paper highlights the importance of regional analyses of climate change to detect areas where effects may run counter to a global average. This is the first data-based study to consider regionally-specific ecosystem responses on a global scale, says Schimel. The results show that ecosystems are sensitive to temperature perturbations.
Co-authors of the paper include Schimel, and Rob Braswell, Ernst Linder and Berrien Moore, of the University of New Hampshire (UNH).