Plant Growth Surges After Global Temperature Spikes, Scientists Report

     El Nino events or volcanic eruptions can boost or depressglobal temperatures within months, but their strongest impacts onthe earth's biosphere may not occur until one to three yearslater, according to a paper published in the October 31st issueof Science.

     Regional analyses show that a global warm spell's initialboost in plant activity is clustered in polar and temperateareas.  On the other hand, heat-stressed tropical and semiaridregions may show an initial drop in plant production.

     The results, reported by scientists affiliated with theNational Science Foundation (NSF)-funded National Center forAtmospheric Research (NCAR) in Boulder, Colorado, lend credenceto the notion that biological effects of global change can varysubstantially across the globe.

     According to NCAR's David Schimel, one of the paper'sauthors, the results highlight the power of new data sets onglobal change, as well as the usefulness of computer models thatconnect the atmosphere and biosphere.

     "We were looking specifically for delayed ecosystemresponses in this study because they had been predicted by themodels," Schimel notes.

     The global temperature record revealed several multiyearpatterns, including warming associated with El Nino events in the1980s.  These patterns were correlated globally with carbondioxide levels and regionally with vegetation growth.  Globalcarbon dioxide levels, which are steadily rising due to humanactivities, tended to rise more quickly over the first few monthsafter a global temperature peak.  The carbon dioxide levels roseat a slower pace during the one-to-three-year period after thetemperature peak, followed by another gradual acceleration.

     The authors studied the temperature-vegetation relationshipby region at data points separated by one degree latitude andlongitude (roughly 85 by 110 kilometers, or 50 by 70 miles).  Atthe peak of a warm period, plant growth tended to increase inpolar and temperate regions and decrease at lower latitudes,including tropical rainforests and drier savanna/grasslandregimes.  "This contrast suggests that . . . temperature may havedirect negative impacts on plant growth, or may increase waterstress in semiarid ecosystems," the authors note.

     However, in the one-to-three-year period after a temperaturepeak, the patterns appear to reverse:  plant growth is enhancedin the warmer and drier regions and limited at higher latitudes.Thus, low-latitude plant growth appears to be driving theenhanced uptake of carbon dioxide during this period.

     The paper highlights the importance of regional analyses ofclimate change to detect areas where effects may run counter to aglobal average.  This is the first data-based study to considerregionally-specific ecosystem responses on a global scale, saysSchimel.  The results show that ecosystems are sensitive totemperature perturbations.

     Co-authors of the paper include Schimel, and Rob Braswell,Ernst Linder and Berrien Moore, of the University of NewHampshire (UNH).