A Hyperactive Biosphere?
Remote sensing of photosynthetic activity is based on how plants use or dispose of solar radiation at different wavelengths. Green leaves absorb 85% of the incoming solar energy in the visible part of the spectrum but only 40% of the energy in the near-infrared; hence, there is a large difference in the reflectivities between these two wavelength bands. By contrast, the reflectivities for bare soils are comparable in these two portions of the spectrum. A commonly used parameter for monitoring photosynthetic activity is the normalized difference vegetation index (NDVI): the difference between the reflectivities in the visible and near-infrared bands, normalized by their sum. A high NDVI is indicative of vigorous photosynthetic activity. Recent results from satellite data suggest that on average terrestrial photosynthesis has increased by approximately 10% between 1981 and 1991 at mid to high latitudes, that is, between, 40°N and 70°N.
Should we believe in the NDVI trend? There are no "ground-truth" measurements of photosynthesis at northern high latitudes over the same period, and so the accuracy of the trend cannot be established unambiguously. The data from three radiometers onboard three different polar-orbiting meteorological satellites were used to construct the 11-year NDVI time series. The radiometers degrade at different rates, with time, for different wavelength channels. Furthermore, annoying interference by the intervening atmosphere, only partially corrected in the data, is something with which researchers must contend.
It will be a challenge for ecologists to explain how photosynthesis could possibly have increased by approximately 10% from 1981 to 1991. Over this period, there has only been a 4% increase in carbon dioxide levels, from 340 ppmv (parts per million by volume) to 355 ppmv, and therefore, this could not have enhanced photosynthesis at the NDVI rate. Temperature increases, however, may have stimulated photosynthesis directly or accelerated snowmelt and mobilized nutrients previously frozen in soils. The magnitudes of these effects remain to be investigated.
With the revelation of the rollercoaster-like fluctuations of climate, recorded in ice core data, one cannot help but wonder how the biosphere has fared in the ride. Fossil pollen records show recurring rearrangements of the seating in the past 100 thousand years. The new satellite data suggest that the biosphere, at least at northern high latitudes, has been an increasingly restless passenger in recent years. In late summer 1998, NASA is scheduled to launch AM-1, the first platform of the Earth Observing System (EOS). Instruments on board AM-1 will be calibrated; simultaneous observations of aerosols and other atmospheric constituents will allow for a careful removal of atmospheric effects. What new insights into our backyards will be revealed by the forthcoming satellite observations?
Fung, I. 1997. Climate change: A greener north. Nature 386, 659-660.