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See Inside April 2005

The Lion's Share

Measuring the human impact on global resources

In recent years, environmental scientists have turned their attention to the total productive capacity of the earth and the share appropriated by Homo sapiens. Such measurements complement other indicators of global stress, such as atmospheric concentrations of greenhouse gases.

The latest and most detailed analysis comes from a group headed by biologist Marc L. Imhoff of the NASA Goddard Space Flight Center. Using satellite and climate data, the team calculated the worldwide amount of solar energy converted to plant organic matter through photosynthesis. This measure, called net primary production (NPP), is the chief source of food for living creatures. (It excludes other forms of energy, such as fossil fuels or seafood.) The researchers then figured out the human share of NPP based on consumption of plant foods, meat, milk, eggs, and wood used as fuel and in construction.

Imhoff and his colleagues took into account the efficiency of extraction and production methods, which varies considerably by country. In developed nations, for example, a metric ton of milled lumber requires 1.3 metric tons of tree biomass, compared with two metric tons in developing countries. The team derived the numbers for every 0.25 degree of latitude and longitude, equivalent to about 27.8 kilometers square at the equator.

The data show that average annual NPP from 1982 to 1998 was 119.6 billion metric tons of organic matter, of which humans appropriated 20 percent. (The uncertainties are large, resulting in a range of 14 to 26 percent.) The map indicates human appropriation of NPP as a percentage of that produced locally. The human share of NPP varies markedly by region. Large urban areas consume 300 times the amount of NPP generated locally, whereas in sparsely inhabited areas of the Amazon, human appropriation is close to zero.

The increasing human share of plant and fiber could have worldwide implications, including for the composition of the atmosphere and hydrological cycles. It could also reduce biodiversity, which in turn could lead to declining NPP because of the disruption of such processes as crop pollination and pest control. The rate of species extinction already appears to be accelerating.

Preserving diversity is most difficult in the developing nations, where saving species is a distant second to filling the urgent needs of their people. Indeed, if these regions raised consumption to match that of industrial countries, human appropriation of NPP could rise to more than 35 percent.

Studies of NPP are at an early stage. If they become more precise in the future, such analyses will have great value in identifying the changing effect of consumption and technology and in evaluating policies aimed at reducing the human toll on ecosystems.

This article was originally published with the title "By the Numbers: The Lion's Share."

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