Man's
impact on planet enigmaticEnvironmental even keel or global
crisis - no one knows.
21
December 2001
Nature, Science Update
http://www.nature.com/nsu/011227/011227-4.html
TOM CLARKE
Calculating
man's impact on the planet is difficult. So it's hard to
say whether we are on an environmental even keel or hurtling
towards a global crisis, new research concludes.
The
slice of global land net primary productivity (NPP) - a
measure of the food and carbon-containing resources that
land plants make available - that humans consume could be
10 per cent or 55, according to Stuart Rojstaczer and colleagues
at Duke University in Durham, North Carolina1.
"[The
research] highlights the incompleteness of our understanding
of the whole Earth system," says Chris Field, who studies
global ecology at Stanford University in California.
Global
NPP is shared between man and the rest of life on the planet.
"The fraction appropriated by humans indicates the
breadth of our actions and suggests the fraction left for
other species," says Field.
An influential
1986 study put that fraction at 32 per cent - alarmingly
high according to many experts, especially given projected
increases in the global human population2. Taking into account
the share of NPP required by other organisms, man's consumption
seemed close to the total capacity of the planet.
Rojstaczer's
team set out to update this assessment using the latest
satellite data on agriculture, grazing and forestry. Using
the latest satellite data on agriculture, grazing and forestry,
they extrapolated results from small-scale studies of land
use. They also estimated the uncertainty of the measurements
used in their model.
They
too came up with 32 per cent for man's average appropriation
of NPP. But many of the factors that give rise to this figure
are highly uncertain, they warn.
Farming
out
Measuring
agricultural productivity accurately on a global scale is
particularly fraught, the team found. Variations in fertilizer
use, irrigation and crop type mean that productivity can
vary as much as fivefold, they say. Man's use of, and impact
on, tropical forests is also very hard to gauge.
Nonetheless,
the new study "gives the best feel for the most likely
impact", says Field. What's more, it identifies the
areas where hard facts are lacking.
Field
suspects that 32 per cent may still be an underestimate.
Calculations do not yet include factors such as climate
change and pollution, which may significantly alter plant
growth and therefore NPP. But it is clear, says Field, that
humans take a huge slice of NPP - "probably more than
any other species in Earth history".
Estimates
of NPP consumption can only improve as better data become
available from satellites and agricultural ministries. Until
then, says Rojstaczer, "the magnitude of the human
footprint on Earth is open to much speculation".
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References
1. Rojstaczer, S., Sterling, S. M. & Moore, N. J. Human
appropriation of photosynthesis products. Science, 294,
2549 - 2552, (2001).
2. Vitousek, P. M., Ehrlich, P. R. & Matson, P.A. Human
appropriation of the products of photosynthesis. Bioscience,
36, 368 - 373, (1986).
