Man’s impact on planet enigmatic

Environmental even keel or global crisis – no one knows.

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“.

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).