NORTH AMERICANS can breathe a sigh of relief. By at least one measure, they are not the planet鈥檚 environmental pariahs. So concludes a study comparing how much of the planet鈥檚 biological resources countries consume with how much their territory produces through photosynthesis.
Marc Imhoff of NASA鈥檚 Goddard Space Flight Center in Maryland, who led the study, says it shows the 鈥渦neven footprint of human consumption鈥� on the land (see Map). It places Europeans and Asians in the environmental doghouse, but shows North Americans to be relatively light consumers.
Several recent studies have looked at the ecological 鈥渇ootprint鈥� of nations, according to how much of the world鈥檚 resources they consume. Others have investigated the overall biological productivity of the planet鈥檚 ecosystems by measuring net primary production 鈥� that is, plant matter produced by photosynthesis from solar energy. But until now, nobody has combined the two into a detailed, region-by-region balance sheet of who consumes more than they produce, and vice versa.
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First, Imhoff mapped the net primary production of land ecosystems using satellite images of global vegetation. This produced an estimated global biological productivity of 57 billion tonnes of carbon a year. Next, he calculated how much of this resource humans use. The figure came to around 11.5 billion tonnes of carbon per year 鈥� three-quarters of it used to produce food and firewood. That is almost 2 tonnes for each member of the world鈥檚 population. That means about a fifth of all the biological matter produced by photosynthesis on the planet鈥檚 land surface each year is consumed by humans. 鈥淭his is a remarkable level of co-option for a single species,鈥� Imhoff says.
He then produced detailed maps comparing the supply and demand sides of the 鈥渂alance sheet鈥� for each part of the world. The findings are often counter-intuitive.
As expected, rich people use more of the planet鈥檚 biological resources. More striking is the comparison of consumption patterns with domestic biological productivity. Sparsely populated areas consume less than 1 per cent of the productivity of their lands, while large urban conurbations consume many hundred times more than they produce.
Larger regions show more moderate trends, with South America and Africa doing best. The huge biological output of their rainforests ensures that their citizens consume respectively only 6 and 12 per cent of what their land produces.
At the other end of the spectrum are densely populated western Europe, and east and south Asia, where there is little natural vegetation left but vast human demand for food, firewood and fibres. Western Europe consumes 72 per cent of what it produces; east Asia consumes 63 per cent; and south Asia consumes a record 80 per cent.
By comparison, in North America photosynthesis produces 6.7 billion tonnes of carbon a year 鈥� 9 times as much as western Europe 鈥� but its people consume some 1.6 billion tonnes, only 3 times as much as western Europeans. The result is that North Americans consume a relatively modest 24 per cent of their net primary productivity 鈥� which is close to the global mean.