What price nature? At pounds 20 trillion a year it is truly our most precious asset

What price a wild flower, or lark song, or a view of wooded hills and meadows? Poets might ask the question but biologists and economists have now conspired to come up with an answer.

Package all nature into one global job lot and it's worth pounds 20 trillion a year to humanity. That's almost twice the world's entire gross national product of pounds 11 trillion a year.

Nature, then, is very big business indeed. We take most of what it does for us, in making life possible and delightful, for granted. But we could not begin to meet the bill if we were made to pay for it.

The estimate, by a group of US, Dutch and Argentinian scientists is published in today's issue of the science journal Nature. Much of their work was done during an intensive, one-week workshop at a new National Centre for Ecological Analysis and Synthesis at the University of California, Santa Barbara.

The 13 scientists divided the earth's multitude of habitats into 16 broad categories or "biomes". These included the deep oceans, fish-rich coastal waters, coral reefs, forests and grassland.

For each biome, they estimated what an average hectare was worth in terms of providing 17 different "ecosystem services". These included recreation and culture, food and raw material production, absorbing and recycling man-made wastes, preventing soil erosion and regulating the climate. Cities, ice and rock, desert and sub-arctic tundra were excluded on the grounds that they provided negligible or zero ecosystem services.

Each average hectare of open ocean was estimated to be worth pounds 162 per year, while each hectare of tropical forest came in at pounds 1,216. Most valuable of all were swamps and flood plains worth pounds 11,939 per year - most of that through supplying water and controlling floods. Humanity's croplands, however, provided only pounds 56 per hectare per year, nearly all of that in food production. Add all of this largess up for the entire globe and you get the monstrous pounds 20 trillion which, say the scientists, only gives a "crude, initial magnitude" and is almost certainly an underestimate.

But what does it mean? "One way to look at this ... is that if one were to try to replace the services of ecosystems at the current margin, one would need to increase global gross national product by pounds 20 trillion," they write.

But "this impossible task would lead to no increase in welfare because we would only be replacing existing services, and it ignores the fact that many ecosystem services are literally irreplaceable".

The details of their methods and sums are far too big for a Nature article but a six-page spreadsheet and 18 pages of footnotes are available on the journal's web site, http://www.nature.com.

Nature, however, can be extremely costly to humanity too. Another article in the same issue says the earthquake in Kobe, Japan in January 1995 was the most costly natural disaster in history, doing pounds 79bn worth of damage. It lasted only 20 seconds and killed more than 6,000 people.