The 400-page report by the Food and Agricultural Organisation, entitled Livestock's Long Shadow, also surveys the damage done by sheep, chickens, pigs and goats. But in almost every case, the world's 1.5 billion cattle are most to blame. Livestock are responsible for 18 per cent of the greenhouse gases that cause global warming, more than cars, planes and all other forms of transport put together.

Burning fuel to produce fertiliser to grow feed, to produce meat and to transport it - and clearing vegetation for grazing - produces 9 per cent of all emissions of carbon dioxide, the most common greenhouse gas. And their wind and manure emit more than one third of emissions of another, methane, which warms the world 20 times faster than carbon dioxide.

Livestock also produces more than 100 other polluting gases, including more than two-thirds of the world's emissions of ammonia, one of the main causes of acid rain.

Ranching, the report adds, is "the major driver of deforestation" worldwide, and overgrazing is turning a fifth of all pastures and ranges into desert. Cows also soak up vast amounts of water: it takes a staggering 990 litres of water to produce one litre of milk.

The United States is the only country to ever deny funding to UNFPA for non-budgetary reasons. In July 2002, the U.S. Administration announced its decision to withhold $34 million - that had previously been appropriated by Congress to UNFPA - based on false claims that the Fund supports coercive abortion in China. The decision was taken despite recommendations made by a State Department fact-finding team to release the funds. The State Department team investigated UNFPA-supported projects in China and submitted a report in May stating, based on what we heard, saw, and read, we find no evidence that UNFPA has knowingly supported or participated in the management of a program of coercive abortion or involuntary sterilization in the PRC [People's Republic of China]. Indeed, UNFPA has registered its strong opposition to such practices.

The current Administration has, so far, withheld $127 million in funds appropriated by Congress. UNFPA has publicly stated that it has never, and will never, be involved in coercion in China or any part of the world. The Fund is a staunch supporter, and promoter, of the human rights-based approach to family planning.

So if we believe that the size of the human "footprint" is a serious problem (and there is much evidence for this) then a rational view would be that along with a raft of measures to reduce the footprint per person, the issue of population management must be addressed.

Let us assume (reasonably) that an optimum human population level exists, which would provide the physical and intellectual capacity to ensure a rich and fulfilling life for all, but would represent a call upon the services of the planet which would be benign and hence sustainable over the long term.

A scientific analysis can tell us what that optimum number is (perhaps 2-3 billion?).

With that number and a timescale as targets, a path to reach "Utopia" from where we are now is, in principle, a straightforward matter of identifying options, choosing the approach and then planning and navigating the route from source to destination.

World population is expected to increase by 2.6 billion over the next 45 years, from 6.5 billion today to 9.1 billion in 2050. Almost all growth will take place in the less developed regions, where today's 5.3 billion population is expected to swell to 7.8 billion in 2050. By contrast, the population of the more developed regions will remain mostly unchanged, at 1.2 billion.