I cannot avoid the subject any longer. Almost every day I receive a clutch of emails about it, asking the same question. A frightening new report has just pushed it up the political agenda: for the first time the World Food Programme is struggling to find the supplies it needs for emergency famine relief. So why, like most environmentalists, won't I mention the p-word? According to its most vociferous proponents (Paul and Anne Ehrlich), population is "our number one environmental problem". But most greens will not discuss it.

Is this sensitivity or is it cowardice? Perhaps a bit of both. Population growth has always been politically charged, and always the fault of someone else. Seldom has the complaint been heard that "people like us are breeding too fast". For the prosperous clergyman Thomas Malthus, writing in 1798, the problem arose from the fecklessness of the labouring classes. Through the 19th and early 20th centuries, eugenicists warned that white people would be outbred. In rich nations in the 1970s the issue was over-emphasised, as it is the one environmental problem for which poor nations are largely to blame. But the question still needs to be answered. Is population really our number one environmental problem?

The Optimum Population Trust (OPT) cites some shocking figures, produced by the UN. They show that if the global population keeps growing at its current rate, it will reach 134 trillion by 2300. But this is plainly absurd: no one expects it to happen. In 2005, the UN estimated that the world's population will more or less stabilise in 2200 at 10 billion. But a paper published in Nature last week suggests that there is an 88% chance that global population growth will end during this century.

In other words, if we accept the UN's projection, the global population will grow by roughly 50% and then stop. This means it will become 50% harder to stop runaway climate change, 50% harder to feed the world, 50% harder to prevent the overuse of resources. But compare this rate of increase with the rate of economic growth.

Many economists predict that, occasional recessions notwithstanding, the global economy will grow by about 3% a year this century. Governments will do all they can to prove them right. A steady growth rate of 3% means a doubling of economic activity every 23 years. By 2100, in other words, global consumption will increase by about 1,600%. As the equations produced by Professor Roderick Smith of Imperial College have shown, this means that in the 21st century we will have used 16 times as many economic resources as human beings have consumed since we came down from the trees.

So economic growth this century could be 32 times as big an environmental issue as population growth. And if governments, banks and businesses have their way, it never stops. By 2115, the cumulative total rises to 3,200%, by 2138 to 6,400%. As resources are finite, this is of course impossible, but it is not hard to see that rising economic activity - not human numbers - is the immediate and overwhelming threat.

Those who emphasise the dangers of population growth maintain that times have changed: they are not concerned only with population growth in the poor world, but primarily with growth in the rich world, where people consume much more. The OPT maintains that the "global environmental impact of an inhabitant of Bangladesh ... will increase by a factor of 16 if he or she emigrates to the USA". This is surely not quite true, as recent immigrants tend to be poorer than the native population, but the general point stands: population growth in the rich world, largely driven by immigration, is more environmentally damaging than an increase in population in the poor world. In the US and the UK, their ecological impact has become another stick with which immigrants can be beaten.

But growth rates in the US and UK are atypical; even the OPT concedes that by 2050 "the population of the most developed countries is expected to remain almost unchanged, at 1.2 billion". The population of the EU 25 (the first 25 nations to join the union) is likely to decline by 7 million.

This, I accept, is of little consolation to people in the UK, where the government now expects numbers to rise from 61 million to 77 million by 2050. Eighty per cent of the growth here, according to the OPT, is the direct or indirect result of immigration (recent arrivals tend to produce more children). Migrationwatch UK claims that migrants bear much of the responsibility for Britain's housing crisis. A graph on its website suggests that without them the rate of housebuilding in England between 1997 and 2004 would have exceeded new households by 20,000-40,000 a year.

Is this true? According to the Office for National Statistics, average net immigration to the UK between 1997 and 2004 was 153,000. Let us (generously) assume that 90% of these people settled in England, and that their household size corresponded to the average for 2004, of 2.3. This would mean that new immigrants formed 60,000 households a year. The Barker Review, commissioned by the Treasury, shows that in 2002, the nearest available year, 138,000 houses were built in England, while over the 10 years to 2000, average household formation was 196,000. This rough calculation suggests that Migrationwatch is exaggerating, but that immigration is still an important contributor to housing pressure. But even total population growth in England is responsible for only about 35% of the demand for homes. Most of the rest is the result of the diminishing size of households.

Surely there is one respect in which the growing human population constitutes the primary threat? The amount of food the world eats bears a direct relationship to the number of mouths. After years of glut, the storerooms are suddenly empty and grain prices are rocketing. How will another 3 billion be fed?

Even here, however, population growth is not the most immediate issue: another sector is expanding much faster. The UN's Food and Agriculture Organisation expects that global meat production will double by 2050 - growing, in other words, at two and a half times the rate of human numbers. The supply of meat has already trebled since 1980: farm animals now take up 70% of all agricultural land and eat one third of the world's grain. In the rich nations we consume three times as much meat and four times as much milk per capita as the people of the poor world. While human population growth is one of the factors that could contribute to a global food deficit, it is not the most urgent.

None of this means that we should forget about it. Even if there were no environmental pressures caused by population growth, we should still support the measures required to tackle it: universal sex education, universal access to contraceptives, better schooling and opportunities for poor women. Stabilising or even reducing the human population would ameliorate almost all environmental impacts. But to suggest, as many of my correspondents do, that population growth is largely responsible for the ecological crisis is to blame the poor for the excesses of the rich.

monbiot.com