Population policies have little impact on the way a minority of humans use the Earth's resources.

The birth of a baby is usually an occasion for joy. The arrival, however, of the seven billionth person is being awaited with growing trepidation about the devastating impact of humans on the planet. Environmentalists are arguing in circles about who or what is to blame: the total number of people; or the amount of water, food, mineral ores or clean air each demands. Professor Paul Ehrlich, whose book The Population Bomb helped ignite this debate, likens the environmental impact to the area of a rectangle: one side is the size of population, the other their consumption.

Although Ehrlich's rectangle is a neat illustration, the population “problem” for the environment is more accurately described as two rectangles, each representing the number of people on the vertical and their lifestyles on the horizontal: one tall skinny quadrant encompasses billions of people who use very little of Earth's resources; the other a much shorter, extraordinarily long one for the minority of humans who use the vast majority of natural wealth. The World Bank estimates, for example, that the richest fifth of the world has more than three-quarters of the income; the poorest fifth just 1.5 per cent.

Given that populations are barely stable and sometimes falling in most of the rich world, population policy would inevitably have to make noticeable inroads into the tall-skinny many/poor rectangle. Assuming such policies were successful — and excluding the widely unacceptable coercion of China's one child policy or India's mass sterilisations in the 1970s, persuading people to have fewer babies has proved very tricky — the overall reduction in combined environmental impact would be very small.

The more troubling issue, though, is that this calculation assumes that as the tall-skinny rectangle gets shorter, it does not get wider. Experience, however, suggests that, except for extreme cases such as Zimbabwe, it will get fatter.

A better life?

Across time and geography, countries that have reduced birth rates have got richer and so more consumptive: rising incomes, better health and education give men and women the confidence that more of their children will survive into adulthood and help support their families; and as birthrates fall governments can spend more on each person's health, education and jobs, feeding a virtuous cycle of economic development and slowing population growth.

It would be interesting to see a proper assessment of the point at which the benefit of having fewer people consuming is offset and then increasingly dwarfed by their greater consumption. There are some telling pointers. Comparison by the London-based Guardian's James Ball of the CIA World Factbook data for countries' birthrates and average purchasing power of each person shows a pretty strong correlation between the two.

Statisticians are quick to point out that because two things appear to be linked does not mean one causes the other, but on-the-ground evidence suggests rising affluence and declining fertility rates are inextricable.

Time after time descriptions of countries that have successfully reduced population growth show how they have grown notably richer at the same time, even if they are not exactly well—off: Guatemala in central America, Bangladesh in south-east Asia, and the Asian tiger of South Korea.

At the same time, study after study shows environmental damage rises — so far almost always perpetually — with income, and often more steeply as developing countries begin to industrialise. Most dramatically, these forces appear to have come together in China, whose one-child policy — albeit with massive state investment and rapid expansion of the market economy — has coincided with the country's rise to become the world's second biggest economy (and, incidentally, the biggest emitter of greenhouse gas pollution).

Technically speaking, of course, population campaigners are right: environmental degradation can be helped by reducing the number of people and what they use. Population policies are best left to those focusing on poverty and women's rights. For environmentalists, talk of too many people is a dangerous distraction for campaigners and consumers, too many of whom will find it a convenient excuse to ignore the more pressing need for changes to what and how we spend our growing riches. ( Juliette Jowit is the Observer's E nvironment Editor.) — © Guardian Newspapers Limited, 2011