Who carries the greatest responsibility for the deaths of unborn children in this country? I accuse the leader of the Catholic church in England and Wales, His Eminence Cardinal Cormac Murphy-O'Connor. I charge that he is partly to blame for our abnormally high abortion rate.

Let me begin with a point of agreement. "Whatever our religious creed or political conviction," Murphy-O'Connor writes, the level of abortion in the UK "can only be a source of distress and profound anguish for us all". Quite so. But why has it climbed so high? Is it the rising tide of liberalism? The absence of abstinence? Strange as it may seem, the evidence suggests the opposite.

Last week the cardinal sacked the board of a hospital in north London. It had permitted a GP's surgery to move on to the site, and the doctors there, horror of horrors, were helping women with family planning. Though it is partly funded by the NHS, St John & St Elizabeth's is a Catholic hospital, which forbids doctors from prescribing contraceptives or referring women for abortions. The cardinal says he wants the hospital to provide medical help that is "truly in the interests of human persons".

Murphy-O'Connor has denounced contraception and abortion many times. That's what he is there for: the primary purpose of most religions is to control women. But while we may disagree with his position, we seldom question either its consistency or its results. It's time we started. The most effective means of preventing the deaths of unborn children is to promote contraception.

In the history of most countries that acquire access to modern medical technology, there is a period in which rates of contraception and abortion rise simultaneously. Christian fundamentalists suggest the trends are related, and attribute them to what the Pope calls "a secularist and relativist mentality". In fact it's a sign of demographic transition. As societies become more prosperous and women acquire better opportunities, they seek smaller families. In the early years of transition, contraceptives are often hard to obtain and poorly understood, so women will also use abortion to limit the number of children. But, as a study published in the journal International Family Planning Perspectives shows, once the birth rate stabilises, contraceptive use continues to increase and the abortion rate falls. In this case one trend causes the other: "Rising contraceptive use results in reduced abortion incidence." The rate of abortion falls once 80% of the population is using effective contraception.

A study published in the Lancet shows that between 1995 and 2003, the global rate of induced abortions fell from 35 per 1,000 women each year to 29. This period coincides with the rise of the "globalised secular culture" the Pope laments. When the figures are broken down, it becomes clear that, apart from the former Soviet Union, abortion is highest in conservative and religious societies. In largely secular western Europe, the average rate is 12 abortions per 1,000 women. In the more religious southern European countries, the average rate is 18. In the US, where church attendance is still higher, there are 23 abortions for every 1,000 women, the highest level in the rich world. In central and South America, where the Catholic church holds greatest sway, the rates are 25 and 33 respectively. In the very conservative societies of east Africa, it's 39. One abnormal outlier is the UK: our rate is six points higher than that of our western European neighbours.

I am not suggesting a sole causal relationship: the figures also reflect changing demographies. But it's clear that religious conviction does little to reduce abortion and plenty to increase it. The highest rates of all - 44 per 1,000 - occur in the former Soviet Union: under communism, contraceptives were almost impossible to obtain. But, thanks to better access to contraception, this is also where the decline is fastest: in 1995 the rate was twice as high. There has been a small rise in abortion in western Europe, attributed by the Guttmacher Institute in the US to "immigration of people with low levels of contraceptive awareness". The explanation, in other words, is consistent: more contraception means less abortion.

There is also a clear relationship between sex education and falling rates of unintended pregnancy. A report by the United Nations agency Unicef notes that in the Netherlands, which has the world's lowest abortion rate, a sharp reduction in unwanted teenage pregnancies was caused by "the combination of a relatively inclusive society with more open attitudes towards sex and sex education, including contraception". By contrast, in the US and UK, which have the developed world's highest teenage pregnancy rates, "contraceptive advice and services may be formally available, but in a 'closed' atmosphere of embarrassment and secrecy".

A paper published by the British Medical Journal assessed four programmes seeking to persuade teenagers in the UK to abstain from sex. It found that they "were associated with an increase in the number of pregnancies among partners of young male participants". This shouldn't be surprising. Teenagers will have sex whatever grown-ups say, and the least familiar with contraception are the most likely to become pregnant. The more effectively religious leaders and conservative papers anathemise contraception, sex education and premarital sex, the higher abortion rates will go. The cardinal helps sustain our appalling level of unwanted pregnancies.

But the suffering his church causes in the rich nations doesn't compare to the misery inflicted on the poor. Chillingly, as the Lancet paper shows, there is no relationship between the legality and the incidence of abortion. Women with no access to contraceptives will try to terminate unwanted pregnancies. A World Health Organisation report shows that almost half the world's abortions are unauthorised and unsafe. In East Africa and Latin America, where religious conservatives ensure that terminations remain illegal, they account for almost all abortions. Methods include drinking turpentine or bleach, shoving sticks or coathangers into the uterus, and pummelling the abdomen, which often causes the uterus to burst, killing the patient. The WHO estimates that between 65,000 and 70,000 women die as a result of illegal abortions every year, while 5 million suffer severe complications. These effects, the organisation says, "are the visible consequences of restrictive legal codes". I hope David Cameron, who wants restrictions on legal terminations in the UK, knows what the alternatives look like.

When the Pope tells bishops in Kenya - the global centre of this crisis - that they should defend traditional family values "at all costs" against agencies offering safe abortions, or when he travels to Brazil to denounce its contraceptive programme, he condemns women to death. When George Bush blocks aid for family planning charities that promote safe abortions, he ensures, paradoxically, that contraceptives are replaced with backstreet foeticide. These people spread misery, disease and death. And they call themselves pro-life.

· Bring on the Apocalypse: Six Arguments for Global Justice, by George Monbiot, is available for £10.99 from Guardian Books; to order visit guardianbooks.co.uk or call 0845 606 4232

g.monbiot@zetnet.co.uk