Take hold of your wrist and feel your pulse. Wait for the familiar thump-thump. Then think: every time your heart makes a beat, the world's population grows by two people.
In the time it took to read the paragraph above, at least 25 more people have started living and breathing on this Earth. By the time you finish this article, the world's population will have grown by the size of a small Irish village.
Like your heart-beat, population growth is incessant, relentless. Every year we add about 78 million inhabitants to the planet, the equivalent of a new Germany. Numbers mushroom and multiply to the point of meaninglessness, and no one seems to be in control.
Today is the Day of Six Billion, a somewhat fictional birthday organised by the United Nations Population Fund (UNFPA) to mark the approximate arrival of Earth Citizen Six Billion.
Tuesday's child, wherever he or she is born, will arrive in a crowded, cluttered world, filled unevenly with the bounty of modern technology and 20th-century wealth, and the crass poverty endured by the majority of the planet's population.
It was as recent as 1960 that the Earth's population passed the three billion mark. In 20 years' time, we should be breaching the eight billion barrier, though no one really knows this for certain.
Population forecasting is an inexact science, and tiny changes in women's fertility rates (the average number of children a woman has during her child-bearing years) can make a huge difference in population size further down the road. But all the experts agree on one thing: the only way is up.
Population is the single most important yet overlooked issue determining the future of the world. But the West, with its own population growth under control, wrongly assumes it's an issue for developing countries. The latter plead that poverty must first be eradicated before women start having fewer children. In many states, large young populations serve as cheap labour or cannon fodder for national armies.
Population issues boil beneath the surface of most of the world's conflicts. Serbs are quick to claim that they were in the majority in Kosovo until a few decades ago, after which they were out-bred by the ethnic Albanian community. The genocide in Rwanda and the brutal civil war in neighbouring Burundi are partly attributable to land pressures in two of the world's most densely inhabited countries.
You don't need to go to India or China to witness the effects of growing population. Just look at the effect of a relatively small increase in population in Ireland over the past few years. Suddenly, housing lists are soaring, and Dublin is in gridlock. Meanwhile, the countryside is being consumed by concrete, our rivers and lakes are polluted and CO 2 emissions are increasing.
Sure, life goes on, and society adapts, but what emerges in the new, more crowded Ireland is different from what went before - high rise in place of housing estate, urban anonymity instead of squinting windows, forest trails instead of wild bogland. This may or may not be a good thing, but we should at least be talking about it.
Globally, rising population is the main cause behind the destruction of rain forests, over-fishing, desertification, global warming and just about any environmental calamity currently threatening the Earth.
So why isn't there a greater outcry? One reason is that population growth has defied all interventions thus far. There's a mathematical inevitability to the growth curve. The rate of increase is slowing down and will eventually level off completely, but not before the world's population reaches at least 10 billion.
The second reason is that the doom-sayers of the past have been proved wrong. They were right about the rise in population, but wrong about the world's ability to provide food and water for the extra billions.
The 18th-century thinker, Malthus, was so wrong about Europe's ability to feed itself that his name has become a by-word for pessimism.
"The battle to feed all of humanity is over," declared the leading Stanford University professor, Paul Ehrlich, in his 1968 million-seller The Population Bomb. "In the 1970s, the world will undergo famines; hundreds of millions of people are going to starve to death in spite of any crash programmes embarked upon now."
DR Ehrlich's theories were nonsense - both the US and Europe are still paying their farmers not to produce food - but they were highly influential. Western family planning groups used them to justify coercing poor countries into invasive and untested methods of controlling women's fertility. Force was often used in the dash to reach quotas and targets. But none of this worked, except in China, and the world's population kept rising.
One reason the population debate has been so muted in Ireland is because of the controversial stance of the Catholic Church on birth control. Throughout the 1970s and 1980s the church led the opposition to the use of artificial contraception and abortion to stem population growth in the developing world. With some justice, it can now be seen, it argued that the world could cope with the growing numbers.
Scientists like Ehrlich made the mistake of modelling their predictions on behaviour in the animal kingdom. Biologists talk of the "carrying capacity" of a herd of deer, demonstrating that numbers increase until no more food can be foraged and the population crashes. Then the cycle is repeated.
However, humans can anticipate the future and make changes to their environments to cope with expected changes. New technologies have driven the increases in food yields and now genetically modified organisms may produce further increases in output.
The reason the Earth's population is increasing is quite simple: death rates have declined but birth rates have not. Medicine has found ways to keep people from being born, but their application has encountered many political, cultural and religious objections. These are slowly being removed. Even though the birth rate is falling, the number of births is going up. More women are having fewer babies.
But growth is concentrated in the poorest countries. For a population to replenish itself, women must average about two children each in their lifetime. However, in developing countries outside China, women average four children. In Africa, the total fertility rate is six. Only AIDS is now serving to curb this growth.
Population growth brings with it rapid and massive urbanisation. Three-quarters of the populations of Europe and the Americas live in cities, but the corresponding figure in Africa and Asia is only one-third.
But as the countryside grows ever more crowded, millions of people in the developing world are heading for the cities. Rapid urban growth puts immense strain on the most basic resources, such as water, electricity and sewerage.
The results can be seen in the favelas clinging to the hillsides of Rio de Janeiro, the tombs used as homes in Cairo's City of the Dead and countless other shanty-towns in the poorer quarters of the planet's mega-cities.
International migration is also increasing. Deprived of economic and political strength, the poor of the world are exploiting the only power they have, numerical strength. Refugee numbers are at an all-time high, up to 30 million, and more and more are coming to the rich West in search of a better life.
Thirty years on, Ehrlich is still around, and he has yet to admit his mistakes. But his message today has taken on a green hue, and is directed squarely at the West. Density is generally irrelevant to questions of overpopulation, he asserts, pointing out that Africa's population density is just a fraction of that of Europe or Japan.
Returning to the concept of carrying capacity, he says the key to understanding overpopulation is the number of people in an area relative to the environment's capacity to sustain them.
"By this standard, the entire planet is already overpopulated. Africa is overpopulated because its soils and forests are rapidly being depleted, and that implies its carrying capacity for human beings will be lower in the future than it is now. Europe, Japan and other rich nations are overpopulated because of their massive contributions to the carbon dioxide build-up in the atmosphere, among many other reasons.
"Almost all the rich countries are overpopulated because they are rapidly drawing down stock of resources around the world. They are spending their capital with no thought for the future."
According to Ehrlich, the Netherlands, the most frequently-cited example of a rich and densely-populated country, "can support 1,031 people per square mile only because the rest of the world does not".
The main population problem, Ehrlich concludes, is in wealthy countries. "There are, in fact, too many rich people."
Recognising that coercion has failed, and wary of entering into fresh rows with religious groups, UNFPA now talks in generalised terms about educating girls and improving women's status as ways of slowing population growth. The goal of population control has now been replaced by respect for reproductive rights; helping people limit the size of their families, safeguarding their fertility and the births of their children as they choose.
But is this enough? For poor people everywhere, large families are still the best insurance policy for old age. In conditions of chronic unemployment, low wages and no social security systems, children are more than a blessing, they are a source of income.
According to UNFPA, "unless the rich countries of the industrialised north address the root causes of poverty and all its attendant dangers, the task of reducing population growth will remain an unfair burden on the world's poor".
Tell that to the 2,000 people who have been born since you started to read this article.