Another Life: Leaving it to the fertility gods is tempting destruction

At sunrise on one of the golden mornings the sheep and their pastures sparkled with dew. Tapping briskly forth on the boreen, an ageing squire with an ash stick, I halted at the tracks of a couple of hares that had crossed not long before me. These were printed in dew, crisply black on the gravelly road, and took separate paths between well-worn notches in the field banks. Next to meeting hares eyelash to eyelash, as I do from time to time, their pawprints seemed a nicely casual greeting.

It took me to thinking of Irish hare populations: how they can double from year to year – from four to eight per square kilometre in one recent Irish study – and what sorts their fertility so mysteriously.

All populations in nature are eventually controlled by food supply, adequate space, disease or predation. And that carried me on to consider the mad multiplication of the human race, gobbling up the space of a finite planet and wiping out more and more fellow species of its natural world.

It's no surprise that David Attenborough, the great naturalist, 90 next year, has made the impact of a runaway human population the great cause of his closing years. He does it not least because official reports on planetary problems – climate change, extinctions, sustainable food supply – so often embrace the general taboo on discussing human overpopulation (for video of a key speech go to populationmatters.org).

Anyone who shares his respect for the natural fabric of the planet must surely feel the same way. Pope Francis, however, of whom many had such green hopes, prefers to leave to the fertility gods the destructive sprawl of our race. As Attenborough has warned: "Either we limit our population growth or the natural world will do it for us."

The latest population projection from the UN has raised estimates from those it offered even three years ago. World population, at 7.3 billion this year, is now expected to reach 9.7 billion by 2050 and 11.2 billion by 2100 – even though 48 countries, mainly European, are due some domestic population loss.

Our neighbouring island, however, is expected to add 21 million in the next 65 years, some of which must spill over the Irish Sea, if only for stag nights and rugby finals. The Republic’s population, by official projection, could swell to 6.7 million by 2046, and 10 million seems likely thereafter.

The eight million or so of this island’s pre-Famine years is remembered on our hillside in the mossy foundations of ancient cabins, the corrugations of the grassed-over lazybeds, a handful of stones on a boulder, set in moss since a child left them there.

Whatever happens to comparative birth rates, the new movement of people on the planet is giving national populations the fluidity of mercury. Half the immediate global increase in people is allotted to Africa, but the millions fleeing now from violence and poverty in the Middle East and Africa will be followed by millions more escaping northwards from the droughts of climate change. Our green and temperate island could seem an enduring refuge – until the Gulf Stream, swamped by the melting Arctic, finally withdraws its warmth from the northern Atlantic Ocean.

This year’s UN population report also considered longevity. Between now and the end of the century the rise in life expectancy, from 70 to 83, already widely operable (ha!) in Europe, will become the global average, and the number of over-60s in the world will rise to 3.2 billion. Only Africa will stay rather younger, its demography distorted by fatal plagues.

The normal life expectancy of humans, like that of most other mammals, is a concept torn between statistics and science. When have there ever been the “optimal conditions” for human health and survival in which the deaths of the requisite 10 per cent of the population could be assessed?

There is instead, it seems, something called the Hayflick limit, in which the normal human cell can replicate and divide only 40 to 60 times before it gives up the ghost. But the more or less accepted historical records of oldest people, as summarised for Wikipedia, rise in fairly even jumps from 103 in 1798 to 122 in 1997.

One of the last supercentenarians was Jeanne Caiment of Arles, in southern France – she remembered Vincent van Gogh as very ugly and rude – who rode a bike until she was 100 and stopped smoking only five years before she died.

Above all, apparently, she never let anything worry her, which is why, as a stressed and reckless chain smoker for most of my first 40 years, I find myself living on borrowed time.

Among the many human follies revealed to the elderly is the constant refinement of geriatric repair that is leading societies into bankruptcy, or at least a dire distortion of state spending.

Because I’m worth it – thank you.