We are constantly warned by scientists that our planet is in big trouble, so why can't we change direction? David Suzuki, one of the world's leading ecologists, on how humans have lost the vital skill of foresight

As I approach my 72nd birthday, I have reluctantly achieved the position of elder, and it is mindboggling to reflect on the changes that have occurred in my lifetime. The population of the world has tripled, while technology has exploded from early radio, telephones and propeller planes to the telecommunication revolution, computers, space travel, genetic engineering and oral contraceptives. And stuff! My biggest challenge is to staunch the flow of stuff into my life. But these great successes - economic growth, technology, consumer goods - have come at enormous cost: the degradation of our very life support systems - air, water, soil, energy and biodiversity.

We are now the most numerous species of mammal on Earth and each of the 6.6 billion of us must breathe, drink, eat, be clothed and find shelter. So the mere act of living means our species has a heavy collective ecological footprint. When that is amplified by technology (computers, TV, cars, etc) used on our behalf, our hyperconsumption and the global economy, we have been transformed into a force that is now altering the biological, chemical and physical features of the planet on a geological scale.

Experts estimate that more than 50,000 species now become extinct annually. More than half the planet's forests are gone, and if we continue to destroy them as we are doing there will be no large intact forests left within two decades. We cannot escape the toxic debris of our industrial activity. Recently, three members of parliament in Ottawa were tested for more than 80 toxic chemicals and were shocked to learn that all three carried dozens.

The oceans are being depleted. A global study led by biologist Boris Worm, of Dalhousie University in Halifax, Nova Scotia, predicts that if habitat destruction and overfishing continue, every exploited marine species will be commercially extinct by 2048. And for 20 years, climatologists have warned us that human activity is altering the chemical composition of the atmosphere, with consequent climate change occurring at unprecedented speed.

I could go on listing the depressing litany of problems, but the need to take serious action on all of these issues is converging, and we must make major decisions to change course.

How did we arrive at this moment? As a biologist, I tend to view things from an evolutionary perspective. DNA studies reveal that we emerged as a species in Africa some 150,000 years ago - a mere blink in geological time. The world was a very different place back then. There were still woolly mammoths roaming the Earth, sabre-toothed tigers, giant sloths in North America and 3-metre tall moa birds.

Spectacular trajectory

Let's suppose scientists have discovered time travel, so we take a time machine and hover above the Rift Valley in east Africa 150,000 years ago. The plains are a spectacular sight, covered with immense herds of mammals; vast flocks of birds fill the air; rivers and lakes teem with fish and reptiles. The clusters of furless, upright apes who are our ancestors are not that impressive. Those early humans are not numerous, large, strong or fast, or endowed with special sensory abilities. There is little to indicate the spectacular trajectory this naked ape is about to follow.

But if we watch them for a while, we can recognise their special secret: their behaviour reveals that they are intelligent. The human brain endowed us with a massive memory, insatiable curiosity, inventiveness, and an ability to think in abstracts. These qualities more than compensated for our lack of physical and sensory abilities. That brain created a notion of a future, even though the only reality is the present and our memories of the past. And because that brain invented a future, we recognised that we could affect that future by what we do in the present. If we look ahead, we can see where opportunities and dangers lie, and by following a deliberate path we can avoid the hazards, while exploiting the opportunities. Foresight, I believe, was one of the most important abilities that enabled us to survive and flourish, and continues to underlie our explosive success as a species.

Today, we have all the amplified foresight conferred by scientists, computers, engineers and telecommunications, and for more than 40 years, leading scientists have been looking ahead and warning us that humanity is heading along a dangerous and unsustainable path, while there are benefits and opportunities in moving along a different direction. For example, in 1992, a remarkable document called World Scientists' Warning to Humanity was signed by more than 1,500 senior scientists, including more than half of all Nobel prizewinners alive at that time.

Here is some of what the document said: "Human beings and the natural world are on a collision course. Human activities inflict harsh and often irreversible damage on the environment and critical resources. If not checked, many of our current practices put at serious risk the future we wish for human society . . . and may so alter the living world that it will be unable to sustain life in the manner that we know. Fundamental changes are urgent if we are to avoid the collision our present course will bring about."

The document goes on to list the critical areas of the atmosphere, water resources, oceans, soil, forests, species extinction, and overpopulation. Then the words grow even more urgent: "No more than one or a few decades remain before the chance to avert the threats we now confront will be lost and the prospects for humanity immeasurably diminished. . .

A great change in our stewardship of the Earth and life on it is required if vast human misery is to be avoided and our global home on this planet is not to be irretrievably mutilated."

This is a frightening document; eminent scientists do not often sign such a strongly worded missive. But if the Scientists' Warning is frightening, the response of the media in North America was terrifying - there was no response. None of the major television networks bothered to report it, and both the New York Times and Washington Post dismissed it as "not newsworthy". And even today, when we have been told we could have as little as 10 years to avoid catastrophe, that is considered not worth reporting, while every antic of Paris Hilton or Britney Spears is reported in breathless detail, not for days or weeks but for months and years.

Instead we hear excuses to ignore the warnings: it will ruin the economy; technology will solve the problem; it is not fair when other countries are not included; there are other priorities demanding immediate attention, etc. And so we turn our backs on the very strategy that got us to where we are.

Business as usual

For decades, scientists in the US had pointed out that New Orleans was a disaster waiting to happen. The city lies in an area that is prone to annual hurricanes, half the city is below sea level, and a force 5 hurricane was bound to hit the city, so drastic measures had to be implemented immediately to avoid disaster. All the while, politicians and businesspeople countered that it would be economically ruinous to take precautionary action, and carried on with business as usual - no doubt crossing their fingers that nothing would happen during their tenure. We all know what did happen when Hurricane Katrina hit the city in 2005 and confirmed all the predictions scientists had made.

The need to look ahead and manoeuvre to exploit opportunities and avoid threats continues to be just as critical in modern society. The challenge is to find why we are rejecting foresight, why we can't see what the real threats are that confront us.

· David Suzuki is emeritus professor at the sustainable development research institute, University of British Columbia. Tonight, he gives the Commonwealth Lecture at the Old Theatre, London School of Economics, Houghton Street, London WC2. For details and tickets email: lecture@commonwealth.int