He did not want to be a messenger of doom in his Oxford lecture, but Lord May, one of the UK's leading scientists, said the world was at best facing increasing climate disruption, at worst, devastation

"We don't know how many distinct species of animals and plants are alive on Earth today," said Lord May of Oxford, speaking at the first Annual Oxford Earthwatch Lecture, held at the Said Business School. He continued: "For a ball-park figure on extinction, we can say one species of bird or animal has become extinct per year over the last century.

"This means that, if birds and mammals are typical, we know that, over the past century, extinction rates have increased by at least 100 to 1,000 times the average rate seen over the half-billion-year sweep of the fossil record.

"This suggests we may see a further ten-times increase in extinction rates by the end of the century we are in. It is the kind of acceleration that characterise the big five' mass extinctions in the fossil record, such as the one that saw the end of the dinosaurs."

Leaving aside, for the moment, our knowledge about the diversity of organisms on Earth and the likely consequences of any impending loss, Robert May OM AC Kt, does not equivocate when it comes to climate change. He calls it the biggest problem humans have ever faced.

And this from one of our leading scientists. He is a former Chief Scientific Advisor to the Government (1995-2000) and President of the world's oldest scientific organisation, the Royal Society (2000-2005), and holds a professorship jointly at Oxford University's Department of Zoology and Imperial College, London.

In his lecture, Hard Choices for Tomorrow's World, Lord May looked at what he said were unintended consequences of well-intentioned actions that multiply our impact on the globe.

He ranged over subjects from human population growth to changing infant mortality rates and patterns of health, rural to urban shift and the growth of mega cities, to increasing energy, food and water shortages, and, ultimately, our reliance on fossil fuels and the resulting rise in carbon levels.

The theme fitted well within the ethos of Earthwatch, the environmental organisation that organised the event.

In its 35-year history, Earthwatch has supported thousands of research projects worldwide, from climate studies in the Amazon, to studying orchids in Bengal, to the nocturnal habits of badgers in Wytham Woods in Oxford.

Of the 130 projects supported, 61 focus on threatened species, seven on climate change, and a further seven are looking at the effects of climate change on threatened species.

Lord May, who has received international awards for his pioneering research into ecology andmathematical analysis of biodiversity, touched upon the implications of our "human ecological footprint" for other species, arguing that the question of sustainability could no longer be ignored.

The lecture was timely. Only the week before, the Government's present Chief Scientist, Prof David King, warned that because the world's governments could not agree on cutting greenhouse emissions, climate change was inevitable.

At the same time, politicians are vying for the environmental moral high ground: Chancellor Gordon Brown declared we had a moral imperative to balance emissions and the Prime Minister called for a new international agreement to limit greenhouse gas emissions. The Conservative Party leader and Witney MP, David Cameron, made a trip to Norway to see the vanishing glaciers at first hand.

The warning that global temperatures are set to rise by an average of three degrees centigrade within decades means that catastrophic damage is likely to follow unless governments around the world take urgent action.

Floods, droughts and famines threatening the lives of millions may sound Biblical, but Lord May insists we accept the evidence.

"Although we have to be wary of messages of doom, the worst of times are looming," he believes. We have to confront this: at best we face increasing climate disruptions, at worst, devastation.

Of our profligate energy use, he said: "The average inhabitant of the planet today uses 14 times more energy than is needed for metabolic activities we spend ten calories to put one on the table." Most of this energy subsidy comes from fossil fuels that release carbon into the atmosphere and thicken the carbon blanket around the planet.

The carbon blanket remained remarkably steady over the ages until we started burning fossil fuels around 1780. Humans have undoubtedly affected their world. Lord May suggests we might call the post-1780 period, the Anthropocene, that is, seeing humans as the most important entity.

Populations continue to grow at alarming rates. Within a lifetime the world population has more than trebled in the 1930s, it was two billion, now it is six-and-a-half billion. By 2050, it will be nine billion a growth rate never before seen in history. In addition, by the year 2050, the shift away from rural living means two thirds of the global population will be urbanised.

But what to do about it all? The expert who has worked tirelessly to gain international consensus over global warming admits there are no easy answers.

"There are tremendous barriers to effective action. For one, it is hard to get it into perspective. Global problems are unfolding in the blink of an eye in geological terms, but it is difficult to appreciate for it seems climate change is happening slowly. Whereas serious consequences are the better part of a century away."

Take the three-degree rise in temperature. To get this into perspective, there is only a five-degree difference between the average temperature today and the Ice Age.

We must not be stunned into inaction because of the enormity of the problem, he maintains.

"The problems may be global, but the answers have to be national and, ultimately, local and down to the individual. It is hard to predict what aggregates of individuals, as distinct to large organisations, can do."

And where better to start than the path-setting approach that is part of the Earthwatch culture inspiring individuals to take responsibility themselves for the environment?

Climate Change the Truth on Trial, the annual Earthwatch debate on October 26 is chaired by Sir Crispin Tickell, special advisor to Earthwatch. It is free but tickets must be requested. Earthwatch Institute (Europe) is at 267 Banbury Road, Oxford. For more information visit their website www.earthwatch.org