Crop failures and drought within our children's lifetimes

Children today are likely to reach old age in a world that is 4C warmer, where the 10,000-year certainties of the global climate can no longer be relied on, and widespread crop failures, drought, flooding and mass migration of the dispossessed become a part of everyday life.

This dire scenario could come as early as the 2060s – well within the lifetime of today's young people. It could mark the point when, for the first time since the end of the Ice Age, human civilisation has to cope with a highly unstable and unpredictable global climate.

A series of detailed scientific assessments of this possible "four-degree world", published today, documents for the first time the immense problems posed if the average global temperature rises by 4C above pre-industrial levels – a possibility that many experts believe is increasingly likely.

The international climate negotiations which resume this week in Cancun, Mexico, are aimed at keeping global temperatures within the "safe" limit of a 2C increase. But many scientists believe that, based on current trends, a rise of 3C or 4C is far more likely.

The greatest concern is that a 4C increase in global average temperatures – a temperature difference as great as that between now and the last Ice Age – would create dramatic transformations in the world, leading to water shortages, the collapse of agriculture in semi-arid regions and triggering a catastrophic rise in sea levels in coastal areas.

One of the studies, led by scientists from the Met Office's Hadley Centre, predicts that, unless there is a concerted international agreement to curb dramatically the burning of fossil fuels such as coal, oil and gas, a 4C warmer world is virtually inevitable by the end of this century. This critical threshold could, however, be reached within just 50 or 60 years based on other factors, such as human interference with natural feedback cycles that accelerate global warming, and the sensitivity of the climate to man-made carbon-dioxide emissions.

"Most emissions scenarios have a chance that take us past the four-degree point by the end of the 21st century, but it is down to the strength of the feedbacks and the sensitivity of the climate as to when this actually happens. It's certainly not outrageous to say it could happen in the 2060s," said Richard Betts, the lead author of the Hadley Centre study, published in the Philosophical Transactions of the Royal Society.

Of equal concern to scientists is the speed of climate change. A second study in the series found that a global temperature rise of between 2C and 4C could be so rapid that it would coincide with the expected peak in global population, which is expected to reach nine billion by 2050 before it begins to fall.

This would mean that the problems of water shortages and food production caused by climate change will occur at precisely the same time as the world is having to cope with feeding the greatest number of people in its history. A slower rate of climate change, on the other hand, will see the highest temperature increases occurring after the global population has peaked.

Niel Bowerman of Oxford University, who led the study into the rate of climate change, said it highlighted the urgency of having emissions peak in coming years. "Our study shows we need to start cutting emissions soon to avoid potentially dangerous rates of warming within our lifetimes, and to avoid committing ourselves to potentially unfeasible rates of emissions reduction in a couple of decades time," he added.