When the Earth shifted from icy to tropical periods in the past, fossil records show that species shifted too. Today, climate change is moving the butterflies of Europe northward — as well as holly plants in Britain.

But what happens to those that can't move, or that find major cities or highways block their way? Enter assisted migration, a sort of emergency-relief service that springs species from global warming danger zones.

Studies on the Bay checkerspot butterfly suggest that this climate change will push the insect to extinction. The plants it depends on for food will shift their growing seasons, so that when the butterfly eggs hatch, the caterpillars have little to eat. Many other species may face a similar threat, and conservation biologists are beginning to confront the question of how to respond. The solution they prefer would be to halt global warming. But they know they may need to prepare for the worst.

One of the most radical strategies they are considering is known as assisted migration. Biologists would pick a species up and move it hundreds of miles to a cooler place.

Assisted migration triggers strong, mixed feelings from conservation biologists. They recognize that such a procedure would be plagued by uncertainties and risk. And yet it may be the only way to save some of the world's biodiversity.

There could be distinct risks for particular forest types if they are penned into particular areas by artificial human-made boundaries, with the climate gradually becoming less and less suitable for them, he cautions.

"We can help our forests to move gradually across the landscape through 'assisted migration'," Dr Kirschbaum suggests. "Without drastic cuts in Greenhouse gas emissions, it now seems that a degree of climate change is inevitable, so we need to begin to plan now to set up a series of stepping stones that will move particular forest types to the environments where they will be doing best a century or so from now."