The migration of certain species of butterfly into the North has been used as proof of global warming. But now it seems that some of them have been here before. John Dean reports.

THEY are Nature's great barometers, creatures so sensitive that they can tell if there is a change in the atmosphere almost before electronic monitoring equipment.

But now, they are posing a puzzle for scientists trying to work out the truth behind climate change. Are we witnessing a dramatic man-made, permanent rise in temperatures or simply a cyclical and natural peak?

There are many factors confusing the issue, one of which is the recent migration northwards of butterflies which are more common in the warmer south, but which have been absent from the chilly north for decades.

On the face of it, the butterflies were all telling the same story when they arrived: that the north was warming up, allowing them to move into areas which had previously been inhospitable. They were, it seemed, a bolted-on argument for permanent global warming.

But it is not that simple because most of the species are not arriving in the north, rather making homecomings to old familiar haunts, suggesting that 150 years ago the planet was warm then cooled - and raising the possibility that it may happen again.

Take the comma, a small orange butterfly with black spots. Now widespread in the North-East and a frequent garden visitor, its recovery is regarded as one of the great natural history success stories of recent years. But it was also present in the North-East more than a century ago and it was in Scotland as late as the 1920s before the northern population collapsed. Today, it has moved north once more and recolonised the North-East and North Yorkshire.

Or the gatekeeper, to be found on woodland paths and hedgerows, which has been expanding northwards, heading up through Yorkshire and into the North-East. Another sure sign of global warming? Possibly, except it was recorded in the North-East at Castle Eden Dene, Blackhall Rocks and Hesleden Dene, in east Durham, Wolsingham, in Weardale, Seaton Carew, near Hartlepool, and Newcastle at the end of the 19th century.

Or the orange tip, a denizen of roadside ditches and rough pasture, now spreading throughout the North-East but which seems to have undergone regular cycles of decline and increase since the end of the 1800s. Its current rapid expansion northward began in the 1970s long before anyone was talking about global warming.

Or the speckled wood, which likes partially shaded woodland, and whose range contracted during the late 19th and early 20th centuries, but has spread back since the 1920s and has now recolonised many areas in eastern and northern England and Scotland.

Jane Hill, a lecturer in the department of biological sciences at York University, who has studied butterflies and climate change, says that of the UK's 50 butterfly species, 12 have been expanding northwards.

She says: "We have the Victorians to thank for their records of butterflies. By examining those records you can see few species are expanding for the first time, rather re-expanding. Clearly, during the Victorian age there was a period when it became cooler and they contracted. Now they are expanding again.

"Clearly, the climate has changed before. It has been a lot warmer than it is now and it has been a lot colder than it is now. If you look 10-15,000 years ago, the melting of the ice sheet was followed by the butterflies colonising."

Man's land use may be further confusing the issue by removing important elements for butterfly success, such as feed plants and breeding areas. Over-development of land has created fragmented habitats, which means some species have difficulty expanding, warmer weather or not.

Sam Ellis, the senior regional officer for the charity Butterfly Conservation, based near Durham City, agrees that although climate change may be a factor in species' movements, the lifestyles of butterflies are just as important

Take the specialists, like the dingy skipper and the Durham Argus, which tend to move only short distances and require particular plants. If those areas are fragmented, global warming or not, they will struggle to expand.

He says that some species were moving north in the 1970s and 1980s before climate change was being talked about, adding: "This could be cyclical - and there are other factors to consider, such as habitats - but the feeling is it is down to climate change and that the change may be permanent."

Weardale environmentalist Dr David Bellamy believes that the undoubted warming is part of a natural cycle that will eventually result in cooler temperatures again.

While acknowledging that the past decade has seen records for hottest years, he believes that the planet is obeying its natural rhythm. It would certainly explain the confusing situation of scientists talking about global warming while predicting that we are heading towards the next Ice Age.

Dr Bellamy says: "Climate change has been happening over the last 4-6 billion years. If you go back 10,000 years to the end of the last Ice Age, that was global warming and since then the climate has gone up and down. In that time, I'm sure butterflies have been going backwards and forwards. It is a difficult question but I think butterflies have been migrating for centuries."

He also points out that the Romans grew grapes in Yorkshire, as further evidence that the planet was warmer before cooling, all supporting his idea that the 0.6C (1F) rise in the earth's temperature over the past century is part of a natural weather cycle.

While not suggesting we scrap efforts to reduce global pollution, he feels it is just as important to replace landscapes which naturally soak up emissions of the greenhouse gas carbon dioxide. He believes the current restoration of habitats across the UK is a key reason for the recent return of some species.

He says: "If we put back the landscape, reduce overgrazing, replace the soils we have eroded, replace the peat we have burned, replace the trees we chopped down to replace with conifers, which don't do much, then we can have all the wildlife we want. And that is starting to happen in this country."

In the meantime, the butterflies still head north, leading some scientists to question the basic assumptions about climate change.