By Environment Correspondent Alex Kirby

A warming world should benefit some creatures, say researchers from the Norwegian University of Science and Technology in Trondheim.

They have found clear evidence of the way a species responds to changes in climate.

The researchers, whose work is reported in the magazine Science, looked at a population of dippers, Norway's national bird, in the south of the country.

They found that while the number of birds fluctuated between 1978 and 1997, it followed an upward trajectory, and that this trend was closely linked with the North Atlantic Oscillation.

The NAO is an atmospheric pressure system which affects much of the northern hemisphere. In its high phase it brings warm, wet winters to northern Europe, and bitter cold in its low phase.

Birds to benefit

For the dipper, warm years are good years. But when streams are covered with ice in cold years, the bird cannot dive to hunt for food on the stream bottoms.

After warm years, the researchers found, the dipper's numbers increased, because of increased immigration and a higher birth rate in the local population. They estimate that a long-term warming of 2.5 degrees C should increase dipper numbers by 58%.

The mathematical model the Norwegian team used separates the effects of climate change from those of density dependence, a phenomenon where mortality rates tend to rise and birth rates fall as a population's size increases.

Peter Kareiva, a biologist with the US National Oceanic and Atmospheric Administration, quoted in another Science report, says the model - which takes into account random population fluctuations and temperature changes - can also be applied to other species.

Variable effects

But not all will find a warmer world to their liking in the way the dippers appear to.

Another Norwegian team, from the University of Oslo, analysed 15 years of data on northern mammals, and found that nine of 11 ungulate populations - including caribou, musk ox, moose, feral goats and Soay sheep - declined after warm NAO winters.

But the apparent contradiction between the NAO's effects on birds and on mammals does have an explanation.

In coastal areas, the Oslo team found, the survival rates of the goats and sheep improved during mild winters, which meant increased competition for food and consequent population declines the following spring.

Further inland, high NAO years were both warm and more snowy than usual, making it harder for the animals to forage, and also making them easier prey.

Camille Parmesan, a biologist at the University of Texas, Austin, has analysed distribution patterns of 57 non-migratory butterfly species across Europe.

Climate implicated

In the last century about two-thirds of them moved their ranges northwards by as much as 240 kilometres, and some are now appearing in Nordic countries that were formerly too cold for them.

She told Science: "We ruled out all other obvious factors, such as habitat change, that could alter distributions. The only factor that correlated was climate".

The magazine also reports evidence from Costa Rica that rising surface temperatures in the Pacific cause more mist-free days in the mountains, and that this is damaging several amphibian species.