Researchers have found that because of a rise in temperature, caused by an increase in greenhouse gas emissions by humans, the common brown butterfly now emerges from its cocoon 10 days earlier than it did 65 years ago.

"This is the first study in Australia, and one of the first studies around the world, that has linked changes in a natural system to regional climate change, and shown that the change in regional temperatures are due to increases in greenhouse gases in the atmosphere,'' said an author of the study, David Karoly, of the University of Melbourne.

Until now, many studies have only been able to demonstrate "links" between climate change and observed changes in flora and fauna. It has been hard to prove that climate change was the direct cause of such changes.

In the case of the brown butterfly, observations around Melbourne over the past 65 years have suggested it has been emerging earlier in spring each year. The butterfly is also found in South Australia, and the east coast of NSW.

Melbourne's weather over that period has been getting warmer, said the lead author, Michael Kearney, also of the University of Melbourne, whose research is published in Biology Letters.

To determine if the two changes were linked, Dr Kearney and his graduate student measured how fast a group of common brown caterpillars developed at different temperatures. They then compared their lab experiments with temperature records for Melbourne over the past 65 years, to predict when the butterflies would have emerged each year.

Dr Kearney said these predicted emerging times ''matched'' the actual butterfly emergence times that had been observed and recorded by scientists.

It was then left to the leading climate scientist, Dr Karoly, to discover if the rise of almost 1 degree since 1944 as recorded by the Bureau of Meteorology was caused by greenhouse gas emissions released by humans.

Using multiple climate models, Dr Karoly was able to show that the increase in temperature observed in Melbourne was outside the range of natural climate variability. The rise in temperature could be explained only when the effect of greenhouse gas emissions were added to the models, he said.

Dr Kearney said man-made climate change probably had a similar effect on other butterfly species.