Australian scientists say they have uncovered a "causal link" between the early emergence of a common butterfly and human-induced global warming.
Dr Michael Kearney of the University of Melbourne and colleagues report their study on the butterfly Heteronympha merope in this week's issue of Royal Society journal Biology Letters.
"It's now coming out about 10 days earlier than it was 60 years ago," says Kearney.
"When you look at the air temperatures over that time, it's getting warmer."
Kearney says the local Wurundjeri Aboriginal people have traditionally defined one of their seasons as beginning when they see the male of the common brown butterfly on the wing.
"That part of their calendar would be shifted 10 days earlier," he says.
Kearney says that while previous studies have found a correlation between global warming and animals coming out earlier in spring, this study is the first to provide evidence of a causal link between this phenomenon and human-induced global warming.
He says, his team has carried out laboratory experiments to quantify the physiological effect of rising temperatures on butterflies and has also shown the measured temperature increases are not due to natural climatic variation.
"It's causal all the way through," says Kearney.
For the laboratory work, team member, Natalie Briscoe spent hours in the lab, feeding caterpillars under different conditions to see how temperature affected their emergence into winged butterflies.
"The warmer it is, the faster they will emerge," says Kearney.
This enabled the researchers to calculate how many days it would take a caterpillar to emerge given a particular temperature.
They then combined this laboratory evidence on butterfly physiology with historical temperature records, to predict how soon butterflies would have emerged each year between 1944 and 2005.
Kearney and colleagues found these predictions matched with butterfly emergence times as stated in museum records.
Records showed an increase of approximately 0.14°C per decade in the region and the shift in emergence date had shifted 1.6 days per decade over the same period, says Kearney.
The final step taken by the researchers was to link the regional temperature changes with human-induced global warming.
Team member, climatologist, Professor David Karoly applied global circulation models to the Melbourne region, taking into account local factors that influence climate.
This suggested that the regional temperature changes observed over the decade were unlikely to be observed without the influence of human greenhouse emissions, says Kearney.
He and colleagues used temperature records from the Laverton weather station, located on Melbourne's outer edge.
This weather station was used to avoid the "urban heat island" effect of the city of Melbourne on temperature records, says Kearney.
The research is part of an Australian Research Council-funded project to predict the response of species to shifts in climate.
Kearney says the team hopes the findings from the butterfly study can be applied to other less common species.