For the first time, scientists have established a causal link between climate change and the timing of a natural event — the emergence of the common brown butterfly.

Although there have been strong correlations between global warming and changes in the timing of events such as animal migration and flowering, it has been hard to show a cause-and-effect link.

Now, an international team, led by the University of Melbourne, has established such a link — the scientists compared temperature changes in Melbourne, where the butterfly is common, with recorded observations of the first brown butterfly to be seen in the spring since the 1940s.

“With each decade, the butterflies emerged 1.6 days earlier and Melbourne heated by 0.14 degrees Celsius. Overall, the butterfly now emerges on average 10.4 days before it did in the 1940s. And we know the rise in air temperature links to butterfly emergence in a cause-and-effect pattern,” team leader Michael Kearney was quoted by the New Scientist as saying.

The scientists are confident in the cause-and-effect relationship for two reasons. First, they placed eggs of the butterfly, Heteronympha merope, in chambers where temperature could be controlled and found that each larval stage has a different response to warmer-than-normal conditions.

“Each stage has adapted to best survive the season that it finds itself in. The egg, laid in late summer, and the first larval stage do well at high temperature,” said Kearney.

The second to the fifth stages occur in winter and can’t survive high temperatures. The knock-on effect is that the caterpillar pupates earlier and the butterfly emerges sooner.

Second, the scientists made a mathematical model combining these physiological effects of temperature on development with climate data. The model precisely matched the observed changes in butterfly emergence date.

“There is a very high likelihood that the locally observed climate change is human—caused,” said Kearney.

“Summer to lay her eggs, ensuring that the larvae’s development stages align with the seasons. If summers are longer, the female must ‘wait around’, and it’s unknown if her lifespan can cope. It also may be too warm for the developing butterfly to get through the larval stages,” Kearney said.

The findings have been published in the latest edition of the Biology Letters journal.