The study found that mean emergence date for adults of the Common Brown butterfly (Heteronympha merope) has shifted 1.6 days earlier per decade in Melbourne, Australia. Early emergence is causally linked with a simultaneous increase in air temperatures around Melbourne of approximately 0.14°C per decade, and this warming is known to be human-induced.
Researchers raised caterpillars of the Common Brown Butterfly in the laboratory to measure the physiological impact of temperature on its rate of development. They used this information to model the effect of observed historical climate trends in Melbourne on the speed of the butterfly's development.
They combined this with global climate model outputs for the Melbourne area over the same period to examine whether natural climate variability or human influence on climate was more likely to have caused the air temperature change seen in Melbourne.
"Shifts in these seasonal life cycle events represent a challenge to species, altering the food and competition present at the time of hatching. Studies such as ours will allow better forecasting of these shifts and help us understand more about their consequences," says Michael Kearney from the Department of Zoology, University of Melbourne.
"Scientists have previously observed that biological events are happening progressively earlier in spring over the past few decades. This new work has tied the earlier emergence of butterflies directly to a regional temperature increase, and has tied the temperature increase very strongly to increases in greenhouse gas concentrations caused by humans," says David Karoly from the School of Earth Sciences, University of Melbourne.
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