Over recent decades, the timing of many spring events has occurred progressively earlier year on year. Though linked to rising temperatures, attributing these seasonal shifts to climate change has proven difficult. Now, a new study, published in the journal Biology Letters, attributes the early emergence of the common brown butterfly in the city of Melbourne, Australia, to human-induced warming. <img alt=“RH_OH3.jpg” src=“http://blogs.nature.com/climatefeedback/RH_OH3.jpg” width=“300” height=“215” align=“right”//>
A team led by Michael Kearney of the University of Melbourne, Australia collected information on the emergence date of Heteronympha merope, the common brown butterfly, from 1941 to 2005. During this 65 year period, the average date on which the butterflies emerged occured 1.6 days earlier with each passing decade. During this same period, average air temperature increased by 0.14°C per decade.
In the laboratory, Kearney and colleagues calculated the development rate of eggs, larvae and pupae of the butterfly at various temperatures, using 10 field-collected females. Combining the data on sensitivity to temperature with historical monthly climate data, they used a microclimate model to predict that the emergence date would have shifted by 1.3 days earlier per decade over the same period, close to the observed rate.
Results from four global climate models show that the temperature trend over the 65 year period was very unlikely to result from natural climate variability alone. From this, the authors deduce that the seasonal shift in emergence of the butterfly is directly linked to human-induced warming.
Image: Paul Sunnucks