Even when the pair considered habitat destruction or other possible underlying causes for behavior changes in plants, animals and other wildlife, the analyses still pointed to global warming.
"Allowing there to be non-climatic explanations in the model requires the global climate change pattern to be stronger to stand out," said Dr. Camille Parmesan, an assistant professor of integrative biology. "But even then, we still came up with the same overall conclusion – that these changes are linked to general global warming."
The research of Parmesan and Dr. Gary Yohe, an economist at Wesleyan University in Middletown, Connecticut, was published in Nature. The two met in 1998 as participants in the Intergovernmental Panel on Climate Change commissioned by the World Meteorological Organization and the United Nations Environmental Program. Parmesan was the lead author of the panel’s 2001 publication on the current impact and future effects produced by climate change.
Economists such as Yohe, with whom Parmesan worked on the publication, had difficulty believing that global warming was the only answer to the physical and biological changes in wildlife behavior that biologists were measuring.
"I realized that if we couldn’t convince these economists, who are very intelligent, are highly trained and have worked in the environmental field, then we were going to struggle to convince the general public or politicians that this change is real," Parmesan said.
In the extensive, global statistical analysis on climate change that they published, the two took steps to ensure that only the best studies were analyzed with a critical eye.
Their most rigorous analysis of 99 species in North America and Europe showed that the range of territory of wildlife such as birds, butterflies and alpine herbs has shifted northward at an average of 6.1 kilometers (3.79 miles) per decade, or to higher altitudes by an average of 6.1 meters (about 20 feet) per decade.
Global warming has increased temperatures slightly (by about one degree), which would be expected to cause temperature-sensitive wildlife to favor cooler locations that are further north or higher in elevation.
The same approach was used to analyze 172 species and showed that migratory birds, amphibians and other animals were breeding earlier in the spring, and plants and flora blooming earlier.