Two prolonged periods of global warming might have been the cause of the mass extinction 250 million years ago. Shucheng Xie at China University of Geosciences in Wuhan and his co-workers have evidence that two prolonged periods of global warming at that time, most probably due to volcanic eruptions, might be the cause of the ecological crisis.

http://www.naturechina.com.cn/nchina/2008/080110/full/nchina.2007.288.html