"Last century a typical forest in Siberia had about 100 years after a fire to recover before it burned again," said study leader Professor Heiko Balzter of the University of Leicester. "But new observations ... have shown that fire now returns more frequently, about every 65 years. At the same time annual temperatures in Siberia have risen by almost two degrees Celsius, about twice as fast as the global average. And since 1990, the warming of Siberia has become even faster than before."

The scientists observed 18 years of satellite images, determining from 1982 to 1999 nearly all Siberian ecosystems showed an earlier onset of spring.

"Planet Earth is always more complicated than you think," said Balzter. "The lengthening of the growing season that has been described in the scientific literature is a non-linear phenomenon. It is influenced by feedbacks between the atmosphere and the forest, which responds to rising greenhouse gas levels and higher temperatures."

The study is detailed in the Journal of Climate.