Researchers are worried this situation will only grow worse with rising temperatures and that the forests themselves will release massive quantities of carbon as the die-off continues.
"In the future, forests might store less carbon than they do at present, and it also introduces the possibility that western forests could become net sources of carbon dioxide to the atmosphere, further speeding up the pace of global warming," said study co-author Dr. Phillip van Mantgem of the U.S. Geological Survey.
"That may be our biggest concern," said Nathan Stephenson, a study coauthor and USGS research ecologist. "Is the trend we're seeing a prelude to bigger, more abrupt changes to our forests? Society needs to discuss policies that will help adapt to the changes that are well underway."
The study published in the prestigious journal Science found that tree mortality throughout western North America has doubled in the last few decades, regardless of forest type, location or elevation. The pace of the die-off is also accelerating.
Since 1995, mortality has doubled every 17 years in the Pacific Northwest and every 29 years in the US interior. Worse yet, trees were perishing in stands considered healthy – not just in areas affected by the mountain pine beetle.
Warmer temperatures mean that more precipitation falls as rain instead of snow, it evaporates more quickly, the snowpack is shrinking and summers are longer. Forest pests like the mountain pine beetle that would normally be killed off during the winter are thriving this these new conditions.
The changes we are seeing now are a result of the fairly small changes in temperature. The researchers blamed the die-off we have seen on warming of less than 0.5 degrees Celsius per decade – an increase that will greatly accelerate in the coming years.All of this is an excellent example of just how finely tuned individual ecosystems are - and just how dumb it is to continue playing with the thermostat of the planet.