Guardian: Trees in the western United States are dying twice as quickly as they did three decades ago and scientists think global warming is to blame. In their surveys, ecologists found that a wide range of tree species were dying including pines, firs and hemlocks and at a variety of altitudes. The changes can have serious long-term effects including reducing biodiversity and turning western forests into a source of carbon dioxide as they die and decompose. That could lead to a runaway effect …