Historically, population outbreaks of native bark beetles occurred periodically in western North American forests. In recent years, outbreaks of some species are more common, and warming winter and summer temperatures associated with human-caused climatic changes are implicated. The mountain pine beetle, for example, caused tree mortality across 3.3 million ha between 1997 and 2010, and the amount of carbon stored in trees killed by this insect exceeds that of carbon in trees killed by fire. These numbers highlight the importance of this insect to forest ecosystem dynamics and carbon cycles. Knowing specific physiological pathways influenced by temperature helps predict how mountain pine beetle responds in a continually changing climate. The extensive mountain pine beetle range occurs from Baja California Norte to northern British Columbia and Alberta, Canada. Across the range within the U.S., researchers found significant genetic differences among populations in response to temperature. Populations in the north develop significantly faster than populations in the south when reared at the same temperature in the laboratory, most likely an adaptation to the cooler temperatures and shorter period for growth in northern forests. Recent field studies suggest that different selection pressures on northern and southern populations allow mountain pine beetle to maintain a similar and highly successful development pathway across the western U.S. even though temperature varies greatly from north to south. These same selection pressures, however, limit the insects' ability to decrease the overall time required to complete a generation in warmer habitats. Scientists believe the genetic differences seen in this insect in its ability to respond to temperature indicate populations across the range will respond differently to changing climate. In addition, the potential for adaptation to rapidly changing conditions could result in tree mortality in warm areas of the southwest U.S. where this insect has historically not been a major cause of tree mortality.