Trees in the Sierras are dying at rates faster than at anytime in at least 15 years. In fact, scientists say they haven't seen it this bad since they started doing surveys back in 2001.

The cause? California's ongoing drought and persistent pests known generally as bark beetles. Those are the findings from the latest aerial survey from the U.S. Forest Service, which estimates that more than 27 million trees died in 2015.

For more, we're joined by Jeff Moore, regional aerial survey program manager for the Forest Service.