It was revealed in June that California’s southern Sierra Nevada is now stocked with an estimated 66 million dead trees, all killed directly or indirectly by the state’s ongoing drought.

The number is staggering and difficult to comprehend. Even more difficult is understanding what it means. In short, the southern Sierra – the highest section of California’s majestic mountain range – is undergoing ecological changes on a scale never seen before in human history. The estimate was obtained by the U.S. Forest Service using surveys conducted during airplane flights over the range.