natgeo Big Larch Forest, Yoho NP, Canada. Photo by @peteressick on assignment for @natgeo. Fall is coming to Yoho Np and the forecast for today is a high of 78F, pretty hot for 51 degrees north on Sept. 22. In the high sub-alpine zone around Lake O'Hara there are larch trees that turn a bright yellow-orange in the autumn. My local guide, Paul Sylvestre, keeps track of the date when the colors peak each year. He said the time has been remarkably consistent in recent years, always occurring on Sept. 20. He also said a friend, who is a tree expert, believes that the trees changed color based on the time of year, not a drop in temperature. I was skeptical, and even joked that maybe the trees had a notch in the rock - a la Stonehenge- that marked when the sun passed the fall equinox. When I did a story for National Geographic on climate change in 2004, I learned that plants and trees respond to warming temperatures. In the past 30 years as temperatures rose globally, spring has been arriving on average 3 days earlier and fall 3 days later. Since 2012 has been the hottest year on record, I made a calculated gamble and booked my backpacking trip to Lake O'Hara to photograph the larches to begin on Sept. 23, three days later than peak time last year. But a few days ago, I was looking up at some larches on a ridge near Lake O'Hara and they looked really yellow. I started to get a feeling not quite of panic but of concern that I might be missing the peak colors. I quickly called my contact at Parks Canada and yesterday was able to arrange a day trip to Lake O'Hara. It turned out that up close some of the larches had turned to yellow, but many were partially green. The best guess is that peak color is still 5-6 days away. This is a small, but significant finding. For many scientists who study climate change, it is one more data point that points to our uncertain future based on dangerously high carbon dioxide levels in the atmosphere. For all of us, it should be a warning that we are ignoring a probable coming catastrophe that will affect most severely our children and grandchildren. 2y

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