"I'm still optimistic that humans will decide to deal with greenhouse gases in a realistic fashion," he said.

To illustrate why that's important, he pointed me to an online video that shows the death of a young polar bear cub. It's linked here but, as a Mother Jones writer put it, "Be forewarned: this is graphic and ghastly." I wouldn't watch it unless you absolutely feel the need to do so. What's more important is Derocher's reaction. For him, the video echoed his experiences in the Arctic, surveying bears that are struggling because of sea ice melt.

"When you watch that," he said, "you have to, I think, be pretty hardhearted to think that maybe humans aren't treading a little too heavily on this planet."

Follow @CNNOpinion on Twitter.

Join us at Facebook/CNNOpinion.