I interviewed Dr. Earle to ask her whether, having seen what she has seen, she thinks we have time to mitigate climate change. (The following has been condensed and edited for clarity.)

What is the single most profound change you’ve witnessed in a lifetime diving?

There was a period in the 1970s when I dived in an area called Lee Stocking Island, in the Bahamas. We got to know the individual fish and there was one giant boulder, a big brain coral, that was just thriving with life. It was a destination: “Let’s go see Rainbow Reef and admire that monstrous coral.” It was in 1980 that it turned into a snowball [it bleached]. It was shocking. That was climate change in action.

What do you say to people who are not convinced by the evidence?

I think about poor Galileo, 500 years ago or so, when he had evidence that Earth is not the center of the universe and was ostracized. Now, we have evidence that we are totally dependent on the natural systems that hold the planet steady within the temperature range that is safe for us. You can measure the shrinking Arctic and Antarctic ice. Half the coral reefs have either gone or are in a state of sharp decline. I think it’s getting easier, because the ocean is beginning to speak for herself.

You helped persuade President George W. Bush to create an ocean monument 100 times the size of Yosemite. Now, the Trump administration is trying to shrink similar protected areas. How does that make you feel?

Quoting a former Republican president who was not widely celebrated for his environmental ethic, Ronald Reagan: Protecting the environment is not a liberal or conservative thing, it’s just common sense. He at least could see the connection between the economy and the environment. When you lose the quality of a place environmentally — the trees, the water, the space, the air — there are economic and human social consequences. It’s taken a longer time for people to realize that the ocean is vast and resilient, but it’s not too big to fail.

What do you tell people who tell you that they don’t have hope, or they don’t know what they can do?

This is the best time ever to be around, because we have the power of knowing. Look in the mirror to recognize your personal power. It’s a matter of using what you’ve got. Take a kid out to some wild place and see the world through that child’s eyes. Look at the future and imagine you’re there, 50 years out. We’ve reached a time when we really have to choose between our wants and our needs, and we need a planet that works.