Though she doesn’t go so far as to call herself hopeful, Dr. Ayana Elizabeth Johnson is one of those experts trying to change the mood. She’s a marine biologist and a founder of the Urban Ocean Lab, a think tank focusing on climate and coastal cities. She has also worked with the Environmental Protection Agency and advised lawmakers on climate policy. Additionally, Johnson, who is 43, is a leading climate activist and communicator. She was an editor of the best-selling climate anthology “All We Can Save,” and her next book, “What if We Get It Right?” which will be published this summer, is a collection of interviews with leaders from various fields about promising climate possibilities.