Dr. Boyle said that most of the association’s 224 members now have some sort of climate message.

The form varies from subtle to pointed. The zoos in Cincinnati and Toledo, Ohio, for instance, have installed prominent solar arrays over their parking lots to power exhibits and set an example. The San Diego Zoo and the Brookfield Zoo near Chicago have made their exhibits of polar bears and other Arctic species more direct about the threats posed by global warming.

So far the feedback has largely been positive, officials at most zoos say.

Ariella Camera, a counselor with a summer program run by Boston Rising, an antipoverty group, said some of her charges recently took part in a game at the New England Aquarium that taught them what emits carbon dioxide (many factories, most cars) and what absorbs it (trees and the ocean). They were then challenged to balance the two.

Afterward the students struck up a lively conversation about their carbon footprints, Ms. Camera said. “It was a very engaging presentation,” she said.

Such anecdotes gratify Howard Ris, the aquarium’s president. “We would like as many people, if not everyone, to leave encouraged to take action,” he said.

Others are dubious that it will work. “Zoos have been making claims about their educational value for 150 years,” said Jeffrey Hyson, a cultural historian and the director of the American studies program at St. Joseph’s University in Philadelphia. The zoos “say a lot more about what they think they are doing than they can really demonstrate.”

Zoo managers acknowledge that they initially struggled with the challenge of delivering bad news.

In the 1980s and ’90s, Dr. Boyle noted, some zoos and aquariums made a big push to emphasize threats like the depletion of the earth’s ozone layer, the razing of rain forests by loggers and farmers and the overfishing of the Pacific. Electronic boards toted up the numbers of acres being cleared, and enlarged photographs depicted denuded landscapes.