The creation of Earth Day is widely considered the beginning of the modern environmental movement.
As climate activists marked the event this year, an unplanned experiment is changing the planet.
Many people continue to stay at home to stop the spread of the new coronavirus. As a result, people are making less pollution, and the air has become cleaner.
Smog stopped covering New Delhi, one of the world's most polluted cities. Nitrogen dioxide pollution in the northeastern United States has dropped 30 percent. Air pollution levels in Rome have dropped 49 percent compared to a year ago. Stars seem more visible at night.
People also report seeing wild animals in unusual places. Coyotes have been observed walking in downtown Chicago and near San Francisco's Golden Gate Bridge. A puma was seen in the streets of Santiago, Chile. Goats entered a town in Wales and showed no interest in leaving.
When people stay home, the Earth becomes cleaner and wilder.
Stuart Pimm is a scientist at Duke University in the United States. He says the stay-at-home orders worldwide are "giving us this quite extraordinary insight into just how much of a mess we humans are making of our beautiful planet."
Pimm told The Associated Press that the situation is providing a chance to "see how much better it can be."
Scientists, stuck at home like the rest of us, say they are interested in studying unexpected changes in plants, insects, weather, noise and light pollution.
Researchers have been observing sharp drops in traditional air pollutants, such as nitrogen dioxide, smog and tiny particles. These kinds of pollutants kill up to 7 million people a year worldwide, says Dan Greenbaum, president of the Health Effects Institute.
Cleaner air has been most noticeable in India and China. On April 3, people living in Jalandhar, a city in north India's Punjab, woke up to a sight not seen for many years: snow-covered Himalayan mountains more than 160 kilometers away.
Cleaner air means stronger lungs for asthmatics, especially children, said Mary Prunicki. She is a doctor and director of air pollution and health research at the Stanford University School of Medicine in California. She noted early studies have linked coronavirus severity to people with weak lungs and those living in more polluted areas.
Animals take over
In Australia, police shared a video of a kangaroo on social media. It showed the animal jumping around a mostly empty neighborhood in downtown Adelaide. Several wild jackals recently occupied a city park in Tel Aviv, Israel.
We are not being invaded, Duke's Stuart Pimm noted. The wildlife has always been there, but many animals only come out when people are not around.
Human activity usually makes it difficult for sea turtles to leave their eggs on sandy coastlines. Nesting turtles need to be undisturbed. After they come out of their eggs, baby turtles might have problems finding their way to the water because of bright lights, said David Godfrey. He is executive director of the Sea Turtle Conservancy.
But with lights mostly off and people in their homes, the sea turtle nesting seems much better this year -- from India to Costa Rica to Florida, Godfrey said.
"There's some silver lining for wildlife in what otherwise is a fairly catastrophic time for humans," he added.
I'm Ashley Thompson.