How did Earth Day become an international event in 1990?

Earth Day became an international event in 1990 through the organizational efforts of Denis Hayes, who had previously helped coordinate the inaugural 1970 Earth Day in the United States alongside Senator Gaylord Nelson.1

By expanding the scope of the event beyond the United States, Hayes successfully mobilized approximately 200 million people across more than 140 countries to participate in the 1990 observance. This global expansion transformed Earth Day into a worldwide platform for raising awareness about pressing ecological issues, such as global warming and the necessity of renewable energy. Since this pivotal year, the event has remained international in scope, characterized by diverse activities including educational lectures, citizen science initiatives, and environmental demonstrations.1