In 1969, over 3 million gallons (11 million liters) of oil were spilled into the ocean in California following an accident at an offshore drilling platform. Inspired by media attention drawn to the pollution, as well as a growing public interest in green issues, US Senator Gaylord Nelson devised an environmental "teach-in" at college campuses in April 1970. This event became the first Earth Day.

The 1970 Earth Day was the world's largest single-day protest ever. Since then, the day has become an international event to educate and highlight environmental issues. In 2016, the landmark Paris Agreement to limit global temperature rise was signed on Earth Day by more than 170 countries. Diplomats marked the day by planting a cherry blossom tree at the United Nations Headquarters in New York.

The 50-year anniversary in 2020 was supposed to see the biggest Earth Day ever. But, with much of the world in lockdown because of the COVID-19 pandemic, protests and public meetings were impossible. The events went online — and organizers say more than 100 million people took action virtually. Swedish climate activist Greta Thunberg held an open discussion online with climate scientists.

The pandemic also meant that the UN's climate conference, COP26, scheduled for December 2020, has been postponed. In 2019 thousands of protesters at COP25 called for governments to do more — a demand that has partly been heeded — Since Earth Day 2020, several more countries have made ambitious pledges for carbon cuts, including China, Japan and, following the election of Joe Biden, the US.

One of Biden's first acts in office was to rejoin the Paris Climate Agreement, from which his predecessor, Donald Trump, had withdrawn the US. In his first days in office, Biden also announced a major climate summit on Earth Day 2021 where he is expected to strengthen climate reduction pledges. Activists hope the United States' new commitment to sustainable policies will influence other countries.

Organizers have chosen the theme for Earth Day 2021 of "Restore Our Earth." In addition to a number of online and real-life protests and educational events, people are encouraged to organize cleanups in their own communities. Governments, organizations and individuals all have a part to play in the future of our planet — and Earth Day is one way to get involved.

One of 2021's key restoration projects is the Canopy Project with the aim of helping reforestation. Planting more trees and preserving existing forests is important to carbon capture, Earth Day organizers say. Since 2010, funds raised on Earth Day have helped plant tens of millions of trees.

Key among the goals of Earth Day 2021 and for many climate activists is the improvement of climate literacy for all. Younger generations are the ones who will make good on many of the pledges made by current world leaders. Education about climate change must be ingrained around the world for all generations, Earth Day organizers say.