First celebrated in the United States in 1970, Earth Day went international in 1990 with events held in over 140 nations.

Over 190 countries worldwide take part in Earth Day celebrations by organizing various events every year. To see what these countries are doing to mark the day this year, click here.

The United Nations is observing the day by having discussions on promoting sustainable development and the use of renewable energy sources throughout cities and communities.

"As our population grows we have to recognize that our consumption of the planet's resources is unsustainable. We need a global transformation of attitude and practice. It is especially urgent to address how we generate the energy that drives our progress," UN secretary general Ban Ki-moon said in a statement.

Nasa is observing the 44th anniversary of Earth Day through a week-long series of online and live activities until 27 April.

The space agency is holding a two-day exhibition at Union Station in downtown Washington with displays demonstrating how it uses satellite technology to better understand the changing planet.

To further engage the public in celebrating environmental awareness, Nasa has launched Global Selfie Event in which people can post their Selfie taken against their local environment as a backdrop on Twitter, Instagram or Google+ using the hashtag #GlobalSelfie. All the photos will be later used to create a mosaic image of Earth.