(CNN) - Tomorrow, April 22, is Earth Day.
Environmental groups around the world organize rallies and demonstrations to help raise awareness about environmental concerns.
One of the biggest challenges in recent years is how to combat climate change and greenhouse gas emissions.
The first Earth Day on April 22, 1970 is credited with starting the modern environmental movement.
It was the brainchild of Democratic Wisconsin Senator Gaylord Nelson.
An estimated 20 million Americans participated in the first Earth Day.
Later that same year, the government passed an expanded clean air act and created the EPA.
Now, more than 40 years later, Earth Day is observed by a billion people in 180 nations around the world.