04/06/2007 at 03:00 PM EDT
With her third child due this summer
and twins Hazel and Finn about to turn 2½, Julia Roberts has good reasons to want to save the planet.
"It sounds corny, but it's true: It's our absolute responsibility to take care of our children so that we don't just throw up our hands and say, 'Wow, we pretty much f---ed this up. Good luck to ya in 50 years.' " Roberts, 39, tells Elle
in its May "Green" issue.
"Because we did create the problem little by little and that's the way we're gonna have to fix it, little by little, the best we can."
Roberts, who won an Oscar for her performance as an activist in 2000's Erin Brockovich,
has herself gotten political with her efforts to promote biodiesel fuel, especially for school buses.
"I just feel that any parent who's informed cannot put their kid on a school bus without saying to their school district or their congressman, 'Hey, man, you gotta clean this up for us!' Because it's too easy not to fix," she says.
She also took her message to The Oprah Winfrey Show.
"I've been in this business for 20 years, so I understand for better or for worse the media frenzy and the power of celebrity," she says. "And until you say something of true value on Oprah
, you don't really realize how many people watch her."
Roberts also knows that small changes can have a big impact on the environment.
"Every time I go to the grocery store carrying my grocery sacks back from the time before, I feel happy," she says. "All the tiny things, I think, really do make a difference."