09/21/2009 AT 08:55 AM EDT
has a new gig: United Nations goodwill ambassador in a field that's very familiar to her – the environment.
"I'm really honored to have received a call from the UN to spread the message about climate change in the world," the supermodel, 29, told reporters during an appearance in Manhattan's Washington Square, where her new position was announced Sunday.
Casually dressed in jeans and a T-shirt, Bündchen, who in December is expecting her first child
with husband Tom Brady, 31, displayed just the slightest baby bump at the event, where she discussed what inspired her activism.
"I started paying attention to what was happening with the environment when I had a visit to the Xingu area of the Amazon in 2003. I spent a week with an Indian tribe and everyone was complaining they were getting sick because the waters were polluted," she said.
"All these pesticides were going into the rivers because there were no trees to protect the rivers. The fish were sick. The water was polluted. I was like, 'Who is doing something about this?' "
She decided to take action, blogging her green tips on her Web site
and creating a charity, Agua Limpa, whose aim is to plant trees along the Amazon to help purify the water. Now, she's joining forces with the United Nation's Environmental Programme to spread the good word.
"I couldn't have asked for a better childhood,” Bündchen said. “We must act now, so future generations have the same opportunity. Mother Earth is our fundamental life-support system, and by becoming aware and responsible now, we can assist in preserving the planet."