McNamara, who started surfing at 11, has made a name for himself for riding waves from collapsing glaciers to pioneering tow surfing and breaking the world record for the biggest wave ever surfed, in the Portuguese town of Nazare.
"Everything I have came from the ocean and I feel it's time to give back," McNamara, 51, told Reuters during the launch of his anti-plastic campaign in Lisbon. "The government needs to declare a state of emergency and mandate laws now."
Originally from Massachusetts, McNamara discovered the biggest wave in the world off Nazare, 120 km north of Lisbon, and set his first record in 2011 when he rode a 78-foot (24 metres) wave. Two years later he beat it by riding a 100-foot (30 metres) wave at same spot.
McNamara's surfing career slowed in 2016 after he fell off his board and badly fractured the humerus bone in his left arm. He questioned his place in the surfing world and is now pursuing a "bigger purpose", he said.
With the new initiative, he hopes to encourage world leaders and companies to reduce waste and production of plastic.
Many environmental campaigners see plastic as the biggest threat to the ocean. According to the United Nations, more than eight million tonnes of plastic leaks into the ocean every year, costing the lives of one million seabirds and 100,000 marine mammals.
"I would like to start at the root, where it's being produced and where it's being given out," he said. "Big corporations are a perfect place to start."
So far the surfer and his team have convinced outdoor clothing company Thule to make a backpack from recycled ocean plastic. He hopes to enlist a total of 20 companies by the end of 2020.
He is also organising beach cleanups, awareness events and using social media to encourage people to reduce their own plastic footprint.
(Reporting by Catarina Demony, Editing by Axel Bugge and Angus MacSwan)
