(Adds British plan, quotes by Leonardo DiCaprio)
By Ayesha Rascoe and Lesley Wroughton
WASHINGTON, Sept 15 (Reuters) - The United States on Thursday joined more than 20 countries in creating 40 new marine sanctuaries around the world to protect oceans from the threat of climate change and pollution.
President Barack Obama designated the first U.S. marine reserve in the Atlantic Ocean: 4,913 square miles (12,724 square km) known for underwater mountains and canyons off the coast of New England.
Britain said it would double the area of ocean under marine protection around its overseas territories to about 2.5 million square miles (6.5 million square km), an area greater than the landmass of India.
The various sanctuaries, unveiled at a high-level conference in Washington, limit commercial fishing, oil and gas drilling, and other human activities that affect ocean ecosystems.
Altogether, countries at the two-day oceans conference will announce new sanctuaries covering nearly 460,000 square miles (1.19 million square km) of ocean, an area around the size of South Africa.
The area designated by Obama includes canyons as deep as the Grand Canyon in Arizona and underwater mountains higher than any in the United States east of the Rockies, according to Environment America, a federation of state-based environmental advocacy organizations. Environment America said it and other groups were pushing for Obama to designate more land-based national monuments before leaving office in January.
Obama, who recalled body-surfing in the Pacific Ocean while he was growing up in Hawaii, called the pledges at the conference a "pretty good down payment" but said bolder international action was needed.
"The notion that the ocean I grew up with is not something that I can pass on to my kids and my grandkids is unacceptable, it's unimaginable," Obama told the conference.
Last month, Obama expanded a massive reserve off the coast of Hawaii, the world's largest such protected area, as he works to cement his environmental legacy.
He traveled to the remote Midway Atoll in the reserve, and told the conference what it was like to snorkel among purple and orange coral as endangered monk seals sunned themselves on nearby rocks.
"I saw it. It was right there, evidence of the incredible power of nature to rebuild itself, if we're not consistently trying to tear it down," Obama said.
Opponents of the new Atlantic reserve have said it threatens the commercial seafood industry in the region. Obama said it was designed to respect the fishing industry's role in the region's economy and history.
Britain's announcement was made by British Foreign Office minister Alan Duncan, who said fully protected marine reserves are to be set up around the Pitcairn Islands in the South Pacific, and St. Helena, Tristan da Cunha and Ascension islands in the South Atlantic.
This will involve the permanent closure of around 520,000 square miles (1.3 million square km) to commercial fishing, Duncan said.
Actor Leonardo DiCaprio, who has championed environmental causes for years, called for action by world leaders and communities to protect vital marine ecosystems.
"Warming waters, acidification, plastic pollution, methane release, drilling, overfishing and the destruction of marine ecosystems like coral reefs are pushing our oceans to the very brink," the Oscar-winning actor said.
(Reporting by Ayesha Rascoe and Lesley Wroughton; Editing by Andrew Hay and Alistair Bell)