For Earth Hour 2010, record-breaking millions of businesses, homes, and landmarks around the world will turn off their lights Saturday evening for the sake of the planet, conservationists say.

Now in its fourth year, Earth Hour—which takes place from 8:30 to 9:30 local time on March 27—will be bigger than ever this year, said Leslie Aun, a spokesperson for the conservation nonprofit WWF, which organizes the annual event.

"We're off to a terrific start," Aun said.

Earth Hour began in Sydney, Australia, in 2007 with about two million participants. The voluntary one-hour blackout has since grown into an international event involving hundreds of millions of people as a show of support for action against global warming.

Earth Hour's energy-saving impact is limited, however: It does very little to reduce the greenhouse emissions that contribute to global warming. But WWF maintains that Earth Hour's real value is symbolic.

New Landmarks to Go Dark for Earth Hour 2010

Thousands of cities in a record 121 countries—34 more than in 2009—are set to officially participate in Earth Hour 2010.

In the U.S., 27 states have signed on for official participation in Earth Hour 2010, nearly four times more than were on board last year.

"We've had three states join up in the last 24 hours," Aun said. "We're telling people there's still plenty of time. We'll take them until the end."

The celebrity endorsements of Earth Hour that began last year will also continue for Earth Hour 2010. New England Patriots quarterback Tom Brady; his wife, model Gisele Bündchen; actor Edward Norton; and others have recorded public service announcements for television to help raise awareness of the event.

Landmarks to go dark for Earth Hour for the first time in 2010 include South Dakota's Mount Rushmore, Niagara Falls at the U.S.-Canada border, the Leaning Tower of Pisa in Italy, the Great Pyramids in Egypt, Table Mountain in South Africa, and the Hiroshima Peace Memorial in Japan.

Family Focus for Earth Hour 2010

While cities and famous landmarks get the most attention, families and individuals are responsible for much of Earth Hour's momentum, Aun said. In a WWF survey conducted after Earth Hour 2009, 80 million U.S. citizens said they had participated, according to Aun.

"Cities and businesses are the ones you're always hearing about, because they have the skyscrapers and the big marquees. But Earth Hour has always been an event about families and individuals as well," Aun said.

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