Leaders from the United Nations' three largest member nations will not attend this week's climate summit, which begins Tuesday.

More than 120 world leaders are expected, but China, Russia, India, as well as Germany and Australia, will not be present in leadership roles.

Meanwhile, some are questioning the science behind the climate change alarmism.

"There's a real problem with the computer models for climate change," Dr. Pat Michaels, director of The Center for the Study of Science at the Cato Institute, said.

"By last count there were 42 separate explanations in the scientific or public literature on why it hasn't been warming. When there are 42 explanations for one phenomenon, I can tell you what that means: scientists don't know what they're talking about," Michaels said.

But that hasn't stopped thousands from marching in New York, London, Berlin, Rio de Janeiro and cities around the world, calling for more action to stop global warming and for a global climate treaty.

New York City teacher Patrick Callahan was among those marching in the Big Apple. Callahan told reporters, "The message here is to get the word out that climate change is real, that science backs it up."

Celebrities Emma Thompson and Peter Gabriel were among those marching Sunday, along with U.N. Secretary General Ban Ki-moon.

Struck by the crowd, Ban said, "I am overwhelmed by such a strong power and energy and voice of people."

"You have to get everyone who knows what's going on, all of them, to take action," Thompson urged.