Watch the CNN/YouTube debate on CNN International at 2100 GMT (1600 ET) on Wednesday.
London, England (CNN) -- Thousands of people tuned in to watch the CNN/YouTube climate change debate, which took place Tuesday at the COP15 climate conference in Copenhagen, Denmark.
CNN's Becky Anderson was joined by a distinguished panel of guests who answered questions posted on YouTube in the weeks leading up to the debate.
The panel included Yvo de Boer, Executive Secretary of the United Nations Framework Convention on Climate Change; U.S. journalist and author Thomas L. Friedman; Bjorn Lomborg, author of the "The Skeptical Environmentalist;" Hollywood actress and environmental activist Daryl Hannah and former U.N. Secretary-General Kofi Annan.
The panel tackled a range of climate change issues head on, addressing some of the major concerns of YouTube's users.
John from the Republic of Ireland wanted to know whether the panel thought that world leaders were taking climate change seriously enough.
The message from Yvo de Boer was clear.
"I've never seen a moment in history when so many world leaders have taken an interest in this topic," de Boer said. "We have 115 coming on Friday to get a strong resounding result out of this week."
Daryl Hannah agreed: "This is definitely a defining moment for our politicians and world leaders to take a stand to make real change from a fossil fuel economy to a new energy independent economy."
But Bjorn Lomborg questioned whether world leaders would sign an agreement.
"One-hundred-and-fifteen leaders are coming to Copenhagen but what are they going to agree on?" he said. "Don't they want to do something smarter that will actually work this time."
Thomas Friedman urged the United States to take a lead on climate change. "Right now," he said, "that is very much up in the air. The uncertainties surrounding climate change are not a reason for inaction. They are a reason to act."
The panel answered a range of questions from YouTube users all over the world from the United States to Bangladesh and Brazil.
A questioner from Morocco wanted to know if African countries had a loud enough voice at the climate talks. Kofi Annan was in no doubt that the continent did not.
"What is clear," he said, "is that Africa and the least developed countries, which are not so much responsible for the problem of climate change, are bearing the greatest brunt."
Annan, who joined the debate via a live satellite link from Geneva, Switzerland, said the whole world was in the same boat.
"We must come together to protect the planet," he said. "It is our collective responsibility. We cannot resolve climate change in one nation, or one continent ND not in another," he continued.
The Web broadcast was attended by an audience of delegates who are attending the U.N. climate conference at the Bella Center.
The program also showcased the winners of the Raise Your Voice video competition and featured a recorded video message from Archbishop Desmond Tutu, who warned about the impacts of climate change.
"The consequences could be conflict and instability, which are things we must avoid at any price," he said.