Former Vice President Al Gore testifies about global warming during a hearing held by the House Energy and Commerce Committee on Capitol Hill in Washington March 21, 2007. (Xinhua/Reuters Photo) Photo Gallery>>>

    WASHINGTON, March 21 (Xinhua) -- Former U.S. Vice President Al Gore testified at a congressional hearing on climate change Wednesday, warning that the world "faces a true planetary emergency."

    "Global warming is real, and human activity is the main cause," he testified at a joint hearing on climate change by the House Energy and Commerce Committee and House Science and Technology Committee.

    "Today I am here to deliver more than a half-million messages to Congress asking for real action on global warming," said Gore, who brought with him boxes containing over half a million letters asking Congress to act on global warming.

    He would testify before the Senate Environment and Public Works committee later Wednesday.

    Gore called on U.S. lawmakers to begin a program to sharply reduce emission of carbon dioxide and other warming gases by 90 percent by 2050 to avoid a crisis.

    "We do not have time to play around with this; we do not have the luxury of making it a political football," said Gore, whose climate change documentary "An Inconvenient Truth" recently won two Oscars.

    A former lawmaker from Tennessee, Gore had served both chambers of Congress before he was elected Bill Clinton's vice president in1992.

    Gore was the Democratic presidential nominee in 2000, but was defeated by George W. Bush in one of the most controversial presidential elections in U.S. history.