NEW: Is human activity a substantial cause of global climate change?

– NEW: Is human activity a substantial cause of global climate change? – Read pro and con arguments from the National Academy of Sciences, the George C. Marshall Institute, Al Gore, Dr. Michael Mann, the Intergovernmental Panel on Climate Change (IPCC), Dr. Richard Lindzen, the Oregon Institute of Science and Medicine, Dr. Tim Ball, and many other scientists, activists, and organizations.