U.S. President Barack Obama mocked congressional Republicans' popular "I am not a scientist" demurral on climate science, and vowed to prevent Congress from undermining his administration's steps to reduce greenhouse gas emissions. <br /> <br />Obama elevated global warming to the top of the list of challenges facing us, saying, "And no challenge — no challenge — poses a greater threat to future generations than climate change." <br /> <br />Here's his retort to the "I'm not a scientist" rhetoric, used by Republican Sen. Marco Rubio and House Speaker John Boehner, among many others. Obama first auditioned these lines during a speech at the University of California at Irvine last year.
