There is no doubt that climate change has the most urgent political and ethical significance today among science issues. While there has been much discussion about environmental rhetoric, political speeches on the climate change issue have generated less interest. I explore President Obama's speech at Georgetown University in 2013 in terms of rhetoric by weaving together the proofs of logos, ethos, and pathos under concept of cultural rationality. Obama's climate change speech illustrates the available means of persuasion for stimulating the policy-making for cutting carbon pollution and establishing global environmental leadership. In addition, I discuss about how he establishes his ethos and pathos, and analyze how he optimizes his rhetorical strategies for the domestic and international audiences, respectively. Finally, I compare his speech with James Inhofe's speech "The Science of Climate Change" in 2002. Since James Inhofe is one of politicians who deny the claims of global warming proponents, this comparison will give a general conclusion about political speeches on climate change.