Pointing the person to evidence from broadly respected sources on the risks of human-caused climate change is one obvious way to respond. But this approach has its shortcomings—people frequently defend their pre-existing beliefs by questioning, discounting, or ignoring evidence. One alternative to emphasizing the reality of climate change to those who deny it is to highlight short-term problems with sources of greenhouse gas emissions (such as air pollution or health risks). It may help to follow general guidelines for talking to someone with whom you disagree—such as avoiding personal attacks, sarcasm, and condescension, expressing basic respect, and not assuming suspect motives.