The United States need to show the world that we are a leader in combating threats common to all of humanity. That includes things like nuclear proliferation, dictatorships, basic human rights, and climate change. By staying in the Paris climate accord we will be helping ourselves avoid billions in damage and thousands of American deaths due worsening to severe weather, including more and stronger tornados and hurricanes, frequent flooding and polarizing droughts. If we do not do commit our country to combatting it our allies will begin to see the US - not just as a weakened power and influence, but also a problem that they will have to combat. Americans abroad will be thought of as dirty polluters and could be treated poorly even in countries where we now enjoy favorable opinion and are allowed to travel to safely. All of this is more important than the upfront cost of shifting away from fossil fuels.