Since the beginning of industrialization, people have been putting carbon dioxide and other greenhouse gases into the atmosphere. Those trap heat, which increases the odds of higher temperatures, sea level rise, dangerous weather, with broad consequences for the environment, for our economies, and for national security. Those broad consequences are what put together is climate change. Countries have a host of dilemmas that they face when they’re trying to address climate change. Cutting emissions today can be costly, but failing to cut emissions today can be even costlier down the road in terms of climate damages. On top of that, when one country takes strong efforts to cut emissions and another doesn’t, it can disadvantage that first country in its industrial competitiveness. And that’s exacerbated by the fact that poorer countries will insist that richer countries, including the United States, have a big historical obligation because of their past emissions to do more in cutting emissions today. What makes it really tough as an international issue is that emissions anywhere affect people everywhere. So in order to cut into the problem and reduce it, we need action from pretty much everyone. The United States has three big challenges when it comes to dealing with climate change. The first is that Americans need to agree on some serious policies, and pursue those to cut emissions. The second is, whatever the United States is doing, it needs to persuade others that it is taking serious steps to deal with the problem to strengthen its hand in trying to mobilize a bigger international response. The third piece is that the United States needs to work effectively with other countries to pull together that international response, to increase countries’ confidence that everyone is taking serious steps to deal with the problem. What matters most is that countries not only commit to ambitious actions, but actually successfully follow through with those.