In their last presidential debate Monday night, the two presidential candidates began with Libya and stayed in the Islamic world for almost the entire evening. They talked about “divorcing” Pakistan, arming Syrian rebels and rallying allies against Iran. In this exchange, Romney offered few serious counterproposals to Obama’s current policy, and Obama offered little more about his vision for the next four years. The president promised to keep Americans safe, insisted that the United States needs to “rebuild at home” and attacked Romney as too fickle and inexperienced to run foreign policy. Romney tried to convince people that he is a calm hand who wouldn’t start another boots-on-the-ground war. Romney’s failure to contrast his plans with the president’s record and Obama’s attacks on Romney’s promiscuous attitude toward policy added up to a win for the president.

But, as in previous debates, the victory rings hollow for all the issues that the candidates did not address. Aided by the moderator’s questioning, they spent exactly no time on one of the greatest challenges the world’s governments must face, and foremost among them the United States’. This problem threatens the lives and livelihood of millions, particularly in poor countries, but, left unchecked, it also poses hazards to plenty of Americans and American interests. It will require possibly very expensive choices for developed and developing countries and delicate international negotiations. This challenge is climate change.

Global warming is, well, global. Emissions here count just the same as emissions anywhere else. So, ultimately, facing up to the challenge will require many countries moving in the same direction with enough speed. Without adequately discussing climate change in a global context, the candidates’ foreign policy agendas and even their economic plans, so preoccupied as they are with energy policy, are utterly incomplete.

Romney’s plan is to concentrate on keeping fossil fuels plentiful and domestically sourced. Under his scheme, reducing America’s carbon emissions would be nice, but that wouldn’t be the primary goal. Achieving any emissions cuts of significance with existing technology would be expensive, the argument goes, and since China, India and others might not also clamp down, emissions could continue to build up too-rapidly, regardless of what the United States does. But Romney must explain why American leadership stands to fail in the case of climate change, yet seems to be his solution to every other major global crisis. Why is the best option to wait and see, even though the scientists tell us we shouldn’t?

Obama has to make the mirror-image case. He favored a cap-and-trade bill early in his first term, and he has since shifted to less economically rational methods of wringing carbon out of the economy, such as Environmental Protection Agency regulation, direct subsidies and federal clean energy mandates. These are expensive measures. Why does he think they will make a dent in the global carbon budget? How will China respond? What will India do? Can American action really convince increasingly dirty developing countries to sacrifice some growth now to address a problem that will proceed slowly, over the course of decades? How does he account for the serial failure of international climate negotiations to get major emitters anywhere near where scientists say they should be? What’s the new strategy to improve that record?

The Middle East is important. Pakistan is important. Iran’s nuclear ambitions are important. But so is climate change, and the debates should have reflected that.