We should approach Syria in terms of the U.S.-Russia nuclear situation because that's by far the bigger issue. Syria's population is about 22.5 million. This is less than half of one percent of the total human population (about 7.1 billion). And it is a miniscule fraction of the total population of all humans who will ever live. Even if the Syrian civil war killed everyone in that country -- an outcome much worse than anyone expects -- it would be an imperceptible blip in the trajectory of human civilization.

The same cannot be said for U.S.-Russia nuclear war. The two countries still hold the overwhelming majority of the world's nuclear weapons: About 4,000 active weapons and 16,000 total, making for probably several gigatons of explosiveness. If launched, they could cause hundreds of millions of immediate deaths and billions more in the ensuing nuclear winter. Human civilization may never recover. Such a global catastrophe overwhelmingly dwarfs the Syrian civil war, making it by far the larger priority.

This focus is not to say we shouldn't care about the human tragedy in Syria. To the contrary: it's precisely because we do care about the Syrians, and everyone else around the world, and all future generations of humans who would love the chance to experience life, that we must prioritize reducing the risk of U.S.-Russia nuclear war, or any other civilization-ending global catastrophic risks.

There are at least two other global catastrophic risks that could be affected by the Syrian civil war. One is biological weapons, which are especially worrisome because (unlike chemical and nuclear weapons) they can be released in one location and spread worldwide. The other is global totalitarianism, which would be hard to dislodge. These two global catastrophic risks are worth being mindful of, but I believe nuclear war is the bigger concern with Syria.

So what role could Syria play in reducing the risk of U.S.-Russia nuclear war? Risk is, in simplest terms, probability times magnitude. The probability of U.S.-Russia nuclear war can be reduced by lowering U.S.-Russia tensions. The two countries are fortunately not on the brink of war as they were during the Cold War. But some tensions have always lingered, and they've risen recently over Syria and also Edward Snowden, to whom Russia granted temporary asylum. Tensions also contribute to the risk of false alarms causing inadvertent nuclear war, such as the Stanislav Petrov incident exactly 30 years ago.

U.S.-Russia cooperation over Syria's chemical weapons may be able to help lower U.S.-Russia tensions, thereby making nuclear war less probable. Already the situation has increased civil dialogue. Following through on the diplomatic agreement to remove chemical weapons from Syria could build trust between U.S. and Russia, trust that future relations can build on. Or it could worsen tensions, if the Syria agreement ends poorly.

​The magnitude of U.S.-Russia nuclear war can be lowered via nuclear disarmament. Fewer weapons means less total damage should the war occur. Despite Washington's current partisan gridlock, disarmament is likely harder for Moscow. For better or worse, Russia's nuclear stockpile is one of the last vestiges of its former glory. It has lost Soviet territory, its economy is no longer preeminent, its conventional forces cannot compete with America's, and even its population has declined, but it still has one of the two great nuclear arsenals.

Syrian diplomacy may be able to help here too. If Russia can emerge as a key actor in resolving the civil war, that could earn it some of the self-confidence it needs to continue its nuclear disarmament. And Russia's nuclear weapons never played a role in Syrian diplomacy. If it doesn't need the weapons to achieve its geopolitical goals, it might as well relinquish them, with the U.S. following suit.

Some caution is warranted here. No one can predict with certainty how the Syrian civil war will play out, and how it could affect the U.S.-Russia nuclear weapons situation. But what is certain is that safely resolving this situation is essential for the long-term success of human civilization. For the sake of all of humanity, let us strive to get this one right.

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