President Ronald Reagan famously used to discomfit his advisors by bringing up a favorite thought experiment. What, he wondered, would the nations of the world do if extra-terrestrial aliens invaded our planet? Wouldn't we put aside our differences and unite against the common threat? And if that is true, then shouldn't we put aside our differences now, to unite against that which threatens all of life on earth, the scourge of nuclear weapons?

He brought the subject up with Soviet Premier Mikhail Gorbachev at their summit in Iceland, and again in a speech to the General Assembly of the United Nations, because for him it was not an idle speculation, but a very serious matter. We would, he was sure, unite against a common enemy that threatened us all in the most total way imaginable. So why couldn't we see the risk of a nuclear exchange as a similar kind of common enemy, and unite against it?

Reagan was so serious about the threat of nuclear weapons that he quite seriously proposed to Gorbachev that they eliminate them entirely. And Gorbachev indicated a possible agreement — if Reagan would agree to eliminate the still-hypothetical defense against ballistic missiles known as the Strategic Defense Initiative as well, which Reagan firmly refused to consider. S.D.I. was, for him, not a weapon that would enable America to win a nuclear war, nor (as most of his own foreign policy team thought) a clever bit of vaporware to be traded away for more concrete concessions from the Soviets in arms negotiations. On the contrary: Missile defense was the technological answer to the nuclear threat, the system that would make ICBMs obsolete, and thereby defeat the common danger. That's why he sincerely offered to share the largely-mythical technology with the Soviets.

Of course, Reagan's vision never came to pass. The Cold War ended, not because America and the Soviet Union put aside our differences but because the Soviet side collapsed. Far from abandoning nuclear weapons that they could ill afford, the Russian Federation has clung to them as a vital signifier of superpower status, while the United States has, under Bush and Obama and now Trump, explored ever more-seriously using them on the battlefield. Worst of all, nuclear technology is now in the hands of a regime as terrifying as North Korea. If the fear of a general nuclear exchange has receded considerably, the prospects of international cooperation to actually end the threat feel further away than they were at the height of the Cold War.

I was thinking about this history in light of the much-discussed recent doom-crying article on climate change by David Wallace-Wells for New York magazine.

Wallace-Wells' premise in writing the article is similar in its way to Reagan's: that if people understood the nature and scope of the common threat, they would unite against it. Most people probably don't realize just how catastrophic the consequences of climate change could be, just as most people probably didn't realize that mutually-assured destruction really did mean that the human race itself was at risk if deterrence ever broke down. While much of the press since Wallace-Wells' article came out has cautioned that the worst-case scenarios are unlikely and that real progress is actually being made, it's also true that the composition of the atmosphere has already changed enough that some serious consequences are already baked in, and that predictions get harder the further out into the tail of the probability distribution we get. Even under more hopeful scenarios, the potential consequences of climate change are severe enough to outweigh virtually any of the petty concerns that dominate our politics.

So why can't we unite against this threat?

Well, consider that, when Reagan was a young man, the closest thing to an alien invader did threaten all of humanity. His name was Adolf Hitler. And the world did not simply unite to defeat him. Italy and a number of smaller revisionist European states allied with Hitler, and a host of countries from Ireland to Argentina maintained a neutral stance that barely masked hopes for a British defeat. Britain, unprepared for a war that Hitler eagerly sought, bought time at the price of Czech independence and industrial might. The Soviet Union signed a non-aggression pact with Hitler and joined him in dismembering Poland. A reluctant United States had to be dragged into Lend Lease, and only went to war after Japan's sneak attack on Pearl Harbor. And the course of the war was repeatedly affected by divergent interests among the allies in the character of the post-war settlement.

Even those with the clearest view of the threat Hitler posed, in other words, understood that he was not the only threat in the world, and never stopped watching their backs even when standing shoulder-to-shoulder with their allies in the fight.

So it is today with climate change. The Paris Accord symbolized global unity, but at the price of being a largely toothless document. Ironically, in the wake of the Trump administration's withdrawal, America may still meet its climate targets because of the falling price of renewable energy. But the more feasible a low-carbon economy is, the more different countries will use that fact to jockey for advantage — for example, imposing onerous tariffs on products of carbon-intensive manufacturing processes. And the very fact that, if we are able to avoid the most extreme planet-wrecking scenarios, climate change poses a greater threat to poorer regions of the planet gives richer countries an overwhelming advantage in the struggle to respond. New York City can afford to adapt to protect itself from rising seas and more severe weather. Dhaka likely cannot.

It's easy to get demoralized by this selfish dynamic. But it is far more productive to accept it and take advantage of it by pitting countries and companies against each other to compete in a low-carbon-intensity world rather than relying on global solidarity as a precondition to progress. Narrow self-interest can also be enlightened, and is a lot easier to inspire. And if the consequences exacerbate existing economic inequalities — and they likely will — then that becomes another good fight to fight, and not a reason to hold the planet hostage.