At what point will the sun make earth uninhabitable for humans?
It nearly happened on May 23, 1967.
According to this history paper released yesterday, summarized nicely here on Space.com, at the American Geophysical Union (paper publisher), and in many other media outlets, a powerful Earth-directed solar flare on that date jammed radar of the US Air Force's Ballistic Missile Early Warning System. Radar jamming like that, especially across all three sites, was considered an act of war, with the Soviet Union as the obvious culprit. Nuke-laden planes prepared to go destroy the enemy and set off the nuclear war that likely would have rendered Earth uninhabitable for humans.
Fortunately for all of us, the US had invested resources in monitoring the sun several years earlier, and somebody figured out that this was a solar flare with just enough time to stop the nuclear deliveries and keep the Cold War cold.
Popular Mechanics points out:
If that bulletin had been delayed a few minutes, those nuclear aircraft could have launched, and the solar flares would have made it impossible to communicate in the air. 989
If those aircraft had launched, there would have been no way to call them back.
It also nearly happened on September 26, 1983.
With the superpowers again at hair trigger tensions, a rare alignment of sunlight glinting off high altitude clouds and the highly elliptical orbits of Soviet early-warning satellites caused the detection system to report that five intercontinental ballistic missiles, likely carrying nuclear warheads, had been launched by the United States against the Soviet Union. With only a short time to act on such a report, Soviet leaders likely would have launched a large nuclear strike against the US in retaliation for the detected attack. However, the human on duty to receive this detection report was civilian-trained <PERSON>, who was skeptical of the detection and correctly classified this as a false alarm. (Five nuclear missiles seemed small for the kind of attack he expected from the US.) This decision has been credited with preventing a large-scale nuclear war, which otherwise would have been triggered by misinterpreted sunlight.
So a possible answer is, "the next time some tense socio-technical system we've built misinterprets the Sun."
Thanks to user <PERSON> for the example from September 1983, and to all those who happened to be in the right place at the right time and helped keep us safe. 714