That's easy: abrupt climate change, the sort of thing where most of the earth returns to ice-age temperatures in just a decade or two, accompanied by a major worldwide drought. Then, centuries later, it flips back just as quickly. This has happened hundreds of times in the past.
The earth's climate has at least two modes of operation that it flips between, just as your window air-conditioner cycles between fan and cool with a shudder. And it doesn't just settle down into the alternate mode: the transition often has a flicker like an aging fluorescent light bulb. There are sometimes a half-dozen whiplash cycles between warm-and-wet and cool-and-dusty, all within one madhouse century. On a scale far larger than we saw in the El Nino several years ago, major forest fires denude much of the human habitat.
To the extent the geophysicists understand the mechanism, it's due to a rearrangement in the northern extension of the Gulf Stream. A number of computer simulations, dating back to 1987, of the winds and ocean currents have shown that gradual global warming can trigger such a mode switch within several centuries, mostly due to the increased rainfall into the northern North Atlantic Ocean (if the cold salty surface waters are diluted by fresh water, they won't flush in the usual manner that allows more warm water to flow north and lose its heat). Meltwater floods from Iceland and Greenland will do the job if tropical-warming-enhanced rainfall doesn't.
This has been the major story in the geophysical sciences of the last decade. I've been puzzled since 1987 about why this story hasn't been widely reported. A few newspapers finally started reporting the story in some detail two years ago but still almost no one knows about it, probably because editors and readers confuse it with gradual climate change via greenhouse gases. This longstanding gradual warming story seems to cause the abrupt story to be sidetracked, even though another abrupt cooling is easily the most catastrophic outcome of gradual warming, far worse than the usual economic and ecological burden envisaged.
How would I report it? Start with the three million year history of abrupt coolings and how they have likely affected prehuman evolution. Our ancestors lived through a lot of these abrupt climate changes, and some humans will survive the next one. It's our civilization that likely won't, just because the whiplashes happen so quickly that warfare over plummeting resources leaves a downsized world where everyone hates their neighbors for good reason. Fortunately, if we get our act together, there are few things we might do to stabilize the patient, buying some extra time in the same manner as preventive medicine has extended the human lifespan.