The Deep Freeze

In 2102, in an attempt to cure Earth's global warmth problems, COAL had written into legislation that atmospheric cryoseeding of the thermosphere was to begin to help cool the planet. For eight years the cryoseeding seemed to be working well with no harmful consequences; the atmosphere was thickening restoring its protective properties pertaining to radiation from the sun, the Earth became more humid and supportive of life, and the temperatures around the planet had been slowly but stably lowering to more comfortable levels. Then in 2110, something happened with the cryoseeding probes causing them to eject more coolant pods than required. It is estimated over fifteen times more pods than are normally used were jettisoned by every single cryoseeding probe. This caused a near-instant ice age over 95% of the Earth's surface. All of the lakes, rivers, and oceans were frozen almost completely solid. People near the poles died nearly instantly due to drastic temperature changes and torrential precipitation. The ice and snow fell faster than the most advanced technology could counter; fuel, food, and shelter became scarce. Survivors were forced to flee towards the equator where it was slightly warmer, but many cities had already fallen. Three bastions of humanity remained due to their massive population, pollution, technology, and warm climates; Cairo, Beijing, and Mexico City. Within 2 years Earth's population had fallen from nearly 9,800,000,000 to 500,000,000; and the three aforementioned 'capitals' made up 98% of the planets Human population.