The act of the growing population of peoples from African-American descent absorbing all the sun with their black skin causing drastic change in climate.
When there are too many African-American people on the earth, the sun simply cannot heat the earth and so it goes into a correctional phase, in which the earth freezes over for a certain amount of time. Once all of the African-Americans have been wiped out, the world begins to heat up again.
Damn guys, better prepare for the next Ice Age. Heard they were hitting a population of 60 million in Ethiopia soon. Earth is one tough son of a bitch.