It's all a result of farting cows, decomposing baby nappies and coal-fired plants.
Or else it's just the regularly scheduled irresistable end of the interglacial, right on time the way it always is, every 11,500 years like a Swiss clock.
Without Ice Ages to cull the herd, imagine what sort of people would dominate the earth. It'd be like Mike Judge's Idiocracy except those people would be like rocket scientists in comparison. Actually, there's already a place where there are no Ice Ages to cull the herd. It's called Africa and the average IQ there is 70, which is 21 points behind Koko the Sign Language Gorilla on the exact same test.