Greed. Humanity has always been driven by greed, but the hope was that it would recognize and overcome this sin and get to the next level. This hasn't happened. I have a weird feeling that humanity (or rather this civilization that started about 10k years b.c.) has just missed the critical point when it needed to take off. The greed was too heavy and we've started losing altitude. In practice, losing altitude manifests as the raising temperatures: the only way to solve this problem is to overcome greed.