sorry, it will be overpopulation, with the consequences that food and potable water will be in short supply. Disease will play a role, but I would describe it more that the human race will burn up the planet, pollute it to the point that you can't live there.

I did see a program last night that suggested the magnetic field of the Earth may be shifting polarity. I imagine if that were to happen, at some point during the process there would be a period where the radiation that is pouring out of the sun and going around us because of this field would start taking a more direct route. The Aurorae Borealis and Astralis move South and North respectively and every one gets a terminal sunburn, though that might just kill everything not underground far enough when it happens.