The climate getting worse means that many years, their crops won't grow: there will be too much rain, not enough rain; things will change in ways their fragile environment simply can't support.

Hmmm. If only we had the technology to move water from one location to another. Nah let's just kill everybody so they don't starve.

Seriously though, if you want to reduce the population of developing nations, just stop sending foreign aid and interfering with their economy/culture. These populations are dependent on a level of technological and cultural maturity that they did not earn and cannot maintain.

The climate getting worse means that many years, their crops won't grow: there will be too much rain, not enough rain; things will change in ways their fragile environment simply can't support.

Hmmm. If only we had the technology to move water from one location to another. Nah let's just kill everybody so they don't starve.