If the crisis were severe enough to warrant deliberately killing off billions to save the rest, the only question is whether we can do it while minimising suffering. That may not be possible.
Ideally, by controlling birth rates, which to a large degree requires education and better economic situations, which in turn leads to greater use of technology and pollution. Otherwise you actively have to kill people.
Well, yeah. My suggestion is only for an extreme scenario. Hopefully it won't get to that, but I suspect that it'll happen on its own anyway (wars, famine, etc.) But in extreme circumstances sometimes ethics have to give way to a greater principle.