In the abstract we have to pick one crisis or the other, but the real world is messy. Global warming has momentum, and so does population despite what look like reliable projections of population decline in many places. In other words we could see negative impacts from both without them canceling each other out.
> In other words we could see negative impacts from both without them canceling each other out.
See William Gibson's concept of Jackpot.
Demographic momentum means population continues to increase for a while after the total fertility rate drops well below replacement. Once the decline starts the momentum works in the opposite direction: decline continues for a while even if TFR is somehow increased again.