Because higher temperatures aren’t the problem, the rate of change is. I assume the worst because we’ve seen it before in the fossil record. The best comparison is the Triassic-Permian extinction. Rapid change in temperatures led to global ecological collapse and the death of 85% of all life on earth. Now, during the Triassic-Permian extinction CO2 levels rose from 400 ppm to ~2500 ppm over the course of ~50,000 years, with an estimated rate of change of around .05 ppm per year. We’re starting out lower at 280 ppm before the industrial revolution, but we’ve already hit 420, and we’re now adding about 2.5 ppm every year, with that number increasing every year. So we’re currently experiencing warming that’s 50 times faster than the most devastating extinction event in Earth’s history.

The fact that our entire food industry is based around genetically engineered monocultures is just another point of failure. It’s a constant game of cat and mouse to continually keep each generation of plants protected against changing diseases and pests, and because the vast majority of the seed is coming from one company, if something does adapt to overcome the engineered defenses, it’s devastating to the entire global population of that crop.