What warming scenario on Earth would lead to the end of complex life forms? You realize that humans colonized most of the planet thousands of years ago with stone-aged technology, survived ice ages, lived in all sorts of climates? That the dinosaurs evolved on a world far warmer than we're likely to see? Complex life forms didn't all go extinct back then.
>>goatlo+(OP)
A snowball earth with a thin band of warm enough weather in the tropics is easier to survive on than an earth with 6 months of warm spring to tropical weather and 6 months of pitch black winter darkness.