Sperm lose their motility and vitality when ambient temperatures reach average human body temperature, 98.6°F. When the Earth gets hot enough, this should have some measurable effect on human birth rates, but I fear it is going to devastate terrestrial animal populations. We should be less concerned about all the people and more concerned about permanently losing Earth's precious diversity.
The only animals that are going to survive are domesticated cats and dogs. In the USSR an experiment with foxes successfully transformed foxes to behave as dog within 60 years. The snout became shorter, curly tail and doglike friendly playful behavior made it possible to keep these foxes as a pet. The last century lions living in the wild decreased from 240.000 to 20.000. You could argue it will take another 100 or 200 years but they will go extinct within this millennium. I wonder if lions could be domesticated by an extensive breading program to save them.
Orca’s, our equivalent in the water has a hard time finding fish. In captivity we can make them do tricks no human can perform in the water. Orca’s hunt on white sharks by flipping them upside down in which sharks are in a paralyzed sleep mode. The pod of orca’s then snatches the liver out of the shark and leave the carcass for other sea creatures to feast on. Orca’s have not discovered the breading ground of white sharks around the Easter Islands. Would it be possible to transfer this knowledge to them?
Humans evolved 5-7 million years ago during the Pliocene. The Mid-Pliocene is the most recent period with atmospheric CO2 comparable with the present (ca. 400 ppmv), with mean annual surface temperatures approximately 1.8 °C to 3.6 °C warmer than preindustrial temperatures.
Modern humans evolved 200K-300K years ago. In the last 650K years, Earth's climate was characterized by large climate swings as the ice ages advanced and receded triggered by changes in Earth's orbit relative to the Sun. Slow changes to the shape of the Earth’s orbit and wobble of the Earth’s axis caused cooling that was amplified by natural effects, including the growing ice sheets and the reduction of carbon dioxide by being absorbed in the oceans. Over tens of thousands of years the amplifying feedback caused the ice ages. The warm climates of the interglacials, the periods between ice ages, were similar to today, but probably a little cooler than right now. A few interglacials were a little bit warmer; some were a little bit cooler.
