If the Earth rotated at half its current speed of 1,037 mph at the equator, the days would be longer. If a day were say 48 hours long as compared to 24, plants on the sunny side of the planet would dry up during the day, especially in hotter regions, lakes begin evaporating and animals die from heat exhaustion. During hotter times of the year, people could only survive in air conditioned shelters.
Also, if the nights were twice as long, the night side of Earth would grow very cold, necessitating a way to stay warm for the remainder of the night once the heat of day was gone.
If the Earth rotated more slowly, our magnetic field would grow weaker as well because the speed of the Earth’s rotation is what causes the solid inner and liquid outer cores of the planet to rotate like a dynamo and the speed at which they do is what generates the strength of the magnetic field. A weaker magnetic field would allow more radiation to reach the surface of the Earth with a corresponding increase in damage to the environment. And us.
Conversely, if the Earth rotated at twice its current speed, our days would be only 12 hours long. The weather would be considerably wilder, and temperatures lukewarm, not having enough time to get hot or cold. Tides would be more violent. There might be more earthquakes and volcanoes with the increased pressure on the Earth’s mantles from the speed of the rotation. More,
“But if we were rotating 100 mph faster than usual, a day would be about 22 hours long. For our bodies, that would be like Daylight Savings on crack. Instead of setting the clocks back by an hour, you’d be setting them back by two hours every single day, without a chance for your body to adjust. And the changing day length would probably mess up plants and animals too…
“At 1 mph faster, the water around the equator would get a few inches deeper within just a few days.
“At 100 mph faster, the equator would start to drown. ‘I think the Amazon Basin, Northern Australia, and not to mention the islands in the equatorial region, they would all go under water,’ says Fraczek. ‘How deep underwater, I’m not sure, but I’d estimate about 30 to 65 feet.
“If we double the speed at the equator, so that Earth spins 1000 miles faster, it would clearly be a disaster,’ says Fraczek. The centrifugal force would pull hundreds of feet of water toward the Earth’s waistline. ‘Except for the highest mountains, such as Kilimanjaro or the highest summits of the Andes, I think everything in the equatorial region would be covered with water.’ That extra water would be pulled out of the polar regions, where centrifugal force is lower, so the Arctic Ocean would be a lot shallower.” PopSci
Bottom Line: If the Earth rotated at half the speed it does now, our magnetic field would cease to function. The atmosphere might float out into space, blown there by the solar wind and radiation from space would reach the surface of the planet. The side of the Earth facing the sun would roast while the other side would freeze
If the Earth rotated at twice its current speed, that would mean tough climatic changes every single day. Super storms might become the norm. To maintain a stable environment without wide temperature swings, the length of our day/night cycles must remain at or very near the 24 hours we enjoy today. Thankfully for life, it does.