Predict how the tropical rain forest and desert biomes would change if earth were 30 degrees celsius cooler than it is today. what phenomenon keeps this from happening?

 Desert animals tend to be cold blooded, so they would generally die out (they can't survive in weather much cooler than what they're accustomed to, at least not without hibernation. 30 degrees celsius would definitely mean instant death, though). The rain forest would see less wildlife activity. The precipitation itself wouldn't change at all, because it would still be well above freezing. But the environment wouldn't be hot and muggy like it is now, so insect life wouldn't flourish as much, which would in turn affect other animals that eat insects. So really, the whole food chain would be affected. The desert area would also begin to take on characteristics of tundra if there was precipitation at night. 

It allows water to resist freezing and thawing cycle.