I live in the desert near the Superstition Mountains, east of Phoenix, Arizona. Not far from my house is a gorgeous Saguaro cactus that I drive by daily. I’ll be honest: I had no clue how big those things were until I stood beside one and looked up! This particular monster has eight arms growing from its 60-foot-tall body.
A Saguaro doesn’t even get its first arm until it’s at least 75 years old, and it can live for 200 years or longer. Judging by its splendor, this guy has been around long enough to see European settlement in the Valley of the Sun grow from a few buildings along the Salt River into a sprawling metropolis of 4.6 million people spanning 9,000 square miles.
The Sonoran Desert is the only place on Earth where Saguaros grow. That’s because the cactus and the desert grew up together. As the climate around here became what it is today, plants that fared better survived, reproduced, and passed along their genes. Less successful plants didn’t. Generation after generation, as the climate changed, the Saguaro evolved to keep up.
But wait a minute! Why is this a desert at all? That has a lot to do with the physics of water and convection. Sunlight heats the tropics, driving planetwide convection that carries much of that thermal energy toward the poles. As warm, moist tropical air rises, it also cools, dumping much of its moisture as rain. As water vapor condenses, it releases heat. By the time that once-moist tropical air completes its upward path, it is both dry and a lot warmer than it might have been.
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