Arid. To most of us, when we hear the word “arid,” we get pictures of the desert or droughts. But do we truly understand how land ever becomes dry? Do we understand how land ever gets its opposite—wetness?
For land to become arid, we need to have low humidity. Warmer air can hold more moisture, but because it’s warmer, that doesn’t automatically mean it’s humid or moist. It can be extremely dry. This happens when hot, moist air rises from the tropics and moves poleward (northward or southward from the equator). As the wet air rises, it cools, becomes clouds and rains. This precipitation dries out the air. When the air gets to the mid-latitudes, it descends and the increased pressure warms it up again. But this time, the air which can hold much more moisture has been squeezed dry by the earlier rain.
In the polar regions, precipitation is relatively rare. Why? Because cold oceans don’t evaporate much. There’s not much moisture in cold air. And cold air does not rise to become clouds and precipitation. This is why Antarctica is considered to be the largest desert in the world. It gets some snow, but on a par with the rain received by the Sahara.
Global Warming Reduces Droughts
Contrary to popular belief, global warming does not create more droughts. Arid lands actually become less arid with greater global warmth. During the far warmer Holocene Optimum, the Sahara desert was green for nearly 3,000 years. Nomadic tribes took their herds across those once dead lands. There was so much additional rain that today’s wimpy Lake Chad was, back then, a robust inland sea which rivaled the Caspian Sea in its heyday. It took a significant measure of global cooling to turn the Sahara back into a deadly desert.
The reason for this contrary fact is quite simple. We merely need to ask ourselves: How does land ever get water in the first place? As we already know, cold oceans don’t evaporate much. It takes a great deal of warming energy to evaporate water from the ocean. That water has to come down sometime. While the oceans keep evaporating, clouds keep forming and rain keeps falling. Increase the warming, and you increase the clouds and the potential for rain. Certainly, warmer climate dries out the land faster, but land would never have water in the first place if it weren’t for warmth. As long as we have oceans, we’ll have water to cool down the land with nourishing rain.