This NASA article posits that the “Mayans did it to themselves.” Among other things, the article suggests that deforestation – required to clear land to plant crops to support a growing population – led to lower rainfall, which of course led to fewer crops, which of course required more deforestation, lather, rinse, repeat, until eventually the Mayans died out.
So how does deforestation affect rainfall?
I guess it’s basically the same thing that happened in North Africa. When part of the plantlife is removed there is nothing left holding moisture on the ground, so the rainwater runs down in deep aquifers and there is very little evaporation in the area, which in turn leads to less rain and even less plantlife. A century or so later what you have is practically a desert since very little can grow in dry earth.
Deforestation leaves the ground vulnerable to rapid erosion – the water is no longer held in the roots and the plants/trees themselves, so when it rains, the water rapidly washes away, resulting in the overall moisture leaving the area and going far away, eventually to the ocean. The main point is that it doesn’t have time to soak into the ground and stick around locally, so it won’t evaporate and create clouds locally, and so it will no longer rain as often … it’s just no longer in the area.
Oops! Looks like Telperion explained it while I was typing.
It’s not that trees ‘hold’ moisture, but that they put it into the air. For leaves to work, the have to be both moist and exposed to the atmosphere, so lots of water evaporates from leaves. I can’t look it up right now, but something like a a third to a quarter of the rainfall on a typical temperate forest is pulled out of the ground by trees and evaporates. I imagine in a more tropical area that percentage is even higher. Crops are going to have less leaf area that trees, so less evaporation. With less water being put into the air, there’s less rain downwind.
To some degree, crops are going to have more surface run-off (and more erosion), but as far as affecting local rainfall, the leaf evaporation is going to be the big effect.
Though my shallow understanding was that soils in tropical forests are extremely vulnerable to erosion and nutrient depletion, so that their productivity for crops declines dramatically after a few years. To my non-expert perspectice, this seems more likely to be a problem than shifting rainfall patterns due to land cover changes. [And looking at the article, they certainly mention this, too]