In #12 we saw that soil moisture droughts - agricultural and ecological droughts - have increased globally. I’ve been following the flow of AR6 in their discussion of recent trends. They do go on to discuss hydrological droughts without much that’s definitive so perhaps we’ll have a brief look at that in another article, as I’m something of a completionist.
Thanks for digging up and presenting these conflicting hypotheses about drought. Awesome. One thing about climate change I was confident about was that aridity on land would increase. Now I'm not so sure. Perhaps the following thought experiment is useful:
Imagine a totally flat planet with no oceans. The total precipitable water vapor in the atmosphere is 2.5 cm and there is a much larger amount of water in the ground (to mimic the reservoir found in the ocean). Now imagine rising GHGs raise the temperature of this planet by 1 degC or some multiple thereof. The amount of water vapor in the atmosphere increases by 7%/K, but otherwise the planet can't become more arid because of conservation of matter: The water that evaporates must fall somewhere as precipitation! The predicted increase in transpiration ignores the fact that the evaporated water must come down somewhere. We understand this principle in rain forests, where enormous amounts of water evaporation from an enormous number of leaves. That evaporated moisture falls as rain in a thunderstorm later the same day.
However, in arid regions climate modelers may be wrongly assuming the water lost to transpiration "disappears". In arid regions where precipitation is infrequent or highly seasonal, the amount of water vapor in the air tomorrow depends on how much water vapor is brought in by prevailing winds or turbulent mixing. In California, where it never rains in the summer, the absolute amount of moisture in the air over the relatively cold ocean is relatively low and when that air warms over land, can become pleasantly or unpleasantly warm, but never sticky like elsewhere in the summer. The grass on the hills turns brown. However, we know there is still significant humidity in the air, because the danger of fire shoots up dramatically on rare days when really dry winds blow from the interior towards the coast. In a single day, those dry winds can suck "all" of the moisture out of the dry grass and fallen debris from trees, making it much more flammable. When those dry winds stop blowing, the danger goes away, presumably because the dry grasses and debris suck moisture from the Pacific out of the air without any rain falling????
Thanks for digging up and presenting these conflicting hypotheses about drought. Awesome. One thing about climate change I was confident about was that aridity on land would increase. Now I'm not so sure. Perhaps the following thought experiment is useful:
Imagine a totally flat planet with no oceans. The total precipitable water vapor in the atmosphere is 2.5 cm and there is a much larger amount of water in the ground (to mimic the reservoir found in the ocean). Now imagine rising GHGs raise the temperature of this planet by 1 degC or some multiple thereof. The amount of water vapor in the atmosphere increases by 7%/K, but otherwise the planet can't become more arid because of conservation of matter: The water that evaporates must fall somewhere as precipitation! The predicted increase in transpiration ignores the fact that the evaporated water must come down somewhere. We understand this principle in rain forests, where enormous amounts of water evaporation from an enormous number of leaves. That evaporated moisture falls as rain in a thunderstorm later the same day.
However, in arid regions climate modelers may be wrongly assuming the water lost to transpiration "disappears". In arid regions where precipitation is infrequent or highly seasonal, the amount of water vapor in the air tomorrow depends on how much water vapor is brought in by prevailing winds or turbulent mixing. In California, where it never rains in the summer, the absolute amount of moisture in the air over the relatively cold ocean is relatively low and when that air warms over land, can become pleasantly or unpleasantly warm, but never sticky like elsewhere in the summer. The grass on the hills turns brown. However, we know there is still significant humidity in the air, because the danger of fire shoots up dramatically on rare days when really dry winds blow from the interior towards the coast. In a single day, those dry winds can suck "all" of the moisture out of the dry grass and fallen debris from trees, making it much more flammable. When those dry winds stop blowing, the danger goes away, presumably because the dry grasses and debris suck moisture from the Pacific out of the air without any rain falling????