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????
I’ll try and address some of this in a follow up article. Globally, precipitation must equal evaporation over a decent time period. But evaporation can be higher over the ocean than land.
IF I wasn't clear above: The consensus's understanding of drought shows that warming on our planet must be accompanied by an increasing deficit in moisture in the soil. However, if you apply that understanding in a thought experiment to a planet with no ocean, the consensus's understanding appears to be wrong. In this case, the additional water lost by transpiration must fall additional precipitation and the soil doesn't dry with warming.
On our planet, things are more complicated. What fraction of evaporation comes from the land vs the ocean? What of precipitation falls on the land vs the ocean? WIll these fractions change? My thought experiment skips these complications.
There is also a Soden paper I never fully understood that predicts from theory that wet areas will get wetter and dry areas will get drier on a warmer planet. Perhaps this renders my thought experiment (with a homogeneous surface with homogeneous rainfall) inappropriate.
I don't need a reply if you want to push forward with your series and perhaps answer my comments in an orderly fashion.
Condensation is a function of humidity and temperature. More clouds and rain when warm air meets cold. And the places rain falls is dependent on the humidity being transported and cooled. In the real world the relative humidity is decreasing, one degree warming gives less than 7% increased water vapor. This can be an explanation of the decreased cloud cover the last 40 years.
There has been more warming over land than over oceans, and i would guess that rainfall also has increased more over land, as more air is being lifted to higher and colder altitudes.
What goes up must come down. One should think that higher specific humidity, by over 6% should give as much increased precipitation. Could humidity com down in another way? It is logical to assume that dry soil can take up some humidity directly, perhaps together with some plants, bacteria etc. Perhaps some more dew that will not count as rain.
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????
Frank,
I’ll try and address some of this in a follow up article. Globally, precipitation must equal evaporation over a decent time period. But evaporation can be higher over the ocean than land.
IF I wasn't clear above: The consensus's understanding of drought shows that warming on our planet must be accompanied by an increasing deficit in moisture in the soil. However, if you apply that understanding in a thought experiment to a planet with no ocean, the consensus's understanding appears to be wrong. In this case, the additional water lost by transpiration must fall additional precipitation and the soil doesn't dry with warming.
On our planet, things are more complicated. What fraction of evaporation comes from the land vs the ocean? What of precipitation falls on the land vs the ocean? WIll these fractions change? My thought experiment skips these complications.
There is also a Soden paper I never fully understood that predicts from theory that wet areas will get wetter and dry areas will get drier on a warmer planet. Perhaps this renders my thought experiment (with a homogeneous surface with homogeneous rainfall) inappropriate.
I don't need a reply if you want to push forward with your series and perhaps answer my comments in an orderly fashion.
Condensation is a function of humidity and temperature. More clouds and rain when warm air meets cold. And the places rain falls is dependent on the humidity being transported and cooled. In the real world the relative humidity is decreasing, one degree warming gives less than 7% increased water vapor. This can be an explanation of the decreased cloud cover the last 40 years.
There has been more warming over land than over oceans, and i would guess that rainfall also has increased more over land, as more air is being lifted to higher and colder altitudes.
What goes up must come down. One should think that higher specific humidity, by over 6% should give as much increased precipitation. Could humidity com down in another way? It is logical to assume that dry soil can take up some humidity directly, perhaps together with some plants, bacteria etc. Perhaps some more dew that will not count as rain.