Something I didn't realize until I read it on Twitter is that average precipitation for the continental US over the last 12 months is at a record high:
Rainfall is supposed to increase as the planet warms, due to increased evaporation of ocean waters. I looked for worldwide precip over time, but failed to find it.
Unfortunately, many naive people think that global warming has magical properties, that can cause just about any type of harm. In particular, some people attributed the recent Californian drought conditions to climate change, although there's no scientific basis for that belief. In fact, with increasing rainfall, it would seem that, if anything, droughts should become less of a problem.
David in Cal said... "In particular, some people attributed the recent Californian drought conditions to climate change, although there's no scientific basis for that belief."
“In a new study published in PNAS, Diffenbaugh et al. now add weight to the accumulating evidence that anthropogenic climatic changes are already influencing the frequency, magnitude, and duration of drought in California (2). The authors show that the increasing co-occurrence of dry years with warm years raises the risk of drought despite limited evidence of a trend in precipitation itself, highlighting the critical role of elevated temperatures in altering water availability and increasing overall drought intensity and impact.”
https://www.pnas.org/content/112/13/3858.short
(2) Diffenbaugh NS, Swain DL, Touma D (2015) Anthropogenic warming has increased drought risk in California. Proc Natl Acad Sci USA 112:3931–3936. https://www.pnas.org/content/112/13/3931?ijkey=163568051d26bf1c48ce1ddea5020094ed45df6d&keytype2=tf_ipsecsha
DiC: Rainfall is supposed to increase as the planet warms, due to increased evaporation of ocean waters
Evaporation can happen on land as well as sea.
DiC: Unfortunately, many naive people think that global warming has magical properties, that can cause just about any type of harm. In particular, some people attributed the recent Californian drought conditions to climate change,
See figure 7 in Manabe and Wetherald (1975). Notice what happens to precipitation and evaporation at latitudes 34-40 after a doubling of CO2?
That is the result of a very early GCM, but shows that this is not news.
Layzej, good catch in the M&W paper. I had read it in the past, but never realized that there was a calculated drop in precipitation at the latitude you pointed out. So here are my questions: Is this drop a manifestation of the poleward expansion of the Hadley circulation or is it something else? If it's due to the HC, is this the first paper (once again!) to make this prediction?
By the way, here's a nice write-up by Richard Seager of Columbia on increasing drought in the US southwest due to the HC expansion: http://ocp.ldeo.columbia.edu/res/div/ocp/drought/science.shtml
David, As I remember there wasn't unanimous agreement among climate scientists as to the Diffenbaugh paper. Two of the world experts on drought, the above-mentioned Richard Seager as well as Martin Hoerling, said the drought in California during 2011-2014 was dominated by natural variability https://journals.ametsoc.org/doi/full/10.1175/JCLI-D-14-00860.1
Meanwhile Mann and Trenberth disagreed with Seager and Hoerling: https://www.nbcnews.com/science/environment/global-warming-isnt-causing-california-drought-report-triggers-storm-n263941
Depends on how one defines "drought". The paper doesn't say low precipitation years are more likely due to global warming. It says that low precipitation years are a bigger problem if it's warmer. Obviously global warming increases that probability.
"t's in the right neighbourhood for HC expansion..."
It is, but does anyone know for sure?
The M&W papers don't even mention Hadley, but I did find this from Rind and Rossow, https://journals.ametsoc.org/doi/pdf/10.1175/1520-0469%281984%29041%3C0479%3ATEOPPO%3E2.0.CO%3B2
"The Hadley circulation is of central importance to the distribution of climate regimes on Earth and to the determination of atmospheric dynamics on other planets. Some scenarios, which envision the northward expansion of the subtropical arid zones into the breadbasket regions of the United States and the Soviet Union in response to climate warming by increasing CO2 abundance (Manabe and Wetherald, 1980), in effect hypothesize the poleward expansion of the Hadley circulation subsidence regions."
The M&W (1980) paper is a continuation of their 1975 paper.
What I'm not sure about is the use, by R&R of the term "in effect". Are M&W the first to hypothesize the poleward expansion of the HC, or not?
What I mean is, a given drought today (lack of precipitation by whatever inches in whatever time frame) is worse than a given drought in the past (same lack of precipitation by whatever inches in whatever time frame) because, besides the lack of precipitation, there's more evaporation in a warmer world than than there was in the before-warming world.
David and Layzej -- we seem to all be in agreement. Here's another point to consider. If global warming really does increase average rainfall, and if warmer weather makes droughts worse, what is the magnitude of the two effects? Which one is larger? What is the net impact?
It's not just that evaporation increases in warmer weather and that the atmosphere can hold more water vapor. It's that the atmospheric circulation changes as a function of a warming climate. Some regions in the world will have less precipitation and other regions will have more. As I now realize, even Manabe and Wetherald realized back in 1975 that the circulation changes. It's why local regions in north Africa, southern Europe, the SW part of the US and locations in Australia will dry out in a warmer world, but other parts of the planet will get wetter. The expression that in a warming world the dry parts get dryer and the wet parts get wetter is too simple, but basically true.
As a starting point, take a look at a map of the world's deserts https://www.quora.com/Why-are-most-of-the-world%E2%80%99s-deserts-located-on-the-Western-margins-of-continents-in-the-subtropics
Notice how the world's deserts more or less line up with the 30 degree parallel north and south of the equator? That's your Hadley circulation in action.
From what I gather, M&W knew over 40 years ago that the circulation will expand poleward.
16 comments:
Rainfall is supposed to increase as the planet warms, due to increased evaporation of ocean waters. I looked for worldwide precip over time, but failed to find it.
Unfortunately, many naive people think that global warming has magical properties, that can cause just about any type of harm. In particular, some people attributed the recent Californian drought conditions to climate change, although there's no scientific basis for that belief. In fact, with increasing rainfall, it would seem that, if anything, droughts should become less of a problem.
Cheers
David in Cal said...
"In particular, some people attributed the recent Californian drought conditions to climate change, although there's no scientific basis for that belief."
DiC, you sure about that??
DiC, this took < 10 seconds to Google:
“In a new study published in PNAS, Diffenbaugh et al. now add weight to the accumulating evidence that anthropogenic climatic changes are already influencing the frequency, magnitude, and duration of drought in California (2). The authors show that the increasing co-occurrence of dry years with warm years raises the risk of drought despite limited evidence of a trend in precipitation itself, highlighting the critical role of elevated temperatures in altering water availability and increasing overall drought intensity and impact.”
https://www.pnas.org/content/112/13/3858.short
(2) Diffenbaugh NS, Swain DL, Touma D (2015) Anthropogenic warming has increased drought risk in California. Proc Natl Acad Sci USA 112:3931–3936.
https://www.pnas.org/content/112/13/3931?ijkey=163568051d26bf1c48ce1ddea5020094ed45df6d&keytype2=tf_ipsecsha
Why can't you do such Googling, David?
DiC: Rainfall is supposed to increase as the planet warms, due to increased evaporation of ocean waters
Evaporation can happen on land as well as sea.
DiC: Unfortunately, many naive people think that global warming has magical properties, that can cause just about any type of harm. In particular, some people attributed the recent Californian drought conditions to climate change,
See figure 7 in Manabe and Wetherald (1975). Notice what happens to precipitation and evaporation at latitudes 34-40 after a doubling of CO2?
That is the result of a very early GCM, but shows that this is not news.
Layzej, good catch in the M&W paper. I had read it in the past, but never realized that there was a calculated drop in precipitation at the latitude you pointed out. So here are my questions: Is this drop a manifestation of the poleward expansion of the Hadley circulation or is it something else? If it's due to the HC, is this the first paper (once again!) to make this prediction?
By the way, here's a nice write-up by Richard Seager of Columbia on increasing drought in the US southwest due to the HC expansion: http://ocp.ldeo.columbia.edu/res/div/ocp/drought/science.shtml
David, As I remember there wasn't unanimous agreement among climate scientists as to the Diffenbaugh paper. Two of the world experts on drought, the above-mentioned Richard Seager as well as Martin Hoerling, said the drought in California during 2011-2014 was dominated by natural variability https://journals.ametsoc.org/doi/full/10.1175/JCLI-D-14-00860.1
Meanwhile Mann and Trenberth disagreed with Seager and Hoerling: https://www.nbcnews.com/science/environment/global-warming-isnt-causing-california-drought-report-triggers-storm-n263941
I'm not sure whether this was ever resolved.
So here are my questions: Is this drop a manifestation of the poleward expansion of the Hadley circulation or is it something else?
It's in the right neighbourhood for HC expansion...
Depends on how one defines "drought". The paper doesn't say low precipitation years are more likely due to global warming. It says that low precipitation years are a bigger problem if it's warmer. Obviously global warming increases that probability.
Cheers
"t's in the right neighbourhood for HC expansion..."
It is, but does anyone know for sure?
The M&W papers don't even mention Hadley, but I did find this from Rind and Rossow, https://journals.ametsoc.org/doi/pdf/10.1175/1520-0469%281984%29041%3C0479%3ATEOPPO%3E2.0.CO%3B2
"The Hadley circulation is of central importance to the distribution of climate regimes on Earth and to the determination of atmospheric dynamics on other planets. Some scenarios, which envision the northward expansion of the subtropical arid zones into the breadbasket regions of the United States and the Soviet Union in response to climate warming by increasing CO2 abundance (Manabe and Wetherald, 1980), in effect hypothesize the poleward expansion of the Hadley circulation subsidence regions."
The M&W (1980) paper is a continuation of their 1975 paper.
What I'm not sure about is the use, by R&R of the term "in effect".
Are M&W the first to hypothesize the poleward expansion of the HC, or not?
I can't find anything explicit. To me, "in effect hypothesize" means it was a result of the model even if not formally acknowledged.
DiC, my understanding is that warming always makes a drought worse, because it increases evaporation.
I think DiCs point is that drought is often measured by change precipitation without regard for change in evaporation.
Yes, that's what I thought too.
What I mean is, a given drought today (lack of precipitation by whatever inches in whatever time frame) is worse than a given drought in the past (same lack of precipitation by whatever inches in whatever time frame) because, besides the lack of precipitation, there's more evaporation in a warmer world than than there was in the before-warming world.
No?
Fair point.
David and Layzej -- we seem to all be in agreement. Here's another point to consider. If global warming really does increase average rainfall, and if warmer weather makes droughts worse, what is the magnitude of the two effects? Which one is larger? What is the net impact?
Cheers
It's not just that evaporation increases in warmer weather and that the atmosphere can hold more water vapor. It's that the atmospheric circulation changes as a function of a warming climate. Some regions in the world will have less precipitation and other regions will have more. As I now realize, even Manabe and Wetherald realized back in 1975 that the circulation changes. It's why local regions in north Africa, southern Europe, the SW part of the US and locations in Australia will dry out in a warmer world, but other parts of the planet will get wetter. The expression that in a warming world the dry parts get dryer and the wet parts get wetter is too simple, but basically true.
As a starting point, take a look at a map of the world's deserts
https://www.quora.com/Why-are-most-of-the-world%E2%80%99s-deserts-located-on-the-Western-margins-of-continents-in-the-subtropics
Notice how the world's deserts more or less line up with the 30 degree parallel north and south of the equator? That's your Hadley circulation in action.
From what I gather, M&W knew over 40 years ago that the circulation will expand poleward.
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