[Keith_McClary]

(Note that this is quite old--1997. I suspect now much larger areas of, for example, northern China would be considered under threat.)

More maps here:
https://www.google.ca/search?q=desertif ... isch&tbo=u

[dohboi]

dol, like I said, it's an oldish map. More recent projections would doubtless have greater risk in some places and probably less in others.

But really, in many places (though probably not SE England) increased risks from flooding are likely to go, somewhat paradoxically, hand in hand with increased threats of desertification--when it rains it pours, and all that.

[dohboi]

In southern New Mexico, the transformation has gone one step further — from sagebrush to weeds to sand-blown desert — and biologists say the pattern is likely to be repeated across the West.

http://www.latimes.com/local/la-me-drou ... tml#page=1

[KingM]

A warmer earth will be a wetter earth. That doesn't mean the climate zones won't shift, but there will be less desert in the future, not more.

[farmlad]

Loss of soil organic matter, loss of biodiversity, high levels of atmospheric CO2, desertification, and global climate change, are all caused, mostly by mismanaged agriculture. Agriculture depending on how it is managed is able to either sequester or else release tremendous amounts of atmospheric CO2. Time is running out and the city people will suffer the most as our climate gets worse and the power for change is in the hands of the cities, as they are by far the majority, to change the ridiculous ag subsidies and instead start paying farmers for environmental benefits , such as for providing clean water, carbon sequestration etc. if you're serious check out these http://achmonline.squarespace.com/ http://www.amazingcarbon.com/ http://soilcarboncoalition.org/ http://brownsranch.us/?id=1

[dohboi]

KM, you are right that the average global atmospheric humidity has been and will continue to rise. But the same heat that allows that to happen also dries out soils, especially those that are already partly dry. The general pattern is that relatively already dry areas will get drier while wet areas will tend to get wetter. But as you say zones will also shift, sometimes in unpredictable ways. The 'stuck' jet stream waves are also likely to cause certain areas to get very dry and others very wet. Neither are good for agriculture--more crops are lost to floods than to drought, iirc.

fl, good points about the powerful effects, positive or negative, that agricultural practices can have on soils. As pstarr has pointed out, very healthy soils seem to be more resilient in the face of drought, at least for a time. But even well managed lands need some rain to grow crops. And of course most lands have not and won't any time soon be managed wisely. So the effects of GW on most farm and range land will indeed be devastating--much more devastating much more quickly than if they had been managed well.

[Keith_McClary]

A warmer earth will be a wetter earth. That doesn't mean the climate zones won't shift, but there will be less desert in the future, not more.

I posted this in the Drought Thread. It suggests that a cooler earth was a wetter earth, at least in western North America.

During the Last Glacial Maximum (LGM), giant lakes covered large sections of California, Nevada, Oregon and Utah, including where Salt Lake City is today. Earth scientists have long been puzzled by how these ancient lakes, now completely dry, grew so large. The prevailing theory was there was more rain and snowfall during this time period. But recent evidence from paleoecology and climate model simulations indicates that precipitation rates were actually relatively low compared to later periods.
...
Their analyses revealed that 21,000 years ago, the evaporation rate at Lake Surprise was nearly 40 percent lower than today, with precipitation rates similar to the modern era. These results are consistent with previously run climate simulations that show Earth's climate was cooler during the LGM.
The cooler global temperatures would have reduced evaporation rates, allowing the lakes to gradually grow over time through inflows from streams and rivers.
"Lake Surprise is located in a closed basin. All streams flow into the lake, but there is no outflow. The only way for water to escape is through evaporation," Ibarra said.
http://phys.org/news/2014-06-scientists ... lakes.html

[dohboi]

Arizona Could Be Out of Water in Six Years

Prolonged drought and a rapidly expanding population are pushing Arizona’s water system to its limit

Arizona is bone dry, desiccated by the worst drought ever seen in the state’s 110-year long observational record. The Grand Canyon State has been in drought conditions for a decade, and researchers think the dry spell could hold out for another 20 to 30 years, says the City of Phoenix.

That people have not been fleeing Arizona in droves, as they did from the plains during the 1930s Dust Bowl, is a miracle of hydrological engineering. But the magic won’t last, and if things don’t start to change Arizona is going to be in trouble fast, says the New York Times.

A quarter of Arizona’s water comes from the Colorado River, and that river is running low. There’s not enough water in the basin to keep Arizona’s crucial Lake Mead reservoirs topped up. If changes aren’t made to the entire multi-state hydrological system, says the Times, things could get bad.

http://www.smithsonianmag.com/ist/?next ... 180951814/

[dohboi]

A crucial part of land management is phosporus management, and there just happens to be a book recently published on the subject:
http://www.springer.com/environment/sus ... 007-7249-6
Sustainable Phosphorus Management

[Subjectivist]

KM, you are right that the average global atmospheric humidity has been and will continue to rise. But the same heat that allows that to happen also dries out soils, especially those that are already partly dry. The general pattern is that relatively already dry areas will get drier while wet areas will tend to get wetter. But as you say zones will also shift, sometimes in unpredictable ways. The 'stuck' jet stream waves are also likely to cause certain areas to get very dry and others very wet. Neither are good for agriculture--more crops are lost to floods than to drought, iirc.

You know I see this written a lot, however the higher the humidity goes the harder it is to dry out the soil. Yes higher temperature creates a higher vapor capacity for the atmosphere, but everything has limits in physics and when the relative humidity reaches 100% percipitation forces it to stop rising. If global temperature goes from for example 17 C to 20 C the capacity for humidity in the atmosphere doesn't rise all that much.


http://upload.wikimedia.org/wikipedia/c ... midity.png

[Synapsid]

KingM, dohboi,

The amount of water vapor in the atmosphere (actually in the troposphere, the part with weather where we live) has indeed been increasing. How that manifests in precipitation amount and distribution isn't just a matter of temperature, though, but of tropospheric circulation patterns.

There's more than one way deserts form; some, like the Gobi, are in large part isolation deserts with little in the way of moisture sources upwind, while the Canadian High Arctic and Antarctica's Dry Valleys are dry because the air is so cold that it carries very little water vapor. On a world map we can see that the desert belt north and south of the US border with Mexico lies in latitudes similar to those where we find the deserts of North Africa and the Middle East--which have ocean nearby. In western North Africa you can ride your camel out of the desert into the surf, and Baja California is almost completely surrounded by ocean. These latitudes lie in a zone of descending air that is part of the overall circulation of the troposphere; descending air warms by compression and becomes progressively drier on the way down so that even if it had contained water vapor aloft (it doesn't, that condensed out where the air rose) its decreasing relative humidity would prevent cloud formation and thus precipitation. The result is cloudless days and high temperatures (and cloudless nights and low temperatures) unless there's a source of moisture nearby.

On the west sides of landmasses in that zone of descending air ocean currents are cold (think of the California Current) and the water doesn't moisten overlying air through evaporation to the extent that aridity on adjacent land is overcome, so we get the climate of San Diego and Sonora and the Sahara. On the east side (think Gulf Stream) very warm ocean currents provide plenty of moisture through evaporation so instead of arid climate you get Georgia's and southern China's; monsoons contribute moisture seasonally too.

All this (pant, pant) is in service to the point that there will always be that zone of descending air because the troposphere will go on circulating, so there will always be a desert belt (perhaps poleward of its current position). There's one in the Southern Hemisphere too, by the way, though there isn't a lot of continent in it except in dry, dry Australia, and in South Africa.

This is good news for those of us who love desert.

[Synapsid]

Keith McC,

Lower temperatures in the Great Basin did result in lower rates of evaporation so that lakes grew there, where there are no (large) lakes today--Great Salt Lake is the exception but it's a shrunken remnant of Glacial Lake Bonneville. We have to be careful about extending that to the whole planet though because in some areas, such as interior northern Africa, higher (not lower) temperatures led to lakes forming where there are none today.

There's a summer monsoon that supports forest in West Africa south of the Sahara and Sahel. The forest peters out inland as monsoon moisture decreases away from the Atlantic, its source. After the last glacial ended there was a thermal maximum during which that summer monsoon was stronger and moisture was carried farther inland than it is today, into what's now desert, and lakes and rivers formed where there is only aridity today. There were whole connected drainages in what's now the Sahara (archaeologists use their traces to locate places where people used to live.) The lake beds, now dry, are the source of fine sediment picked up by winds and carried across the Atlantic to South America and the Caribbean, where the dust is a source of phosphorus to the vegetation.

Ahem...sorry. The point is that both higher temperatures and lower have led to large lakes forming in the past in areas that are now desert.

[dohboi]

Sub, to paraphrase/summarize Syn's excellent points--the earth is not a uniform sphere. Increase in average humidity does not imply increase in humidity everywhere. Places where conditions already 'discourage' precipitation will mostly get yet drier and hotter.

Also, as the tropical Hadley Cell continues to expand, places that used to get regular rainfall will rather suddenly stop getting it (or not get it with the same regularity).

[KingM]

Sub, to paraphrase/summarize Syn's excellent points--the earth is not a uniform sphere. Increase in average humidity does not imply increase in humidity everywhere. Places where conditions already 'discourage' precipitation will mostly get yet drier and hotter.

Also, as the tropical Hadley Cell continues to expand, places that used to get regular rainfall will rather suddenly stop getting it (or not get it with the same regularity).

Not really. What will happen is that the rainfall belts will shift. We know that the Sahara was wetter when the climate was warmer because the monsoons shift to the north. At the same time, there will likely be places that then get less rainfall. The average global rainfall will increase due to a warmer, wetter climate, but individual places are going to be all over the place. New deserts will form, old ones will disappear or be moderated, etc. Of course it will be very disruptive, but this view of marching deserts across the globe is wrong.

[dohboi]

"this view of marching deserts across the globe is wrong"

Where did anyone make any such claim?

Yes, rain belt will shift. The monsoons that once fell on India will fall on the ocean or on the Himalayas--neither will be of much use to the billion plus people that will go starving.

Increased rainfall in the Sahara, especially since it will likely mostly come down in torrents, will not instantly turn it into an a agricultural paradise. Meanwhile, the Northern coast of Africa and much of southern Europe will become the new Sahara. I'm sure all the folks there will be happy to learn that there is still rain falling somewhere in the world.

Really, I know perfectly well that gw will lead to major increases in heavy rain fall events on average across the earth. That's why I started the "Deluge/major rainfall events" thread a while back. If you want to talk about those, that is the place to go. This thread is about the other side of that coin, but in no way was it ever intended to imply a claim that there would be more drying, over all, than 'wetting.'

I fear you are sparring with a straw man of your own creation.

[dohboi]

http://climatestate.com/2014/04/03/one- ... d-dry-out/

Warming Temperatures Could Dry Out One Third of Planet

Warming temperatures, scientists say, can tip places into drought conditions by increasing evaporation and sapping soil of its moisture. A new study suggests up to a third of the Earth's land area could be subject to drier conditions because of warmer temperatures, not just changing precipitation patterns, by the end of the century.

The study finds that one of the agricultural heartlands of the United States, the Central Plains, could become drier, even though it may not receive less rain in the future, simply because warmer temperatures will drive evaporation to dry out the rich soils. Drier soil makes it harder to grow the food that feeds the country, which can have knock-on effects on trade and national security.

And the problem isn’t limited to the U.S.: Western Europe and the fertile fields of southeast China show the same drying trend, the study, detailed in the journal Climate Dynamics, found.

[Keith_McClary]

Here's a more recent map from US EPA:

[dohboi]

Thanks for that updated map, KM.

It is notable that the only significant large areas that will see significant increases in precipitation are the far northern latitudes. This is already starting to happen in many places, and will accelerate rapidly as the Arctic Ocean becomes more and more open more and more of the year. The increased snow and (increasingly) rain will not do much to make these areas better for agriculture. But they will insure that the permafrost in those areas melts, and that when it does, most of the carbon released will be in the form of methane--a greenhouse gas some 100 times more powerful than CO2 in decadal time frames.

I note also that this map, at least, shows the Sahara mostly staying the same or getting drier.

[Keith_McClary]

I note also that this map, at least, shows the Sahara mostly staying the same or getting drier.

I think that map shows absolute (not relative to now) drought.

[dohboi]

"drier than current conditions" certainly does sound to me like it's "relative to now."