Desert Expansion

[AbruptSLR]

If the Sahara Desert has expanded 10% since 1920, it is reasonable to assume that many/most other deserts have also expanded a comparable amount in that timeframe:

Natalie Thomas, Sumant Nigam (2018), "Twentieth-Century Climate Change over Africa: Seasonal Hydroclimate Trends and Sahara Desert Expansion", Journal of Climate, 31 (9): 3349 DOI: 10.1175/JCLI-D-17-0187.1

https://journals.ametsoc.org/doi/10.1175/JCLI-D-17-0187.1

Abstract: "Twentieth-century trends in seasonal temperature and precipitation over the African continent are analyzed from observational datasets and historical climate simulations. Given the agricultural economy of the continent, a seasonal perspective is adopted as it is more pertinent than an annual-average one, which can mask offsetting but agriculturally sensitive seasonal hydroclimate variations. Examination of linear trends in seasonal surface air temperature (SAT) shows that heat stress has increased in several regions, including Sudan and northern Africa where the largest SAT trends occur in the warm season. Broadly speaking, the northern continent has warmed more than the southern one in all seasons. Precipitation trends are varied but notable declining trends are found in the countries along the Gulf of Guinea, especially in the source region of the Niger River in West Africa, and in the Congo River basin. Rainfall over the African Great Lakes—one of the largest freshwater repositories—has, however, increased. It is shown that the Sahara Desert has expanded significantly over the twentieth century, by 11%–18% depending on the season, and by 10% when defined using annual rainfall. The expansion rate is sensitively dependent on the analysis period in view of the multidecadal periods of desert expansion (including from the drying of the Sahel in the 1950s–80s) and contraction in the 1902–2013 record, and the stability of the rain gauge network. The desert expanded southward in summer, reflecting retreat of the northern edge of the Sahel rainfall belt, and to the north in winter, indicating potential impact of the widening of the tropics. Specific mechanisms for the expansion are investigated. Finally, this observational analysis is used to evaluate the state-of-the-art climate simulations from a comparison of the twentieth-century hydroclimate trends. The evaluation shows that modeling regional hydroclimate change over the African continent remains challenging, warranting caution in the development of adaptation and mitigation strategies."

See also:

Title: "The Sahara Desert is expanding"

https://www.sciencedaily.com/releases/2018/03/180329141035.htm

Extract: "New study finds that the world's largest desert grew by 10 percent since 1920, due in part to climate change"

[AbruptSLR]

The linked open access reference suggests that due to vegetation-albedo feedback the rate of desert expansion may average 34% from 2009 to 2100.

Ning Zeng and Jinho Yoon (2009), "Expansion of the world’s deserts due to vegetation-albedo feedback under global warming", Geophys. Res. Lett., 36, L17401, doi:10.1029/2009GL039699

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2009GL039699

Abstract: "Many subtropical regions are expected to become drier due to climate change. This will lead to reduced vegetation which may in turn amplify the initial drying. Using a coupled atmosphere-ocean-land model with a dynamic vegetation component that predicts surface albedo change, here we simulate the climate change from 1901 to 2099 with CO2 and other forcings. In a standard IPCC-style simulation, the model simulated an increase in the world’s ‘warm desert’ area of 2.5 million km2 or 10% at the end of the 21st century. In a more realistic simulation where the vegetation-albedo feedback was allowed to interact, the ‘warm desert’ area expands by 8.5 million km2 or 34%. This occurs mostly as an expansion of the world’s major subtropical deserts such as the Sahara, the Kalahari, the Gobi, and the Great Sandy Desert. It is suggested that vegetation-albedo feedback should be fully included in IPCC future climate projections."

[AbruptSLR]

Per the linked article, the United Nations Convention to Combat Desertification (UNCCD) indicates that annually 12 million hectares of arable land are lost to desertification:

Title: "The High Price of Desertification: 23 Hectares of Land a Minute"

https://reliefweb.int/report/world/high-price-desertification-23-hectares-land-minute

Extract: "Desertification is on the march. Many people are going hungry because degraded lands affects agriculture, a key source of livelihood and food in much of Africa. More than 2.6 billion people live off agriculture in the world. More than half of agricultural land is affected by soil degradation, according to the United Nations Convention to Combat Desertification (UNCCD).

It gets worse. The UN body says 12 million hectares of arable land, enough to grow 20 tonnes of grain, are lost to drought and desertification annually, while 1.5 billion people are affected in over 100 countries. Halting land degradation has become an urgent global imperative.

The UN Food and Agriculture Organization (FAO) estimates that by 2030 Africa will lose two-thirds of its arable land if the march of desertification — the spread of arid, desert-like areas of land — is not stopped."

[AbruptSLR]

The linked open access document summarizes extant IPCC knowledge about “climate change, desertification, land degradation, sustainable land management, food security, and greenhouse gas fluxes in terrestrial ecosystems".

IPCC- WGIII [Pete Smith (Chair), Mark Howden, Thelma Krug, Valérie Masson-Delmotte, Cheikh Mbow, Hans-Otto Pӧrtner, Andy Reisinger, Josep Canadell & Phillip O’Brien] (2017), "Special Report on climate change, desertification, land degradation, sustainable land management, food security, and greenhouse gas fluxes in terrestrial ecosystems (SR2) - Background report for the Scoping Meeting"

https://www.ipcc.ch/report/sr2/pdf/sr2_background_report_final.pdf

Forward: "This background documentation provides a summary of the available knowledge from previous IPCC reports and other relevant sources to act as a foundation upon which to build discussion at the Expert Scoping Meeting in February 2017 for the IPCC Special Report on “climate change, desertification, land degradation, sustainable land management, food security, and greenhouse gas fluxes in terrestrial ecosystems”. The aim of the background documentation is not to provide a comprehensive exploration of all of these areas, nor to propose outcomes, but to support participants by providing a summary of relevant information before and during the scoping meeting. We review relevant reports and other outputs before providing a final section on key known unknowns and issues arising since AR5, as a means to stimulate discussion."

[oren]

"The UN body says 12 million hectares of arable land, enough to grow 20 tonnes of grain, are lost to drought and desertification annually" - even knowing nothing about the subject, I knew this statement was off. But after looking into it,  I still can't figure out what it was that they actually meant, or what their error was.
In any case, annual wheat yield is 3 tons per hectare, if anyone was wondering.

[Red]

"The UN body says 12 million hectares of arable land, enough to grow 20 tonnes of grain

A slip I presume 12 million x 3 tonnes = 36 million tonnes not 20

[Tor Bejnar]

If global average wheat production is 2.8 tons/hectare (link)[or 3.1 US 'short' tons/hectare, just to confuse things], I imagine 'marginal lands' about to be desertified will have a much lower yield 'to start with' (maybe 1.7 tons/ha [20/12 in oren's post - I'll presume they left out the word "million" before "tonnes"]?).  Algeria, for example (via USDA):

Last year’s [wheat] crop had a favorable yield of 1.91 tons per hectare [short or long tons?]

I doubt it was all 'marginal land about to be desertified'.

[Disclamer:  I don't know squat about farming. The question intrigued me, so I did some internet searching.]

[charles_oil]

I suspect marginal land on the edge of desertification has a much lower yield then normal, properly cultivated arable land.

[El Cid]

Climate change brings more water vapour. We also know that colder periods were drier with much less rain and bigger deserts, paleodata support this. Think of Green Sahara during the Holocene Climatic Optimum. So I do not believe that global warming creates more deserts, on the contrary, it reduces the size of deserts.

The growth of deserts if and when it happens is actually caused by bad land practices / overgrazing, etc. Overpopulation in Africa is the No1 danger currently causing desertification

[wili]

The usual view is that, while overall there will be more water vapor (already increased by some 7%), there will also be areas that will get drier, especially many places already dry now, and interiors of large continents. Also, as climate zones move poleward, arid regions will move (are already moving) poleward, so in the Northern Hemisphere, places now north of major deserts are likely to become drier.

Yes, ag, overgrazing can make things much worse, but there have always been areas that were drier and areas that were wetter, and the dry areas tend to fall at certain latitudes because of Hadley Cell behavior, which others who know more about such things can certainly elucidate much more clearly than I can.

[El Cid]

...there will also be areas that will get drier, especially many places already dry now, and interiors of large continents. Also, as climate zones move poleward, arid regions will move (are already moving) poleward, so in the Northern Hemisphere, places now north of major deserts are likely to become drier.

That is true. However if that is true, then some places that used to be deserts (eg Southern Sahara) will no longer be. Also according to paleo data - as far as I know - warmer periods had smaller deserts in total... 

[Daniel B.]

...there will also be areas that will get drier, especially many places already dry now, and interiors of large continents. Also, as climate zones move poleward, arid regions will move (are already moving) poleward, so in the Northern Hemisphere, places now north of major deserts are likely to become drier.

That is true. However if that is true, then some places that used to be deserts (eg Southern Sahara) will no longer be. Also according to paleo data - as far as I know - warmer periods had smaller deserts in total...

That is true.  The African Humid Period was a time roughly 10k years ago, in which North Africa teamed with lush vegetation and numerous wildlife.  This corresponded with warmer temperatures and a healthy monsoon season.  Paleo data suggests that the transition to its current death state began about 5k years ago, and was complete in a relatively short timeframe.  In recent times, the most widespread droughts have been associated with colder periods.

[wili]

Probably inevitably, our discussions on this are going to be more simplistic than the full complexity of what actually transpired. Here's wiki's brief take on the relevant periods of North Africa, for what it's worth:

The end of the glacial period brought more rain to the Sahara, from about 8000 BC to 6000 BC, perhaps because of low pressure areas over the collapsing ice sheets to the north. Once the ice sheets were gone, the northern Sahara dried out.

In the southern Sahara, the drying trend was initially counteracted by the monsoon, which brought rain further north than it does today. By around 4200 BC, however, the monsoon retreated south to approximately where it is today, leading to the gradual desertification of the Sahara. The Sahara is now as dry as it was about 13,000 years ago.

So according to this in the first instance it was actually the presence of ice-sheets that allowed North Africa to be relatively green...not exactly a simple--warmer globe/less desert--correlation or causation.

And 'lush' may be overstating it a bit...it was certainly greener than most of the Sahara currently is, but still a rather arid, grassland environment: "During periods of a wet or "Green Sahara", the Sahara becomes a savanna grassland "

(My emphases) https://en.wikipedia.org/wiki/Sahara

[El Cid]

That is not the current scientific explanation to my knowledge:

https://www.nature.com/scitable/knowledge/library/green-sahara-african-humid-periods-paced-by-82884405

"Strengthening summer-season solar radiation causes the North African landmass to heat up relative to the adjacent Atlantic Ocean due the lower thermal inertia of the land surface relative to the upper ocean mixed layer. The warmer land mass creates a broad low pressure zone, driving the inflow of moist air from the tropical Atlantic. The resulting summer monsoonal rains nourish the landscape. During winter, the land cools relative to the ocean and the winds reverse (one definition of a monsoon), returning dry conditions across North Africa. Since precession controls summer insolation, it effectively controls the amount and northward penetration of the monsoonal rains into North Africa. Simple atmosphere-only climate models have shown that a 7% increase in summer radiation, similar to what occurred during the AHP, results in at least a 17% increase in African monsoonal rainfall, and up to 50% if ocean feedbacks are included (Kutzbach and Liu, 1997)."

The warming up (even more than now!) of North Africa during summer caused monsoon rains to appear during the hot season. It is reasonable to think that when North Africa warms up again, the same thing will happen.

Also, every study I have read pointed out that cooler periods had more deserts, warmer periods had less

[wili]

Yes, over all, a wetter atmosphere should generally mean more places getting wetter than places getting drier. I'm just saying turning that into predictions for specific locations can be difficult.

And the two are not mutually inclusive. Many places will see both deeper and longer periods of drought interspersed with ever-more intense rain-bombs that will tend to wash away the drought-killed soil rather than foster new thriving life.

As mentioned earlier, though, the effects that locals have are likely to be at least as important wrt desertification or restoration.

[TerryM]

FWIW
In the American Southwest the dry lakes now found throughout the desert ~uniformly dried up at the end of the ice age, often referred to as the cooler, pluvial period.
Herds of giant Bison and families of Mammoths simply couldn't have survived on the sparse vegetation now extant, but their bones prove their earlier existence.
Terry

[El Cid]

Yes, over all, a wetter atmosphere should generally mean more places getting wetter than places getting drier. I'm just saying turning that into predictions for specific locations can be difficult.

And the two are not mutually inclusive. Many places will see both deeper and longer periods of drought interspersed with ever-more intense rain-bombs that will tend to wash away the drought-killed soil rather than foster new thriving life.

As mentioned earlier, though, the effects that locals have are likely to be at least as important wrt desertification or restoration.

Yes, I can agree with the above. This, however means, that good practices, eg: rain harvesting, soil erosion control, forest/plant cover, etc. could help a lot to avoid desertification even in areas with less rain.

The biggest problem is not less rain as we all know, but possibly changing precipitation patterns and adaptation to that.

[Sebastian Jones]

Yes, over all, a wetter atmosphere should generally mean more places getting wetter than places getting drier. I'm just saying turning that into predictions for specific locations can be difficult.

And the two are not mutually inclusive. Many places will see both deeper and longer periods of drought interspersed with ever-more intense rain-bombs that will tend to wash away the drought-killed soil rather than foster new thriving life.

As mentioned earlier, though, the effects that locals have are likely to be at least as important wrt desertification or restoration.

Yes, I can agree with the above. This, however means, that good practices, eg: rain harvesting, soil erosion control, forest/plant cover, etc. could help a lot to avoid desertification even in areas with less rain.

The biggest problem is not less rain as we all know, but possibly changing precipitation patterns and adaptation to that.

While warmer means wetter, it also means increased evapo-transpiration. This can result in local conditions becoming more desert like despite increased precipitation. For example, here at the northern frontier of the boreal forest, one would assume that a warmer wetter climate would result in better conditions for trees. However, the dominant species, white spruce cannot grow when summer temps rise above a certain threshold, about one degree  more than the current July temperature. Combine this with the higher rate of evapo-transpiration and it is predicted that the climate will soon be more suitable for aspen parkland than for boreal forest. And aspen parkland feels pretty desert like compared to boreal forest!
 

[El Cid]

I see. Still, I would not call aspen parkland a desert 

[Tor Bejnar]

I recall reading (decades ago) about some research somewhere in the Southwest USA concerning the widespread development of arroyos during the 18th and 19th centuries.  [Edit:  apparently there were 'few' arroyos in the 16th and 17th centuries when the first Spaniards explored the region.] The 'prevailing theory' was that overgrazing was the cause.  The researchers wondered why there were arroyos 'everywhere', not just around where settlements were.  They discovered, IIRC, there was a grass that was dying out (regionally) due to a slight increase in rainfall; there were two forces supporting erosion (more water and fewer roots). A different grass migrated in that relatively stabilized the region or had a different sort of root structure that didn't hold soil as well, or something (some details I don't recall!).  Overgrazing, of course, was and remains a force that supports arroyo development.
(arroyo images)

[sidd]

A good paper on hydrological response: apparently plants influence the hydro cycle more than previously thought:

"we here show that the CO 2 physiological response has a dominant role in evapotranspiration and evaporative fraction changes and has a major effect on long-term runoff compared with radiative or precipitation changes due to increased atmospheric CO 2 . This major effect is true for most hydrological stress variables over the largest fraction of the globe, except for soil moisture, which exhibits a more nonlinear response. This highlights the key role of vegetation in controlling future terrestrial hydrologic response and emphasizes that the carbon and water cycles are intimately coupled over land."

"Contrary to previous wisdom, changes in precipitation and radiation do not play the primary role in future drying and moistening in most regions. Rather, biosphere physiological effects and related biosphere–atmosphere interactions (42) are key for predicting future continental water stress as represented by ET, long-term runoff, EF, or leaf area index. In turn, vegetation water stress largely regulates land carbon uptake (43), further emphasizing how tightly the future carbon and water cycles are coupled so that they cannot be evaluated in isolation"

doi: 10.1073/pnas.1720712115

Open access. Read the whole thing.

sidd

[Michael Hauber]

Climate change brings more water vapour. We also know that colder periods were drier with much less rain and bigger deserts, paleodata support this.

Can you back this claim up with specific studies?  Not studies showing higher rainfall when warmer, but rather higher soil moisture/less deserts/more vegetation.  And global, not limited to a specific area such as Sahara.

A warmer world will put more water vapour into the atmosphere.  It will also increase evaporation rates.  A naive expectation is that the two effects would cancel each other out.  However even if the effects cancel each other out, more stress might be placed on plants during dry periods, and wet periods might result in a greater percentage of water running out to sea in rivers.

Also warming now is much faster than known from paleoclimate studies.  The result is that the land warms up faster than ocean.  Global rain will depend more on ocean temps, and plant stress due to evaporation depend on land temps, so global rainfall will not keep up.  However especially for monsoon regions a faster land warming may result in a greater percentage of the global rainfall falling on land.  As models tend to show a reduction in soil moisture in the future, I guess the first factor wins.  Or other factors are at play.  A flip side is that as soon as action is taken to stabilise CO2 concentrations in the atmosphere, we may still have more warming in the pipeline, but the rate will slow down and the land vs ocean temperature difference will reduce.

[El Cid]

Climate change brings more water vapour. We also know that colder periods were drier with much less rain and bigger deserts, paleodata support this.

Can you back this claim up with specific studies? 

Unfortunately I do not have anything saved on my computer, I just read stuff and wrote down what I learnt from my readings. I do not have time for this now, just quickly googled and found this, though it might not be convincing to you (modelling the Eemian which is often used as an analogue of a warmer climate):

https://www.clim-past.net/9/1789/2013/cp-9-1789-2013.pdf

stating:  "Desert areas reduce significantly in the Northern Hemisphere, but increase in northern Australia"

[Alexander555]

https://watchers.news/2018/04/18/massive-dust-storm-sweeps-over-yazd-province-iran/