Deserts are typically defined by low average annual rainfall—usually 100 millimeters (less than 4 inches) of rain per year or less. The researchers analyzed rainfall data recorded throughout Africa from 1920 to 2013 and found that the Sahara, which occupies much of the northern part of the continent, expanded by 10 percent during this period when looking at annual trends.

When the authors looked at seasonal trends over the same time period, the most notable expansion of the Sahara occurred in summer, resulting in a nearly 16 percent increase in the desert‘s average seasonal area over the 93-year span covered by the study.

“Our results are specific to the Sahara, but they likely have implications for the world’s other deserts,” said Sumant Nigam, a professor of atmospheric and oceanic science at UMD and the senior author of the study. Nigam also has a joint appointment in UMD’s Earth System Science Interdisciplinary Center (ESSIC).

The study results suggest that human-caused climate change, as well as natural climate cycles such as the Atlantic Multidecadal Oscillation (AMO), caused the desert’s expansion. The geographical pattern of expansion varied from season to season, with the most notable differences occurring along the Sahara’s northern and southern boundaries.

“Deserts generally form in the subtropics because of the Hadley circulation, through which air rises at the equator and descends in the subtropics,” Nigam said. “Climate change is likely to widen the Hadley circulation, causing northward advance of the subtropical deserts. The southward creep of the Sahara however suggests that additional mechanisms are at work as well, including climate cycles such as the AMO.”

The Sahara is the world’s largest warm-weather desert, roughly equal in size to the contiguous United States. (The Arctic basin and the Antarctic continent—which are each about twice as large as the Sahara—also qualify as deserts due to their low rates of precipitation.) Like all deserts, the boundaries of the Sahara fluctuate with the seasons, expanding in the dry winter and contracting during the wetter summer.

The southern border of the Sahara adjoins the Sahel, the semi-arid transition zone that lies between the Sahara and the fertile savannas further south. The Sahara expands as the Sahel retreats, disrupting the region’s fragile grassland ecosystems and human societies. Lake Chad, which sits in the center of this climatologically conflicted transition zone, serves as a bellwether for changing conditions in the Sahel.

“The Chad Basin falls in the region where the Sahara has crept southward. And the lake is drying out,” Nigam explained. “It’s a very visible footprint of reduced rainfall not just locally, but across the whole region. It’s an integrator of declining water arrivals in the expansive Chad Basin.”

A number of well-known climate cycles can affect rainfall in the Sahara and the Sahel. The AMO, in which temperatures over a large swath of the northern Atlantic Ocean fluctuate between warm and cold phases on a 50- to 70-year cycle, is one example. Warm phases of the AMO are linked to increased rainfall in the Sahel, while the opposite is true for the cold phase. For example, the notable drying of the Sahel from the 1950s to the 1980s has been attributed to one such cold phase. The Pacific Decadal Oscillation (PDO), marked by temperature fluctuations in the northern Pacific Ocean on a scale of 40 to 60 years, also plays a role.

To single out the effects of human-caused climate change, the researchers used statistical methods to remove the effects of the AMO and PDO on rainfall variability during the period from 1920 to 2013. The researchers concluded that these natural climate cycles accounted for about two-thirds of the total observed expansion of the Sahara. The remaining one-third can be attributed to climate change, but the authors note that longer climate records that extend across several climate cycles are needed to reach more definitive conclusions.

“Many previous studies have documented trends in rainfall in the Sahara and Sahel. But our paper is unique, in that we use these trends to infer changes in the desert expanse on the century timescale,” said Natalie Thomas, a graduate student in atmospheric and oceanic science at UMD and lead author of the research paper.

The study’s results have far-reaching implications for the future of the Sahara, as well as other subtropical deserts around the world. As the world’s population continues to grow, a reduction in arable land with adequate rainfall to support crops could have devastating consequences.

“The trends in Africa of hot summers getting hotter and rainy seasons drying out are linked with factors that include increasing greenhouse gases and aerosols in the atmosphere,” said Ming Cai, a program director in the National Science Foundation’s Division of Atmospheric and Geospace Sciences, which funded the research. “These trends also have a devastating effect on the lives of African people, who depend on agriculture-based economies.”

Thomas and Nigam are focused on learning more about the drivers behind desert expansion in the Sahara and beyond.

“With this study, our priority was to document the long-term trends in rainfall and temperature in the Sahara. Our next step will be to look at what is driving these trends, for the Sahara and elsewhere,” Thomas explained. “We have already started looking at seasonal temperature trends over North America, for example. Here, winters are getting warmer but summers are about the same. In Africa, it’s the opposite—winters are holding steady but summers are getting warmer. So the stresses in Africa are already more severe.”

The research paper, “20th-Century Climate Change over Africa: Seasonal Hydroclimate Trends and Sahara Desert Expansion,” Natalie Thomas and Sumant Nigam, was published online March 29, 2018, in the Journal of Climate.

PHYS.org

15 Comments on "The Sahara Desert is expanding—world’s largest desert grew by 10 percent since 1920"

Outcast_Searcher on Fri, 30th Mar 2018 1:34 pm 



Gee. All the people who think the world never changes will be SO upset.

fmr-paultard on Fri, 30th Mar 2018 2:37 pm 



i’m not concerend about desertification. my more immediate is our extinction through war unleashed by the eurotrad’s PMBB axis of power. USA is going down fast. Look at the number of English speakers of 2Billion and french of 40 millions and growing fast.

I’m not able to sleep unless a strong dose of niquil

Dredd on Fri, 30th Mar 2018 2:54 pm 



Actually, Antarctica is the largest desert (5.405 million mi²) compared to the Sahara (3.552 million mi²).

But the Sahara is expanding while Antarctica is shrinking (Antarctica 2.0 – 5).

Cloggie on Fri, 30th Mar 2018 3:04 pm 



Yet at the same time the planet is rapidly getting greener as 50% of the 10 billion ton annual new atmospheric CO2 is turned into new vegetation:

https://www.nasa.gov/feature/goddard/2016/carbon-dioxide-fertilization-greening-earth

Davy on Fri, 30th Mar 2018 3:16 pm 



Sure neder, CO2 is greening the world but that does not mean it is healthy. What about the acidification of the oceans? What about the diminishing effects of CO2? At a certain point CO2 plus increased heat will actually make plants less healthy. If there are benefits they are
minuscule to the damage being done.

BobInget on Fri, 30th Mar 2018 3:47 pm 



Still, a great beach.

Anonymouse1 on Fri, 30th Mar 2018 4:41 pm 



Holy fook are you ever dumb clogg-berg. That link of yours, does nothing to refute the fact that desert are expanding.

Second, the article, as usual, if you actually read it, does not support your AGW-is-a-bolshevik-hoax fantasies.

From the same article clogged-commode uses to ‘support’ his AGW-denialism.

“While rising carbon dioxide concentrations in the air can be beneficial for plants, it is also the chief culprit of climate change. The gas, which traps heat in Earth’s atmosphere, has been increasing since the industrial age due to the burning of oil, gas, coal and wood for energy and is continuing to reach concentrations not seen in at least 500,000 years. The impacts of climate change include global warming, rising sea levels, melting glaciers and sea ice as well as more severe weather events.

The beneficial impacts of carbon dioxide on plants may also be limited, said co-author Dr. Philippe Ciais, associate director of the Laboratory of Climate and Environmental Sciences, Gif-suv-Yvette, France. “Studies have shown that plants acclimatize, or adjust, to rising carbon dioxide concentration and the fertilization effect diminishes over time.”

So no, that report, supports neither of clogged-toilets assertions. Oh, I forget, your a ‘broadcaster’, a ‘skimmer’. Not a details (as in reading), kind of denier. My bad.

Better luck next time, cloggraham.

Cloggie on Fri, 30th Mar 2018 5:07 pm 



“Holy fook are you ever dumb clogg-berg. That link of yours, does nothing to refute the fact that desert are expanding.”

Did I say that, dumbo?

I merely state a fact that global vegetation is getting more dense, refuting the idea some might have that the entire planet is turning in a dry, lifeless desert. It is not. And yes the Sahara is expanding.

“Second, the article, as usual, if you actually read it, does not support your AGW-is-a-bolshevik-hoax fantasies.”

Where do I deny GW? Only in that black hole that passes as your brain. Look, I know that you probably recognise words one-by-one, but you are functional illiterate, you just read into sentences what you want to see. You’re completely unable to grasp the meaning of a text.

Boat on Fri, 30th Mar 2018 7:07 pm 



Cloud seeding by China

https://qz.com/

Sissyfuss on Fri, 30th Mar 2018 7:44 pm 



What a pitiful defensive posture you take, Cloggedports. You forwarded that link to ameliorate the articles premise that deserts are expanding thanks to AGW and then say that wasn’t your point. I see your point and it is the top of your head.

deadly on Fri, 30th Mar 2018 10:19 pm 



Forests are growing along with desert growth.

Forests play an important role in the conservation of biological diversity and the area of forest primarily designated for this purpose is also expanding, according to UNECE. In the UNECE region it increased by almost 25 million hectares – an area slightly larger than the United Kingdom – between 1990 and 2010, and currently accounts for about 8 per cent of the total forest area in the region.

Most of this increase took place in Europe, the Caucasus and Central Asia, where the area of forest specifically managed for conservation of biological diversity has been increasing by almost one million hectares annually over the past 20 years, while in North America it has increased by more than 5 million hectares since 1990.

Increasing climate variability, however, has had a negative impact on forest growth and health, with North America reporting the largest area of forests with insect-related health problems in 2005 compared to any other region.

Problems include outbreaks of the mountain pine beetle, which has devastated more than 11 million hectares of forest in Canada and the western United States since the late 1990s – an unprecedented outbreak aggravated by higher winter temperatures. During the same period damage resulting from storms, wind and snow affected 0.4 per cent of the forest area.

https://news.un.org/en/story/2011/03/369752-forests-expand-europe-and-north-america-still-vulnerable-climate-change-un

Even the UN says the forest biomes are growing.

Can’t argue with that.

DerHundistlos on Sat, 31st Mar 2018 5:42 am 



@ deadly

DEAD WRONG. The only source you provide refers to forest coverage in the US and Europe.

The fact of the matter is world forest cover is shrinking and shrinking fast. The total forest loss between 2000 and 2005 was estimated, based on satellite observation, to be 1,011,000 sq km, the researchers said (note, not hectares).

https://www.smh.com.au/world/world-forest-cover-shrinking-report-20100427-tnm3.html

jawagord on Sat, 31st Mar 2018 7:04 am 



It should be obvious that natural processes dominate our climate, the human factor is a small perturbation on the much larger processes of natural climate variability. From the same publication:

Rainfall patterns in the Sahara during the 6,000-year “Green Sahara” period have been pinpointed by analyzing marine sediments, according to new research.

What is now the Sahara Desert was the home to hunter-gatherers who made their living off the animals and plants that lived in the region’s savannahs and wooded grasslands 5,000 to 11,000 years ago.

“It was 10 times as wet as today,” said lead author Jessica Tierney of the University of Arizona. Annual rainfall in the Sahara now ranges from about 4 inches to less than 1 inch (100 to 35 mm).
Although other research had already identified the existence of the Green Sahara period, Tierney and her colleagues are the first to compile a continuous record of the region’s rainfall going 25,000 years into the past.
The team’s paper “Rainfall regimes of the Green Sahara,” is scheduled for publication in the journal Science Advances on Jan. 18.

The team also wanted to know whether the conditions on land interacted with the atmosphere to affect climate, because most of the current climate models don’t simulate the Green Sahara period well, she said.
The amount of solar radiation the Earth receives during the Northern Hemisphere summer depends on where the Earth’s “wobble,” known as precession, is in its 23,000-year cycle.

At the beginning of the Green Sahara, the Northern Hemisphere was closer to the sun during summer. Warmer summers strengthened the West African monsoon and delivered more rain. Toward the end of the Green Sahara, the Northern Hemisphere was farther from the sun and the West African monsoon was weaker

There’s a feedback between vegetation, dust and rainfall, Tierney said. Right now the Sahara Desert is the planet’s biggest source of dust—but a vegetated Sahara would produce much less dust.

https://phys.org/news/2017-01-green-sahara-ancient-rainfall-regime.html#nRlv

Go Speed Racer on Sat, 31st Mar 2018 6:01 pm 



If Sahara Desert is going up 10% a year,
It’s a good rate of return.
how do I profit from that? Can I buy
shares on the NASDAQ?

We could greenify the Sahara.
We plant a great big lawn to turn the
Sahara green again. and move all
the Mexicans there to mow it.

.. on Mon, 2nd Apr 2018 8:47 am 



Increase the average global temperature by 20 c and you will not have any deserts but just rainforests