April 03, 2018

Climate study shows Sahara getting bigger

But does it? The journal article is much more interesting than the lay article below and the abstract ends with an admission that generalizations are difficult and shaky in this area so maybe I should really just leave it at that. I append the journal abstract to the lay summary below.

The thing that bothers me is that the overall conclusions of this study appears to contradict what we have long heard about the Sahara -- namely that its green border -- the Sahel -- is expanding.  So why the reversal?

There is actually no reversal. The authors admit (highlighted in the rubric below) that the Sahel was expanding up until 2013 -- if you look at it in a long term way. So they are putting a lot of weight onto occasional fluctuations in a way that I frankly do not understand

So why has the Sahel gone into reverse recently?  No great mystery.  Many parts of Africa have been in drought in the last few years and the drought is not yet breaking.  I think we have all heard recently about Capetown's reticulated water supply running dry. So drought in the Sahel sounds very much like just another part of that.

But WHY is a lot of Africa in drought?  It cannot be because of global warming.  The rain would be pissing down if the oceans were warmer. It is almost certainly an El Nino effect.  The recent El Nino was a strong one and much more long-lasting than expected.  And the expected reversal in the form of a La Nina has yet to clearly emerge. But just why El Nino affected Africa so strongly I will have to leave to the climatologists.  It seems likely to me that it coincided with some other natural drying cycle within the African climate system and that Africa got a doubly whammy because of that


Earth’s largest hot desert, the Sahara, is getting bigger, a new study finds. It is advancing south into more tropical terrain in Sudan and Chad, turning green vegetation dry and soil once used for farming into barren ground in areas that can least afford to lose it.

Yet it is not just the spread of the Sahara that is frightening, researchers say. It’s the timing: It is happening during the African summer, when there is usually more rain. But the precipitation has dried up, allowing the boundaries of the desert to expand.

“If you have a hurricane come suddenly, it gets all the attention from the government and communities galvanize,” said Sumant Nigam, a professor of atmospheric and oceanic science at the University of Maryland and the senior author of the study. “The desert advance over a long period might capture many countries unawares. It’s not announced like a hurricane. It’s sort of creeping up on you.”

The study was published Thursday in the Journal of Climate. The authors said that while their research focused only on the Sahara, it suggests that climate changes also could be causing other hot deserts to expand – with potentially harsh economic and human consequences.

Deserts form in subtropical regions because of a global weather circulation called the Hadley cell. Warm air rises in the tropics near the equator, producing rain and thunderstorms. When the air hits the top of the atmosphere, it spreads north and south toward the poles. It does not sink back down until it is over the subtropics, but as it does, the air warms and dries out, creating deserts that are nearly devoid of rain.

“Climate change is likely to widen the Hadley circulation, causing northward advance of the subtropical deserts,” Nigam said.

Nigam and the study’s lead researcher, Natalie Thomas, a doctoral candidate at the University of Maryland, used data from the Global Precipitation Climatology Center to arrive at their finding.

SOURCE 

Twentieth-Century Climate Change over Africa: Seasonal Hydroclimate Trends and Sahara Desert Expansion

Natalie Thomas and Sumant Nigama

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.

SOURCE 







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