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https://www.sciencedaily.com/r.../03/170314111320.htm

New research investigating the transition of the Sahara from a lush, green landscape 10,000 years ago to the arid conditions found today, suggests that humans may have played an active role in its desertification.

The desertification of the Sahara has long been a target for scientists trying to understand climate and ecological tipping points. A new paper published in Frontiers in Earth Science by archeologist Dr. David Wright, from Seoul National University, challenges the conclusions of most studies done to date that point to changes in the Earth's orbit or natural changes in vegetation as the major driving forces.

"In East Asia there are long established theories of how Neolithic populations changed the landscape so profoundly that monsoons stopped penetrating so far inland," explains Wright, also noting in his paper that evidence of human-driven ecological and climatic change has been documented in Europe, North America and New Zealand. Wright believed that similar scenarios could also apply to the Sahara.

To test his hypothesis, Wright reviewed archaeological evidence documenting the first appearances of pastoralism across the Saharan region, and compared this with records showing the spread of scrub vegetation, an indicator of an ecological shift towards desert-like conditions. The findings confirmed his thoughts; beginning approximately 8,000 years ago in the regions surrounding the Nile River, pastoral communities began to appear and spread westward, in each case at the same time as an increase in scrub vegetation.

Growing agricultural addiction had a severe effect on the region's ecology. As more vegetation was removed by the introduction of livestock, it increased the albedo (the amount of sunlight that reflects off the earth's surface) of the land, which in turn influenced atmospheric conditions sufficiently to reduce monsoon rainfall. The weakening monsoons caused further desertification and vegetation loss, promoting a feedback loop which eventually spread over the entirety of the modern Sahara.

There is much work still to do to fill in the gaps, but Wright believes that a wealth of information lies hidden beneath the surface: "There were lakes everywhere in the Sahara at this time, and they will have the records of the changing vegetation. We need to drill down into these former lake beds to get the vegetation records, look at the archaeology, and see what people were doing there. It is very difficult to model the effect of vegetation on climate systems. It is our job as archaeologists and ecologists to go out and get the data, to help to make more sophisticated models."

Despite taking place several thousands of years ago, the implications of humans being responsible for environmental and climatic degradation are easy to see. With approximately 15% of the world's population living in desert regions, Wright stresses the importance of his findings: "the implications for how we change ecological systems have a direct impact on whether humans will be able to survive indefinitely in arid environments."

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Perhaps humans had an influence, but assuming correlation is always causation is a bad precedent in modern science. The only only correlation might be that both events happened at the same time and there was little or no impact from humanity. The Holocene Climatic Optimum (Warmest part of the interglacial we're in.) occurred at the same time and it could be climate change that changed the ecology of the region.

http://www.atmo.arizona.edu/st...s/sec5/holocene.html

Stanky posted:

Perhaps humans had an influence, but assuming correlation is always causation is a bad precedent in modern science. The only only correlation might be that both events happened at the same time and there was little or no impact from humanity. The Holocene Climatic Optimum (Warmest part of the interglacial we're in.) occurred at the same time and it could be climate change that changed the ecology of the region.

http://www.atmo.arizona.edu/st...s/sec5/holocene.html

Good point.  But Wright , an archaeologist, did not definitively stated that humans were the cause.  His hypothesis is that humans their livestock domestication were a possible cause.  The article goes on to say that additional data needs to be collected.

OldSalt posted:
Stanky posted:

Perhaps humans had an influence, but assuming correlation is always causation is a bad precedent in modern science. The only only correlation might be that both events happened at the same time and there was little or no impact from humanity. The Holocene Climatic Optimum (Warmest part of the interglacial we're in.) occurred at the same time and it could be climate change that changed the ecology of the region.

http://www.atmo.arizona.edu/st...s/sec5/holocene.html

Good point.  But Wright , an archaeologist, did not definitively stated that humans were the cause.  His hypothesis is that humans their livestock domestication were a possible cause.  The article goes on to say that additional data needs to be collected.

It is highly unlikely that humans have absolutely no impact on the world's climate, but I think the quantification of that impact could be overstated. As an example, just driving to town will show up to a 5 degrees F difference between country and city values and there is a lot of concrete and asphalt on the surface of the world concentrated in areas such as coastlines. I might also note that heading to town also takes me closer to official weather stations. 

Meteorologists and other researchers will sadly have a large non-laboratory study in South America on the effects of deforestation of the rain forests for pasture. Perhaps the human effect could be quantified, at least in South America. 

The ecology of north Africa will revert to a lush forest in the distant future as a natural part of climate change, I've read.  A more current theory is that the Muslim conquest altered the area from Egypt to Libya over several thousand years.  That area was the granary of Europe, from the time of the Roman conquest until around 700 AD.  Muslims conquered the area around 640 AD, bringing large flocks of goats that ate the grass down to the roots causing desertification.

Another theory for the desertification of the Sahara involves the orbit of the Earth:

Most scientists believe the Sahara dried up due to a change in the Earth’s orbit, which affects solar insolation, or the amount of electromagnetic energy the Earth receives from the Sun. Or to use simpler words, insolation refers to the amount of sunlight shining down on a particular area at a certain time. It depends on factors such as the geographic location, time of day, season, landscape and local weather.

Climate scientist Gavin Schmidt, of NASA’s Goddard Institute for Space Studies, explained that around 8,000 years ago, the Earth’s orbit was slightly different to how it is today. The tilt changed from around 24.1 degrees to the present-day 23.5 degrees.

“Additionally, the Earth had its closest approach to the Sun in the northern hemisphere (with) summer in August,” Schmidt said.

“Today, that closest approach is in January. So, summertime in the north was warmer back then than it is now.”

The changes in the Earth’s orbital tilt and precession (or the wobbling motion) occur because of gravitational forces emanating from other bodies in the solar system.

To understand exactly what happens, picture a spinning top when it is slightly disturbed. Just like a top, the Earth too wobbles slightly about its rotational axis. This tilt changes between roughly 22 and 25 degrees about every 41,000 years, while the precession varies on about a 26,000-year period. These cycles have been determined by astronomers and validated by geologists studying ocean sediment records.

“If you get a long enough time series that can be well dated, you should be able to see frequencies in the data that correspond to the periods predicted by theory,” Schmidt explained.

Although scientists agree that the Sahara was once a green place, it is still widely debated how the transition occurred.

http://www.messagetoeagle.com/...green-sahara-desert/


I might also note that the use of camels in the Sahara didn't happen until the later Roman era, sometime between 200 and 300 AD. Up until then it was possible for donkeys and horses to cross the Sahara

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