Monday, September 18, 2023

When And Why Was The Sahara Green?

We've made lots of progress in understanding African Paleoclimates. 

The two images above are from the paper cited below. The image below is from Wikipedia (see also a list of notable climate events here and here).

There is widespread evidence that the Sahara was periodically vegetated in the past, with the proliferation of rivers, lakes and water-dependent animals such as hippos, before it became what is now desert. These North African Humid Periods may have been crucial in providing vegetated corridors out of Africa, allowing the dispersal of various species, including early humans, around the world.

The so-called ‘greenings’ are thought to have been driven by changes in Earth’s orbital conditions, specifically Earth’s orbital precession. Precession refers to how Earth wobbles on its axis, which influences seasonality (i.e. the seasonal contrast) over an approximate 21,000-year cycle. These changes in precession determine the amount of energy received by the Earth in different seasons, which in turn controls the strength of the African Monsoon and the spread of vegetation across this vast region.

A major barrier to understanding these events is that the majority of climate models have been unable to simulate the amplitude of these humid periods, so the specific mechanisms driving them have remained uncertain.

This study deployed a recently-developed climate model to simulate the North African Humid periods to greatly advance understanding of their driving mechanisms.

The results confirm the North African Humid Periods occurred every 21,000 years and were determined by changes in Earth’s orbital precession. This caused warmer summers in the Northern Hemisphere, which intensified the strength of the West African Monsoon system and increased Saharan precipitation, resulting in the spread of savannah-type vegetation across the desert.

The findings also show the humid periods did not occur during the ice ages, when there were large glacial ice sheets covering much of the high latitudes. This is because these vast ice sheets cooled the atmosphere and suppressed the tendency for the African monsoon system to expand. This highlights a major teleconnection between these distant regions, which may have restricted the dispersal of species, including humans, out of Africa during the glacial periods of the last 800,000 years.

From a Science Daily press release

The paper and its abstract are as follows:

The Sahara region has experienced periodic wet periods over the Quaternary and beyond. These North African Humid Periods (NAHPs) are astronomically paced by precession which controls the intensity of the African monsoon system. However, most climate models cannot reconcile the magnitude of these events and so the driving mechanisms remain poorly constrained. Here, we utilise a recently developed version of the HadCM3B coupled climate model that simulates 20 NAHPs over the past 800 kyr which have good agreement with NAHPs identified in proxy data
Our results show that precession determines NAHP pacing, but we identify that their amplitude is strongly linked to eccentricity via its control over ice sheet extent. During glacial periods, enhanced ice-albedo driven cooling suppresses NAHP amplitude at precession minima, when humid conditions would otherwise be expected
This highlights the importance of both precession and eccentricity, and the role of high latitude processes in determining the timing and amplitude of the NAHPs. This may have implications for the out of Africa dispersal of plants and animals throughout the Quaternary.
Edward Armstrong, Miikka Tallavaara, Peter O. Hopcroft, Paul J. Valdes, "North African humid periods over the past 800,000 years." 14(1) Nature Communications (2023) (open access) DOI: 10.1038/s41467-023-41219-4

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