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But McGee says this interpretation of the sediment cores chafes against climate models, which show that Saharan climate should be driven by the region's monsoon season, the strength of which is determined by the tilt of the Earth's axis and the amount of sunlight that can fuel monsoons in the summer.
To get to the bottom of this contradiction, the researchers used their own techniques to analyze a sediment core obtained off the coast of West Africa by colleagues from the University of Bordeaux—which was drilled only a few kilometers from cores in which others had previously identified a ,year pattern.
The researchers, led by first author Charlotte Skonieczny, a former MIT postdoc and now a professor at Paris-Sud University, examined layers of sediment deposited over the last , years. They analyzed each layer for traces of dust and measured the concentrations of a rare isotope of thorium, to determine how rapidly dust was accumulating on the seafloor.
Thorium is produced at a constant rate in the ocean by very small amounts of radioactive uranium dissolved in seawater, and it quickly attaches itself to sinking sediments. As a result, scientists can use the concentration of thorium in the sediments to determine how quickly dust and other sediments were accumulating on the seafloor in the past: During times of slow accumulation, thorium is more concentrated, while at times of rapid accumulation, thorium is diluted.
The pattern that emerged was very different from what others had found in the same sediment cores. Once the researchers removed this confounding effect, they found that what emerged was primarily a new "beat," in which the Sahara vacillated between wet and dry climates every 20, years, in sync with the region's monsoon activity and the periodic tilting of the Earth.
Now we show that it's primarily these cyclic changes in Earth's orbit that have driven wet versus dry periods. It seems like such an impenetrable, inhospitable landscape, and yet it's come and gone many times, and shifted between grasslands and a much wetter environment, and back to dry climates, even over the last quarter million years.
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That said, there's geologic evidence from ocean sediments that these orbitally-paced Green Sahara events occur as far back as the Miocene epoch 23 million to 5 million years ago , including during periods when atmospheric carbon dioxide was similar to, and possibly higher, than today's levels. So, a future Green Sahara event is still highly likely in the distant future. Today's rising greenhouse gases could even have their own greening effect on the Sahara, though not to the degree of the orbital-forced changes, according to a March review published in the journal One Earth.
But this idea is far from certain, due to climate model limitations. Meanwhile, there is another way to turn parts of the Sahara into a green landscape; if massive solar and wind farms were installed there, rainfall could increase in the Sahara and its southern neighbor, the semiarid Sahel, according to a study published in the journal Science.
Wind and solar farms can increase heat and humidity in the areas around them, Live Science previously reported. An increase in precipitation, in turn, could lead vegetation growth, creating a positive feedback loop, the researchers of that study said. However, this huge undertaking has yet to be tested in the Sahara Desert, so until such a project gets funding, humans might have to wait until the year or longer to see whether the Sahara will turn green again.
Live Science. Image 1 of 6. Image 2 of 6. Image 3 of 6. Image 4 of 6. Image 5 of 6. Image 6 of 6. The team measured carbon and hydrogen atoms with different numbers of neutrons in their nuclei — called isotopes — in waxy substances produced by plant leaves. This leaf wax washed into the ocean and was preserved in ocean sediments laid down during the greening period. The number of neutrons in the atomic nuclei of carbon and hydrogen are valuable tracers of rainfall and vegetation changes.
Dr Pausata and his colleagues found that the green Sahara period was associated with very high rainfall rates recorded in the sediment cores. Their isotope data across all the sites indicated that the green Sahara may have been up to ten times wetter than today. The region had a seasonal tropical climate, with most of the rain arriving in the summer monsoon. Wet conditions extended as far north as Morocco, but persisted for a shorter time here than in the southern Sahara.
The team also discovered a drier interval in the middle of the Saharan greening around 8, years ago. Lasting about years, this pause in rainfall caused humans to temporarily abandon the region, as indicated by archaeological records. The abandonment of the southern Sahara coincides with a cultural shift among early humans. While those occupying the Sahara before the dry period survived by hunting, fishing and gathering, those who returned afterwards had a more diverse diet, subsisting partly on meat and dairy from cattle farming.
Dr Pausata and his colleagues suggest that the changing climate was a powerful motivation to abandon hunting and gathering, which are very vulnerable to environmental change, in favour of cattle herding. The team found that rainfall dramatically increased during the greening of the Sahara. However, mathematical models could not reproduce these high rainfall rates, nor could they accurately simulate the full northward extension of the Saharan greening inferred through sediment analysis.
The researchers set out to understand the complex feedbacks between Saharan desert dust, vegetation, and rainfall in enhancing the intensity of the African monsoon. By incorporating the reduction in dust emissions and the vegetation changes into a mathematical climate model, they could replicate the observed rainfall during the greening phase.
The absence of atmospheric dust allowed more solar radiation to reach the ground and be absorbed by a now darker vegetated soil, increasing the land-sea temperature gradient and strengthening the monsoon. The ENSO is a cyclic warming and cooling of the equatorial Pacific that influences temperature and precipitation worldwide. Using mathematical climate models, the researchers examined the impact of the Saharan greening on ENSO. They adjusted the models to account for the increased vegetation cover and reduced dust emissions in this period and found that modelling the Saharan greening reduced the predicted ENSO variation.
Such changes in the Atlantic could then impact the circulation in the equatorial Pacific, affecting the ENSO state and intensity. Models of future climate change suggest that the West African monsoon could strengthen again in the future, causing changes in vegetation cover and dust emissions in the Sahara and Sahel regions. Dr Pausata and his colleagues have shown that these changes could have a considerable effect on ENSO variability.
Their work shows that future climate change projections will have to consider potential changes in Saharan vegetation and dust. Tropical cyclones are among the most destructive weather phenomena on Earth, claiming thousands of lives globally each year. They are intense, rotating storms that form over warm seas.
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