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Early Civilizations Throughout humanity’s prehistory, Africa had no nation-states and was instead inhabited by groups of hunter-gatherers such as the Khoi and San. The domestication of cattle preceded agriculture. It is speculated that by 6,000 BCE, cattle were already domesticated in North Africa. In 4,000 BCE, climate change led to increasing desertification, which contributed to migrations of farming communities to the more tropical climate of West Africa. By the first millennium BCE, ironworking began in Northern Africa and quickly spread across the Sahara into the northern parts of sub-Saharan Africa. By 500 BCE, metalworking was fully established in many areas of East and West Africa. Copper objects from Egypt, North Africa, Nubia, and Ethiopia dating from around 500 BCE have been excavated in West Africa, suggesting that trans-Saharan trade networks had been established by this date.
One theory for the formation of the Sahara is that the monsoon in Northern Africa was weakened because of glaciation during the Quaternary period, starting two or three million years ago. Another theory is that the monsoon was weakened when the ancient Tethys Sea dried up during the Tortonian period around 7 million years.[24]
The climate of the Sahara has undergone enormous variations between wet and dry over the last few hundred thousand years,[25] believed to be caused by long-term changes in the North African climate cycle that alters the path of the North African Monsoon – usually southward. The cycle is caused by a 41000-year cycle in which the tilt of the earth changes between 22° and 24.5°.[21] At present (2000 ACE), we are in a dry period, but it is expected that the Sahara will become green again in 15000 years (17000 ACE). When the North African monsoon is at its strongest annual precipitation and subsequent vegetation in the Sahara region increase, resulting in conditions commonly referred to as the "green Sahara". For a relatively weak North African monsoon, the opposite is true, with decreased annual precipitation and less vegetation resulting in a phase of the Sahara climate cycle known as the "desert Sahara".[26]
During the last glacial period, the Sahara was much larger than it is today, extending south beyond its current boundaries.[29] The end of the glacial period brought more rain to the Sahara, from about 8000 BCE to 6000 BCE, perhaps because of low pressure areas over the collapsing ice sheets to the north.[30] Once the ice sheets were gone, the northern Sahara dried out. In the southern Sahara, the drying trend was initially counteracted by the monsoon, which brought rain further north than it does today. By around 4200 BCE, however, the monsoon retreated south to approximately where it is today,[31] leading to the gradual desertification of the Sahara.[32] The Sahara is now as dry as it was about 13,000 years ago.[25]
They found that the CO2 levels rose and fell on a regular cycle of around 2,500 - 3,000 yrs.
In July 2020, Air Products & Chemicals (“Air Products”), whose principal business is selling gases and chemicals for industrial use, announced plans to build a green hydrogen plant in Saudi Arabia. The plant will be powered by 4 GW of wind and solar power, making it the world's largest such project. The USD 5 billion plant will be jointly owned by Air Products, Saudi Arabia's ACWA Power, and Neom, a new mega-city planned near Saudi Arabia’s borders with Egypt and Jordan. Due to be operational in 2025 and situated in the city of Neom, the completed facility will produce 650 tons of green hydrogen daily, enough to run around 20,000 hydrogen-fuelled buses.