![]() ![]() This increase could have escalated African monsoonal rainfall by 17% to 50%, according to a 1997 study published in the journal Science. Based on research first published in the journal Science in 1981, scholars estimate that the Northern Hemisphere had a 7% increase in solar radiation during the Green Sahara compared with now. That wobble is what positions the Northern Hemisphere closer to the sun in the summer - what researchers call a Northern Hemisphere summer insolation maximum - every 23,000 years. One such change is a "wobble" in the Earth's axis, he wrote. "The Earth's axial rotation is perturbed by gravitational interactions with the moon and the more massive planets that together induce periodic changes in the Earth's orbit," Peter de Menocal, the director at the Center for Climate and Life at Lamont-Doherty Earth Observatory at Columbia University in New York, wrote in Nature. As animals there prospered, humans did too, eventually domesticating buffalo and goats and even creating an early system of symbolic art in the region, NOAA reported.īut why did Earth's tilt change in the first place? To understand this monumental change, scientists have looked to Earth's neighbors in the solar system. This increased moisture transformed the formerly sandy Sahara into a grass and shrub-covered steppe, according to the National Oceanic and Atmospheric Administration (NOAA). (Usually, the wind blows from dry land toward the Atlantic, spreading dust that fertilizes the Amazon rainforest and builds beaches in the Caribbean, Live Science previously reported.) The increased heat over the Sahara created a low pressure system that ushered moisture from the Atlantic Ocean into the barren desert. The rise in solar radiation amplified the African monsoon, a seasonal wind shift over the region caused by temperature differences between the land and ocean. ![]() This led to an increase in solar radiation (in other words, heat) in Earth's Northern Hemisphere during the summer months. Notice how the Northern Hemisphere is closer to the sun during the winter months (right) than it is during the summer months. ![]()
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