NEW DISCOVERIES hance monsoon strength , with both local and global impacts .
Earth ’ s changing orbit
The fact that the wetter periods in north Africa have recurred every 21,000 years or so is a big clue about what causes them : variations in Earth ’ s orbit . Due to gravitational influences from the moon and other planets in our solar system , the orbit of the Earth around the sun is not constant . It has cyclic variations on multi-thousand year timescales . These orbital cycles are termed Milankovitch cycles ; they influence the amount of energy the Earth receives from the sun .
On 100,000-year cycles , the shape of Earth ’ s orbit ( or eccentricity ) shifts between circular and oval , and on 41,000 year cycles the tilt of Earth ’ s axis varies ( termed obliquity ). Eccentricity and obliquity cycles are responsible for driving the ice ages of the past 2.4 million years .
The third Milankovitch cycle is precession . This concerns Earth ’ s wobble on its axis , which varies on a 21,000 year timescale . The similarity between the precession cycle and the timing of the humid periods indicates that precession is their dominant driver . Precession influences seasonal contrasts , increasing them in one hemisphere and reducing them in another . During warmer Northern Hemisphere summers , a consequent increase in north African summer rainfall would have initiated a humid phase , resulting in the spread of vegetation across the region .
Eccentricity and the ice sheets
In our study we also identified that the humid periods did not occur during the ice ages , when large glacial ice sheets covered much of the polar regions . This is because these vast ice sheets cooled the atmosphere . The cooling countered the influence of precession and suppressed the expansion of the African monsoon system .
The ice ages are driven by the eccentricity cycle , which determines how circular Earth ’ s orbit is around the sun . So our findings show that eccentricity indirectly influences the magnitude of the humid periods via its influence on the ice sheets . This highlights , for the first time , a major connection between these distant high latitude and tropical regions .
The Sahara acts as a gate . It controls the dispersal of species between north and sub-Saharan Africa , and in and out of the continent . The gate was open when the Sahara was green and closed when deserts prevailed .
Our results reveal the sensitivity of this gate to Earth ’ s orbit around the sun . They also show that high latitude ice sheets may have restricted the dispersal of species during the glacial periods of the last 800,000 years .
Our ability to model the African humid periods helps us understand the alternation of humid and arid phases . This had major consequences for the dispersal and evolution of species , including humans , within and out of Africa .
Furthermore , it provides a tool for understanding future greening in response to climate change and its environmental impact .
Refined models may , in the future , be able to identify how climate warming will influence rainfall and vegetation in the Sahara region , and the wider implications for society .
Figure 2 . The Sahara desert . Getty Images
Grassroots Vol 24 No 1 March 2024 28