Source: Race to Save the Planet: "Only One Atmosphere"
As permanent as geographic features such as deserts may seem, Earth's climate and landscape are dynamic and constantly changing. Geologic evidence has revealed that parts of the Sahara were once lush and filled with animal species more often associated with the savanna far to the south. This video segment adapted from NOVA explains how Earth's position relative to the Sun might be responsible for the dramatic shift in the climate of what is now the Saharan nation of Djibouti.
Evidence from ice cores and ocean sediments suggests that Earth's climate has fluctuated many times during our planet's long history. Some of the minor climatic changes observed in climate records have resulted from variations in solar activity or from volcanic activity that blocked the Sun's radiation with clouds of dust and gas. However scientists attribute the cyclic record of alternating global cool and warm periods to three types of periodic variations in the position of Earth relative to the Sun.
One such variation is the shape of Earth's orbit around the Sun, which fluctuates from being nearly circular to slightly elliptical and back over the course of about 100,000 years. In an elliptical orbit, Earth passes closer to the Sun and absorbs slightly more solar radiation during certain times of the year than it does during other times.
Another variation in Earth's position relative to the Sun is the angle of Earth's axis. Earth tilts on average about 23.5° away from its plane of orbit around the Sun. This results in the seasonal changes we observe each year, whereby the northern hemisphere receives more solar radiation than the southern hemisphere for nearly half of the planet's 365.25-day orbit around the Sun, and the southern hemisphere receives more radiation during the other half of the year. However, the angle of Earth's tilt fluctuates from about 22.1° to about 24.5° and back over the course of about 41,000 years. A low degree of tilt causes temperature differences between seasons to be relatively small, and a high degree of tilt causes temperature differences to be relatively large.
The orientation of Earth's axis also varies cyclically, over the course of about 26,000 years. This is known as precession, somewhat analogous to the motion of a spinning top as it begins to slow. One effect of precession is that the pole stars change over this time period - Polaris, for example, will not always be the North Star. The orientation of Earth's axis has little impact on climate when the planet's orbit is nearly circular. However, when Earth's orbit is more elliptical, seasonal contrast is increased in one hemisphere and decreased in another.
Climate experts now think that increased seasonal contrast resulting from the interaction of these three orbital cycles caused warmer summer temperatures over northern Africa and present day Djibouti several thousand years ago. The warmer temperatures increased heating of the African continent, warming the air, which rose and drew additional moisture out of the Atlantic Ocean. The extra moisture strengthened the monsoon, extending it over the southern edge of the Sahara. Although the desert still existed, its southern border moved northward, making it significantly smaller. People and animals living in Djibouti enjoyed conditions more like those currently seen in sub-Saharan Africa.
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