Практикум. Why is the Sahara a desert.

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WHY IS THE SAHARA A DESERT? Fossil pollen, rock art, and other hints indicate that the Sahara was much greener 6000 years ago in the mid-Holocene period. Neolithic peoples seemed to have hastened desertification at the northern and southern edges of the Sahara, but German geophysicists believe the main causes were natural. They point to the fact that precession (wobble) in the Earth’s orbit causes changes in the timing of perihelion (closest Earth-Sun approach) and our planet’s rotational tilt. These combined to promote a milder climate in most regions of the mid-Holocene Northern Hemisphere. Since then the climate has become cooler and more arid. The subtle alterations in Northern Hemisphere cooling, however, were amplified by a feedback between atmosphere and vegetation causing climate change in the Sahara region to be far more drastic than elsewhere. Indeed what occurred was “the largest change in land cover during the last 6000 years,” according to Martin Claussen. He and his colleagues have now confirmed this hypothesis with computer modeling.

INFLUENCE OF COSMIC RAYS ON EARTH’S CLIMATE. Do small changes in solar activity translate into climate change on our planet? One possible linkage is the sun’s influence over the local flux of galactic cosmic rays (GCR); as the solar magnetic field gets stronger, fewer cosmic rays are able to penetrate to the inner solar system and Earth. And because the GCR are the biggest ionizer of air molecules in the lower atmosphere, they might play a role in processes like cloud formation. Henrik Svensmark, a physicist now at the Danish Meteorological Institute ahs studied the connection between GCR flux, solar activity, and climate on Earth. He finds that during the past 11-year solar cycle. Earth’s cloud cover was more closely correlated with the GCR flux than with other solar activity parameters, such as solar radiance, the main energy emitted by the sun. Svensmark concludes that climate seems to be influenced by solar activity via the GCR-cloud connection. In other words, climate is partly affected by processes in deep space.

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