Observant visitors see the evidence all around them that the Middle East was not always so arid as it is today. Mighty canyons carved by raging rivers stand austere and bone-dry today. Bones of jungle predators and great grazing beasts of the grasslands lie buried in clay laid down by lakes and streams in lands where the infrequent raindrops now vanish instantly into windblown sand. Shifting winds uncover abundant stone-age tools in waterless deserts. And in many parts of the Middle East great mounds of man-made rubble attest to vanished cities where not even nomads pass today.
Middle Easterners ponder the forces that halved the population of Iraq, birthplace of the world's first cities more than 5,000 years ago, and of Syria, the breadbasket of the Roman Empire 2,000 years ago. Today, these forces are widening the swath of Sahara sand that separates the grasslands of Central Africa from the fertile coastal strip of North Africa. Is the contemporary highly visible desertification of the Middle East and Africa manmade, or an irresistible force of nature?
Two American scientists, David K. Jacobs of the American Museum of Natural History in New York and Dork L. Sahagian of Ohio State University in Columbus, maintain the answer can be found etched into coastal rocks all over the globe. Writing earlier this year in the scientific journal Nature, they point out that worldwide sea levels vary by as much as 25 feet roughly every 20,000 years.
These variations are not accounted for by ice ages, which do not occur so frequently or so regularly, and which can store enough water in glaciers to reduce ocean levels by up to 300 feet. The smaller and more frequent variations in sea level, the U.S. scientists believe, result from a known 20,000-year cycle of change in the earth's axis of rotation that tips the Northern Hemisphere, where most of the land is, toward the sun.
This slight tilt increases monsoonal rains. As the rains increase, much of the runoff fills lake basins and aquifers instead of draining to the sea. Enough water can be stored in the now dry Tarim basin of western China, Sahagian and Jacobs say, to lower the worldwide sea level by three and-a-half feet.
The Caspian Sea, between Iran and the former Soviet Union, can hold enough water in addition to its present volume to lower worldwide sea levels by one foot. Other known basins and aquifers can catch and hold enough rain to lower the world's seas by many more feet.
It is during such periods of heavier rainfall that much of Syria or the Sudan can produce rain-watered grain crops without irrigation, the Sahara can become savannah grassland, and the great river valleys of the Nile, the Indus and the Tigris and Euphrates can support dozens of cities where only villages stand today.
Then, as the northern hemisphere tilts away from the sun again and the rains slacken, lakes and aquifers begin to empty, the process of desiccation resumes, sea levels rise, and plants, animals and humans begin the migrations and adaptations to shifting rainfall belts that humanity has seen throughout its recorded history.
Much of the physical evidence of ancient climate changes that has vanished under the sands of the Arabian peninsula and other desert areas can be traced from earth satellites, according to Dr. Farouk El-Baz, Egyptian-born director of the Center for Remote Sensing at Boston University. Last March he outlined the course of a vanished river which once originated in the Hijaz mountains of western Saudi Arabia and flowed eastward for more than 500 miles to the Arabian Gulf.
There it formed a delta that is the site of much of present-day Kuwait, according to Dr. El-Baz. He cites as visible proof of his theory the granitic and basaltic gravel, unrelated to local rock formations, found in Kuwait today. This gravel, he said, appears to have been swept along by the river from the distant Hijaz mountains.
Because such hidden rivers frequently prove to be the source of underground water, Dr. El-Baz points out that this one could prove to be such a source for Saudi Arabia. He believes that what he calls the ''Kuwait River" flowed for some 6,000 years, reaching a width of three miles in places, during a relatively moist epoch in the earth's history. That moist period ended some 5,000 years ago, he said, so it is uncertain how much residue of the river may remain under its former bed.
Since most of its course is hidden by sand dunes running on a north-south orientation across central Saudi Arabia, the river's existence was detected only by close examination of satellite images. Because the nearby Nile and the Tigris and Euphrates rivers supported major human civilizations at the time the "Kuwait River" was flowing, Dr. El-Baz speculates, surveys of its course also may reveal archeological remains of important human settlements.