Most of the northern half of Africa is occupied by a broad ocean of trackless, burning, virtually lifeless sand that has been known for centuries as the Sahara Desert, but it was not always so.
There was a time when the Sahara was a hospitable spot, whose flowing rivers teemed with fish and whose grassy valleys were able to sustain a wide variety of animal and human life. Such transformations depend largely on the migratory whims of the Inter-tropical Convergence Zone.
The ITCZ for short is a circumglobal band of relatively low pressure in the general vicinity of the Equator.
It is the zone in which the trade winds meet - the north-east trades from north of the Equator and the corresponding south-east trades which blow north-eastwards from the southern hemisphere.
Since this converging air must escape to somewhere, it ascends and with ascent comes cooling and then condensation, so the ITCZ is characterised by large cumulonimbus clouds, line-squalls, thunder storms and a plentiful supply of rain. North and south of the roughly equatorial ITCZ are zones of high pressure, descending air and generally dry conditions.
The ITCZ moves slightly north and south with the changing seasons of the year, but viewed in a timeframe stretching over several centuries its average position changes very little: it sits over Africa, some distance to the south of the Sahara.
When looked at over the millennia, as ice ages alternate with interglacial periods, the ITCZ shifts north and south by quite significant amounts. For example, 18,000 years ago the ITCZ was in much the same position as it is now and the Sahara was, if anything, more arid than we see it. By 5000 BC, however, the average position of the ITCZ was over northern Africa and provided life-giving rain to the Saharan region.
That which is now a desert had abundant lakes and rivers and tracts of vegetation with a thriving animal life; Stone Age man could settle in the area, hunt and fish and establish a cereal economy. However, 4,000 years ago the waters of the Sahara began to disappear.
The ITCZ had moved back again towards the Equator, rainfall all but ceased, large animals died out and the fragile ecosystem changed dramatically.
By the time of the New Testament, the north African coastal strip was still the bread basket of the Roman Empire, but rainfall had become troublesomely irregular and overuse of the soil was accelerating natural trends, and gradually the Sahara returned to what it is today.