The Indian Ocean - Michael Pearson [19]
Apart from winds, there are also broader climatic changes which have substantially affected the Indian Ocean. Even something as apparently fixed and immutable as the sea level can change over time, true very long time, as a result of climatic change. Some 15,000 years ago the sea level was about 100 metres lower than it is at present, and even only 10,000 years ago it was still some 40 metres lower. The Gulf was more like a river than a sea channel. Australia and New Guinea were linked, and the passage from Sundaland to the north was only a short one, though a claim that one could go from one place to the other dryshod is an exaggeration.36 Between 8,000 and 6,000 years ago the sea rose dramatically, in some places by 100 or even 150 metres. Then change slowed or stopped completely. Since the maximum transgression of the middle of the fifth millennium BCE, sea levels overall have fallen by a global process known as eustatic adjustment, but not by enough to affect history very much, and not uniformly.37 At present we are witnessing what seems to be a new and very major change in sea levels, the first significant one for some 7,000 years. Low-lying Indian Ocean islands are threatened with being submerged as global warming raises sea levels comparatively precipitously.
Rainfall distribution could produce major consequences, another example then of a deep structural element impacting decisively on humans. We know something of the little ice age in the seventeenth century in Europe, but this seems to have been a worldwide event. Rainfall data from Java, based on tree rings in teak forests, show that the first three-quarters of the seventeenth century were very dry. The consequences could include drought, and so famine, as in India in the early 1630s, less flooding in the delta areas and so less fertile soils, and possibly a slight drop in the sea level.38
Three other deep structure matters affected travel by sea. First are ocean currents, which experienced sailors can 'read' and use to their advantage. Generally speaking, the earth's rotation along with winds means that it is in the western parts of the huge circulating gyres that currents are strongest. In other words, currents are more of a problem, or opportunity, off the East African coast than elsewhere. During the northeast monsoon, November to April, a weak counter-clockwise gyre produces a westward current that travels as fast as one knot. It hits the coast of Somalia and then turns south, and then east between 2 and 10° S. During the time of the southwest monsoon this current reverses, going east, and then north along the coast of Somalia, where it becomes the strong Somali Current. The situation below the monsoon zone is quite different. Here, south of 10° S, is a steady anti-cyclonic gyre, which means the South Equatorial Current flows west between 10 and 20° S, and divides at Madagascar. One arm goes north of Madagascar, and then south between Madagascar and Africa. The other branch goes south to the east of Madagascar and then curves back to the east towards South India. The first branch is known as the Lagullas or Agulhas current, and Marco Polo claimed that this meant Muslim sailors never went south of Madagascar, or even Zanzibar, because they thought the current meant there