Restless Environment - Fluctuating Fortunes of the Gulf
Fluctuating Fortunes of the Gulf
On the coastline of Abu Dhabi during summer with blistering humidity and temperatures soaring into the upper forty degrees Celsius; the Gulf's warm waters gently lapping over beaches, the idea of an ‘Ice Age' seems remote. Indeed, even at the peak of the last Ice Ages, the sheets of ice that covered much of the northern hemisphere never reached as far south as the Gulf. Yet, as unlikely as it may seem, it is precisely such episodes of global cooling over geological time that have helped shaped the physical geography of the entire region, also influencing the lifestyle, and fate, of the early human inhabitants of the Gulf. This attractive and bountiful sea, supporting a range of ecosystems and rich in natural resources is an integral part not only of the Emirate's geographic inheritance but also of its cultural heritage over tens of thousands of years. The Gulf today may appear to be timeless, stable and enduring. In fact, it is the product of radical changes and convulsions over geological time and, indeed, its shorelines and islands have not only changed dramatically over the last few thousand years, but continue to change. The history and evolution of this waterway, so important to the Emirate of Abu Dhabi, provides evidence of the restless nature of the environment, not only in the distant and more recent past but, equally, in the present and certainly into the future.
The Arabian Gulf is the second youngest sea in the world with flooding of the basin commencing only between 14,000 and 12,000 years ago, although there is evidence from dating of sand dunes that the Gulf was also a sea during at least three previous interglacial periods when sea levels peaked at around 120,000, 200,000 and 330,000 years ago. Over recent geological time, the Gulf has experienced huge transitions; accommodating desert environments, ancient river valleys and fluctuating sea levels. Sea levels at the last glacial maximum (about 19,000 to 20,000 years ago) were between 120–130 metres below today's levels. The Gulf's exposed floor comprised a gently northwards dipping plain with desert conditions with sand dunes marching southwards driven by the strong shamal winds. As the Tigris Euphrates river system wound through the Gulf basin, it would have passed through a series of lakes. From the northern Gulf, it flowed south-eastward and, upon reaching Qatar, its course shifted closer to the Iranian side of the Gulf, before eventually flowing out into the Gulf of Oman through the Strait of Hormuz. About 14,000 years ago, the Earth's climate began to warm markedly. The glaciers and ice sheets melted rapidly, raising global sea levels. Seawater once again flooded into the Gulf, peaking between 4,000–5,000 years ago at 1–2 metres above today's sea level before dropping to present levels. Between 9,000–5,000 years ago, a period known as the ‘Climatic Optimum', Arabia experienced a dramatic change in climate, with warmer wetter conditions and more gentle winds. Lakes became a common feature of the landscape. Decreasing rainfall, from around 6,000 years ago, saw a return to arid conditions.
The Gulf – Present and Future
The Arabian Gulf occupies a basin shaped like the traditional Arabic khanjar (dagger). This shallow sea is almost totally landlocked by the Zagros Mountains of Iran to the north and east and the Arabian Peninsula to the south and west. It has a total area of 227,000 square kilometres and extends for 1,000 kilometres from the 46 kilometre wide Strait of Hormuz in the east to the Shatt al Arab delta in the northwest. It is 360 kilometres across at its widest point and has an average depth of 35 metres, rarely exceeding 100 metres. The depth gradually increases towards the northeast where a series of near-coastal basins known as the Zagros fore-deep have formed due to crustal loading imposed by the weight of the Zagros Mountains.
Evaporation of water is high, between 144–500 centimetres per year and in shallower waters along the Abu Dhabi coastline this can exceed 2,000 centimetres per year. The only significant supply of freshwater comes from major rivers such as the Tigris, Euphrates and Karun. Marine water enters the Gulf through the Strait of Hormuz and travels in a broadly counter clockwise direction around the basin. As these marine waters travel westwards along the northern shores, the high temperatures and dry winds increase rates of evaporation, thereby increasing salinity. At the head of the Gulf, the input of freshwater partially restores salinity towards more normal levels. However, as the water flow turns south and eastwards along the southern shores, evaporation once again produces high salinities. It takes around two-and-a-half years for the water circulation to complete this cycle. Higher water temperatures and significantly rising sea levels are serious challenges for the future.
Ice on the Globe
Between 710–640 million years ago (during the later part of the Proterozoic eon) the Earth cooled sharply. Winter snows did not melt but compacted into ice. Polar ice caps grew, glaciers advanced and ocean surfaces froze. Scientists call this episode ‘Snowball Earth'. These were not the first Ice Ages, nor the last. Although Ice Ages may last for tens of millions of years, they are not uniform and can have periods of severe cooling, called glacials, and shorter intervals with more temperate conditions, or interglacials. Today, the Earth is in an interglacial known as the Holocene, which began over 10,000 years ago. These episodic glacial and interglacial periods contributed to the rise and fall of sea levels in the Gulf that have, in turn contributed to the formation of the region's modern landscape.
Palaeoclimates and Culture
The Gulf region has experienced dramatic swings in climate and consequent changes in sea level over the last 200,000 years. The changes in sea level that have taken place during recent geological time (during the Quaternary period, which began around 1.8million years ago) have profoundly affected the Gulf's physical landscape. These have coincided with the arrival of humans in Arabia, and have had a major impact on the development of the region's cultural heritage.