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E. Linacre and B. Geerts |
6/'98 |
A controversial article by R.G. Johnson (1) nicely illustrates the complexity of global warming, and therefore the difficulty of estimating its local effects. It also shows the relevance of oceans to climates.
The writer considered effects on climates caused by the outflow from the
Mediterranean Sea along the bottom, over the sill at Gibraltar. This outflow, known as the Mediterranean Outflow water (MOW), is thermohaline. In particular, it results from the density difference of Mediterranean bottom water compared with that in the Atlantic, the difference presently being enhanced (Note 11.B) by an increase of evaporation due to global warming and decreased freshwater inflow, mainly by the damming of the Nile. In fact, 90% of the Nile water that reached the Mediterranean before the Aswan dam was built is now lost in evaporation, either from irrigated fields or directly from the lake. Johnson suggested that greater salinity of the Mediterranean may indirectly cause glaciation in eastern Canada. This is his argument: greater flow of MOW would trigger a westward ocean current towards the Labrador Sea just south of Greenland, by the upwelling of the MOW east of Ireland and Scotland, deflecting the warm Gulf Stream westwards. A warmer Labrador Sea would result. It is already known that the Labrador Sea was relatively warm at the start of the last glaciation 120,000 years ago, presumably causing additional evaporation there and hence increased snowfall over eastern Canada. This could have enhanced the onset of the last Ice Age. On the other hand, the climate in northwestern Europe would be cooled by the deflection of the Gulf Stream. So the ice extends south on both sides of the ocean.Johnson hypothesized that during the
last Ice Age (Section 2.2) evaporation from the Mediterranean Sea was about the same as now, because of competing effects of higher wind speed and lower temperature, yet monsoon rainfalls in Africa were less, reducing the flow of the Nile. These effects then increased the salinity of the Mediterranean Sea, enlarging the MOW, and so on. Applied to the current situation, the paradox arises that global warming in the Mediterranean might tend to cool northern climates. Then the steeper gradient of temperature with latitude would affect the global circulation of winds, with widespread consequences on temperatures and rainfalls.Johnson concluded by considering the possibility of controlling events by building a dam across the Strait at Gibraltar, to throttle the MOW. This would have to be about 20 km long and as much as 250 metres deep, apart from a 1 km gap in the middle. The top of the dam would be below the sea surface, to allow low-salinity surface water to flow in from the Atlantic in response to the lower level of the Mediterranean surface. Such a dam would require international co-operation and funding on a huge scale, which, he suggested, would become harder to achieve if the project is postponed until times when tensions are increased by the eventual petroleum shortage.
The idea for a dam is not new. In 1933, German engineer Hermann Sorgel proposed to build
a continuous dam across the Strait of Gibraltar, then cut a canal in Egypt to flood parts of the Sahara which are below sea level.Various components of Johnson's hypothesis have not been proven, nor has it been shown that the various feedbacks will result in the suggested change in the real climate system. More recently, S. Rahmstorf (2) disproved Johnsons's hypothesis by means of an ocean circulation model: the modeled thermohaline and surface currents in the Atlantic were hardly affected by a change in the salt/water budgets in the Mediterranean resulting from the building of the Aswan dam. In summary, it is unwise to even contemplate a project as massive as a dam across an ocean strait without more in-depth studies (3).
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