(7) Climate System
| 
| Hits: 2331
Dire ocean scenario warns of the effects of climate change
The great salt crisis
About 6 million years ago the gradual tectonic changes resulted in the closure of the Strait of Gibraltar. This led to the transient isolation of the Mediterranean Sea from the Atlantic Ocean. During this isolation the Mediterranean Sea dried out several times, creating vast evaporate (salt) deposits. Just imagine a huge version of the Dead Sea where a few meters of seawater cover a vast area. This event is called the Messinian Salinity Crisis and it was a global climate event because nearly 6 percent of all dissolved salts in the world’s oceans were removed. By 5.5 million years ago the Mediterranean Sea was completely isolated and was a salt desert. This was roughly the same time as palaeoclimate records indicate that the Northern Hemisphere was starting to glaciate.
But at about 5.3 million years ago the Strait of Gibraltar reopened, causing the Terminal Messinian Flood, also known as the Zanclean Flood or Zanclean Deluge. Scientists have envisaged an immense waterfall higher than today’s Angel Falls in Venezuela (979 m), and far more powerful than either the Iguazu Falls on the boundary between Argentina and Brazil or the Niagara Falls on the boundary between Canada and the USA. More recent studies of the underground structures at the Gibraltar Strait show that the flooding channel may have descended in a rather more gradual way to the dry Mediterranean. The flood could have occurred over months or a couple of years, but it meant that large quantities of dissolved salt were pumped back into the world’s oceans via the Mediterranean-Atlantic gateway. This stopped the Great Ice Age in its tracks and was entirely due to how oceans circulate. The Gulf Stream not only keeps Europe warm but also drives the deep-ocean circulation and keeps the whole planet relatively warm. Five million years ago the deep ocean circulation was not as strong as it is today. This is because fresher Pacific Ocean water was still able to leak through the Panama ocean gateway which is discussed below. So the sudden massive increase in salt due to the Terminal Messinian Flood increased the salt in the North Atlantic Ocean ensuring a very vigorous Gulf Stream and sinking water in the Nordic Seas. With all this tropical heat being efficiently pumped northwards the slide into any further great ice ages was halted about 5 million years ago. We had to wait another 2.5 million years before the global climate was ready to try again.
The Panama paradox
Another important tectonic control, which geologists believe to be a trigger for causing great ice ages is the closure of the Pacific–Caribbean gateway. Professors Gerald Haug and Ralf Tiedemann now at Zurich University and the Alfred Wegener Institute used evidence from ocean sediments to suggest that the Panama ocean gateway began to close 4.5 million years ago and finally closed about 2 million years ago. The closure of the Panama gateway, however, causes a paradox, as it would have both helped and hindered the start of the Great Ice Age. First, the reduced inflow of Pacific surface water to the Caribbean would have increased the salinity of the Caribbean, because the Pacific Ocean water is fresher than that of the North Atlantic Ocean. This would have increased the salinity of water carried northward by the Gulf Stream and North Atlantic Current, and as we have seen above this would have enhanced deep water formation. The increased strength of the Gulf Stream and deep water formation would have worked against the start of the Great Ice Age as it enhances the oceanic heat transport to the high latitudes, which would have worked against ice sheet formation. So after the aborted attempt to start the Great Ice Age about 5 million years ago, the progressive closure of the Panama ocean gateway continued to increase the heat transport northward keeping the chill at bay.
But here is the paradox: two things are needed to build large ice sheets-cold temperatures and lots of moisture. The enhanced Gulf Stream also pumped much more moisture northward, ready to stimulate the formation of ice sheets. This meant that the building of large ice sheets in the Northern Hemisphere could start at a warmer temperature because of all the extra moisture being pumped northward ready to fall as snow and to build up ice sheets.
Why 2.5 million years ago?
Tectonic forcing alone cannot explain the amazingly fast intensification of Northern Hemisphere glaciation. Work of mine using ocean sediments suggests that there were three main steps in the transition to the Great Ice Ages. The evidence is based on when rock fragments that had been ripped off the continent by ice were deposited in the adjacent ocean basin by icebergs. First, sheets started growing in the Eurasian Arctic and Northeast Asia regions approximately 2.74 million years ago, with some evidence of growth of the Northeast American ice sheet; second, an ice sheet started to build up on Alaska 2.70 million years ago; and, third, the biggest ice sheet of them all, on the Northeast American continent, reached its maximum size 2.54 million years ago. So in less than 200,000 years, we go from the warm, balmy conditions of the early Pliocene, which Professor Michael Sarnthein of Kiel University called ‘the golden age of climate’, to the Great Ice Ages.
The timing of the start of the intensification of Northern Hemisphere glaciation must have had another cause. It has been suggested that changes in orbital forcing (changes in the way the Earth spins around the sun) may have been an important mechanism contributing to the global cooling. The details of the Earth’s numerous wobbles and how they caused the waxing and waning of individual ice ages are discussed in the next chapter. But though these individual wobbles are on the scale of tens of thousands of years, there are much longer variations as well. For example one of the most important is obliquity or tilt, which is the wobble of the Earth’s axis of rotation up and down-or, put another way, the tilt of the Earth’s axis of rotation with respect to the plane of its orbit. Over a period of 41,000 years, the Earth’s axis of rotation will lean a little more towards the sun and then a little less. It’s not a large change, varying from between 21.8° and 24.4°. We explored before how the tilt of the axis of rotation gives us our seasons. Hence a larger tilt will result in a larger difference between summer and winter. Over a period of 1.25 million years the amplitude of the tilt changes. Both times the Earth tried to glaciate the Northern Hemisphere, at 5 million years and 2.5 million years ago, the variation of tilt increased to its largest value. This made the changes in each season very marked, most important were the cold summers in the north ensuring that the ice did not melt and could develop into ice sheets.
The tropics react to the ice ages
The onset of the intensification of Northern Hemisphere glaciation did not just affect the high latitudes. It seems half a million years after the start of the Great Ice Ages things changed in the tropics. Before 2 million years it seems there was a very slight east-west sea-surface temperature gradient in the Pacific Ocean, but this gradient later grew, showing a switch in the tropics and subtropics to a modern mode of circulation with relatively strong Walker Circulation and cool subtropical temperatures. The Walker Circulation is the atmospheric east-west component of the Hadley Cell and is instrumental in controlling rainfall in the tropics. The Walker Circulation is also a key element in the El Niño–Southern Oscillation (ENSO). So before 2 million years ago, ENSO may not have existed in its modern form because there was a relatively weak Walker Circulation. The development of the Walker Circulation also seems to be linked to early human evolution. The strengthening of the east-west circulation seems to have produced deep, fresh but ephemeral lakes in the East African Rift. It has recently been postulated that this distinctive climate pulse with rapid appearance and disappearance of lakes at about 2 million years may be linked with the evolution of Homo erectus in Africa with an over 80 per cent increase in brain size and our ancestors migrating out of Africa for the very first time.
Mid-Pleistocene Transition
Mid-Pleistocene Transition (MPT) is the marked prolongation and intensification of glacial-interglacial climate cycles that occurred sometime after 800,000 years ago. Prior to the MPT, the glacial-interglacial cycles seem to occur every 41,000 years, which corresponds to the slow changes in the tilt of the Earth. After about 800,000 years ago the glacial-interglacial cycles seem to be much longer, averaging over 100,000 years. The shape of these cycles also changes. Before the MPT the transition between glacial and interglacial periods is smooth and the world seemed to spend about equal time in each climate. After the MPT the cycles became saw-toothed with ice building up over 80,000 years to produce deep, intense ice ages and then rapid deglaciation, with the loss of all that ice within 4,000 years. The climate then stayed in an interglacial period resembling our current climate for about 10,000 years before descending back into an ice age. One suggestion for this saw-toothed pattern is that the much larger ice sheets are very unstable and therefore with a slight change in climate they collapsed rapidly and the whole climate system rebounded back into an interglacial period.
