Ice Ages

Introduction: We live in an era where even politicians have realised the significant threat climate change poses to civilization and indeed to all life on Earth, but it is nothing new and has been a major driving force of evolutionary change for hundreds of millions of years. One species that undoubtedly owes its very existence to climate change is Homo sapiens. It is no coincidence that the earliest human species, Homo habilis, emerged at just about the same time as Earth entered an ice age. The subsequent epochs of advancing and retreating ice sheets have played a pivotal role in the evolution and dispersal of subsequent human species, culminating in that of modern humans, Homo sapiens.

The Current Ice Age: We now know that the Earth has been affected by a number of ice ages in its history. The current Ice Age begun 2.6 million years ago and has been characterised by the advance and retreat of major glaciers and ice sheets in glacial periods punctuated by warmer interglacial periods. The last glacial period – or what is popularly known as the last Ice Age - began 110,000 years ago and ended with the onset of the Holocene epoch 11,600 years ago. The Ice Age is not in fact at an end and barring the effects of global warming, the glaciers and ice sheets will one day return.

The origins of the current Ice Age go back some 50 million years. Throughout this time the Earth’s climate has been cooling. Though the reasons are not fully understood, the collision of India with the Eurasian landmass (48-52 million years ago) and the migration of Antarctica to the South Pole (23 million years ago) are thought to be factors. From the first of these two events arose the mountains of the Himalaya Range, uplifted by the collision. The weathering of this new mountain range sequestered CO2 from the atmosphere, leading to global cooling. Subsequently the presence of a large landmass at the South Pole encouraged the build-up of ice. These ice-sheets reflected more of the Sun’s radiation back into space leading, in turn, to further cooling. The tipping point was reached 2.6 million years ago, with the expansion of ice sheets in the Northern Hemisphere.

Discovery: Perhaps surprisingly, the discovery that the northern glaciers had once been far more extensive, reaching as far south as London, New York and Berlin, is comparatively recent. Not until the 19th Century did geologists began to ponder such anomalies as bones of reindeer in the south of France and granite boulders high up on the slopes of the predominantly limestone Jura Mountains. The German-Swiss geologist Jean de Charpentier suspected that the boulders might have been deposited there by glaciers. He discussed the idea with his friend and fellow geologist Louis Agassiz, who took it up with great enthusiasm.

Meanwhile the naturalist Karl Friedrich Schimper, who was also a friend of Agassiz, was also of the opinion that ice sheets had once been far more extensive than now, and had once lain across much of Eurasia and North America. But he was a man who very rarely put his ideas into writing. He did however lend Agassiz his notes, but to his and de Charpentier’s considerable annoyance Agassiz subsequently took all the credit for the theory, which he put forward in a two-volume work entitled Etudes sur les glaciers (Study on Glaciers), published in 1840.

The main problem with the theory as it stood was that it offered no explanation for the cycles between glacial and interglacial periods. That these might have an astronomical cause was first suggested by Scottish scientist James Croll in 1860, who claimed that cyclical changes in the Earth’s orbit around the Sun might be responsible. Croll’s theory attracted considerable interest at the time, but had been more or less abandoned by the end of the 19th Century. The theory was revived and extended in the 1920s and 1930s by a Serbian engineer named Milutin Milanković.

The Milanković Pacemaker: The Earth’s seasons arise from its axis of spin being tilted rather than upright in relation to the plane of its orbit. When either the Northern or the Southern Hemisphere is tilted towards the Sun, it will experience summer as a result of both longer hours of daylight and the Sun being higher in the sky and more of its heat reaching the ground. The other hemisphere, meanwhile, will experience winter. For the Northern Hemisphere, the day with the longest period of daylight or Summer Solstice occurs on 21 June. The day with the shortest period of daylight or Winter Solstice occurs on 21 December. The spring and autumn equinoxes occur when the Earth is mid-way between the solstice positions, and everywhere receives 12 hours of daylight.

At the present time in the Northern Hemisphere, summers are hot enough to melt the whole of the previous winter’s accumulation of snow, but if this was not the case then the latter would gradually build up and ice sheets would advance into temperate latitudes. The enlarged ice sheets would then reflect more of the Sun’s radiation straight back into space, causing the cooling process to accelerate.

Milanković considered the possible effects of astronomical cycles on the intensity of the seasons, the amount of sunlight received (“insolation”) in the Northern Hemisphere and the possibility that at certain times the summers in the Northern Hemisphere might not be hot enough to prevent ice sheets from building up. He took into account three variables now known as the Milanković Cycles: precession of the equinoxes; variation of the axial tilt (“obliquity”) and changes in the shape of the Earth’s orbit around the Sun (“eccentricity”).

Precession is the long-term oscillation experienced by the Earth in which the spatial orientation of the axis changes with time. The phenomenon may be likened to the wobbling of a spinning-top or a gyroscope and is caused by caused by tidal effects of the Moon and Sun. A complete cycle takes 25,800 years. Precession affects the time of the year when the Earth is at is closest to the Sun (“perihelion”), which in turn will affect the intensity of the seasons. The picture is complicated by the precession of the orbit itself, with the perihelion slowly migrating around the Sun in a 105,000 year cycle. If these are combined with cyclical changes in the shape of the Earth’s orbit, a periodicity of 21,700 years is obtained for perihelion coinciding with summer in each hemisphere.

The Earth’s axial tilt is currently 23.5 degrees, but varies between 21.8 and 24.4 degrees over a period of 41,000 years. The seasons for both hemispheres will be exaggerated when the angle of tilt is high and moderated when it is low.

Finally the Earth’s orbit changes from near-circular (“low eccentricity”) to an ellipse (“high eccentricity”) with a major cycle of 400,000 years and a number of smaller cycles that average out at 100,000 years. At times of high eccentricity, the seasons are exaggerated in the hemisphere experiencing summer close to perihelion, and moderated in the other.

How these differing cycles combine to either exaggerate or moderate the seasons is of course very complicated, and Milanković spent many years laboriously performing the relevant calculations which – in an era before computers – all had to be carried out with the aid of a slide rule and books of tables. Unfortunately his dates for glacial periods did not tally with the then accepted values, and his theory fell out of favour. However in the late 1960s and early 1970s advances in methods for dating proxy evidence (indications of glacial periods) vindicated Milanković’s predictions and his theory gained widespread acceptance.

Although the duration of glacial periods is now seen to correspond closely to expectations, different cycles seem to have dominated at different times. Prior to 800,000 years ago, glacial periods followed the 41,000 year obliquity cycle, but subsequently the 100,000 year orbital eccentricity cycle has been dominant.

Effect upon Sea Levels: During glacial periods, significant amounts of water are locked up in ice sheets and sea levels fall. At the time of the Last Glacial Maximum (LGM), when the ice sheets reached their maximum extent, 20,000 years ago, sea levels were roughly 120 metres below their present-day level. Britain and Ireland were joined to continental Europe and the Indonesian islands as far east as Borneo and Bali were joined to mainland Asia as part of a subcontinental landmass known as Sundaland. Australia was connected to New Guinea and Tasmania and though it remained separate from Sundaland, the gap was small and could be crossed by humans living at that time.

Effect upon Climate: During the LGM, the climate throughout the world was cooler and dryer. The arid conditions were a consequence of so much water being locked up in ice sheets. In some parts of the world such as Southern Australia and the Sahel Belt south of the Sahara, rainfall dropped by up to 90 percent. Throughout the world deserts expanded and rainforest shrank.

During interglacial periods, the climate is warmer and wetter. In Africa, a weather phenomenon known as in Inter-Tropical Convergence Zone (ITCZ), which normally brings monsoons to the tropics, can extend its influence northwards. During such epochs, the Sahara experiences moist wet conditions and savannah climate. The last such climatic optimum was the Holocene Thermal Optimum, which began at the end of the last glacial period and peaked around 4000 BC. Subsequently, Milanković-determined insolation declined, the ITCZ returned southwards and the Sahara rapidly dried up.

References:

Bryson, B. (2003) A Short History of Nearly Everything, Doubleday.
Evans, E.P. (1887) The North American review, Volume 145, Issue 368, July 1887.
Klein, R. (1999) The Human Career (2nd Edition), University of Chicago Press.
Wilson, R.C.L., Drury S.A. and Chapman J.L. (2000) The Great Ice Age, Routledge.

© Christopher Seddon 2008