One lump or two?
ICE. At one extreme, it is the humble cube which puts the clink in your drink on a hot summer’s afternoon. At the other, it is a major cog in the engine which drives the world’s weather. Furthermore, its behaviour over the ages may have played a pivotal role in human evolution.
At the heart of ice’s significance to the planet is that it floats on water. The arrangement of hydrogen and oxygen molecules in ice is more spacious than it is in liquid water, hence ice is less dense and will float on water. In most other substances the solid form is denser than the liquid form and sinks through it.
That water expands when it freezes has a number of important consequences for life on Earth. Ponds, lakes, and oceans freeze from top down, rather than from the bottom up, allowing life forms such as fish, insects and algae to survive in the liquid water beneath the ice.
The phenomenon also has ramifications for erosion. Rainwater seeping into cracks in rocks, then expanding when it freezes, splits the rock, actively promoting erosion. This process plays an important role in revitalising the soil by adding minerals to it that are necessary for plant growth.
When it comes to changing the landscape, however, not much can compete with an advancing glacier as it sweeps soil away and grinds stones to dust. At the moment, most glaciers around the world are in retreat, but in the South Island the Franz Josef and Fox glaciers are still advancing, helped by cooler El Nino weather.
There are several reasons why El Ninio contributes to the glaciers’ advance. Most important is the fact that in El Nino years more snow tends to fall on top of the mountains where the glaciers start. Normally, most of the rain that falls on the West Coast of the South Island falls when temperatures are above average because it is brought by north-west winds. In summer, in particular, some of the north-west airstreams are warm enough so that even on top of the Alps it rains rather than snows.
But during El Nifio, with its predominant south-west winds, these warm rains are rare, and most, if not all, of the precipitation on top of the Southern Alps falls as snow.
Franz Josef Glacier has a very large area of snow feeding into a very narrow, steep valley. Consequently, it is very sensitive to small climate changes, and has a relatively fast response of about five years between increased snow at the top of the glacier and advance of ice at the terminus.
Another contributing factor in glacier advance is that melting at low altitudes may be significantly reduced during an El Nino. One of the best ways to melt a glacier is to surround it with warm air at near 100 per cent humidity, which is precisely what happens during normal summertime north-west rains. At such times some of the water vapour in the air condenses on to the glacial ice, but as it does so it releases heat, which melts some of the ice. Less warm rain means less melting.
Glaciers are complex entities, and are subject to other influences as well. In fine weather, the rocks of the valley walls heat up and then radiate heat on to the ice, again causing some melting. During an El Nino the predominant south-west winds produce more cloudy skies than normal during summer, west of the Alps.
The glaciers on the eastern side of the Alps, however, are retreating, and there are several reasons for this. First, these glaciers are still adjusting to a major warming trend which has been in force since the end of the last century. The adjustment is continuing because the eastern glaciers, such as the Tasman, are much slower-moving than the Franz Josef and Fox, since they travel down flatter valleys, and they have thick layers of stones and gravel on top of them, which act as insulation. Second, small lakes have formed at the snouts of these glaciers, causing much faster melting of the ice than would occur if the glacier terminated on shingle or bare rock.
For most of this century, the eastern glaciers have retreated up their valleys, leaving behind walls of shattered rock and shingle called moraines at their point of maximum advance. The moraines dam meltwater, creating the lakes. Franz Josef and Fox glaciers do not have moraine dams because their valleys are too steep, and the shingle they produce is washed straight away downstream.
The latest advance of Franz Josef Glacier is still a long way short of its position last century when it was several kilometres further down the valley. But that advance pales in comparison with its position in the last Ice Age, when it calved icebergs into the sea, which itself was more than 100 metres below today’s level, and 10 km west of the present coastline.
The fact that icebergs can form only on land was known to Captain Cook, so when he encountered icebergs floating far to the south of Africa in the summer of 1772/73 he was able to deduce the existence of the Antarctic landmass even further to the south.
Ever resourceful, Cook also used some of the smaller pieces of floating ice as a source of drinking water. Taken up in baskets, the ice was let stand on the deck until the salt water had drained off, and was then was melted down and stored in barrels.
One of the few icebergs to reach New Zealand waters during European times received similar treatment when it grounded in shallow water at the Chatham Islands in October 1892. A local resident despatched a rowboat to fetch some of the ice, which was then used to make a pot of tea.
Parochial anecdotes like this have value in tracing past climatic conditions, particularly in areas where instrument readings are sparse. But well documented encounters with teapots are not the only signs that icebergs have been in the neighbourhood. For one thing, icebergs carry small rock fragments embedded in them that were torn from their glacial beds as they slid over the land. As the bergs melt, they drop these tiny stones in trails across the sea-floor.
Cores of sea-floor sediments reveal periods in the past when these stones were plentiful, indicating episodes of iceberg shedding from the continental icesheets. Measurements taken from the North Atlantic seabed and published in 1997 in the journal Science indicate a cycle of cold climate shifts about every 1500 years. One of these cycles coincides with the prolonged cold spell that occurred during medieval times and was known as the Little Ice Age.
The 1500-year cycle has also been found in measurements of soluble impurities in ice cores from Greenland.
Ice cores can tell us about more than just weather. Changes of acidity in the annual ice rings have been used to track down ancient volcanic eruptions. The rings confirm, for instance, that there was a large eruption at the time of Julius Caesar’s death in 44 B.C., when, according to Pliny, the sky over Rome was partially obscured for nearly a year.
Shakespeare makes reference to the same cataclysm when he has Casca say to Cicero on the eve of Caesar’s assassination: never till tonight, never till now, Did I go through a tempest dropping fire. Either there is a civil strife in heaven, Or else the world, too saucy with the gods, Incenses them to send destruction.”
Though it melts away to nothing, ice can be enormously strong. Snow bridges allow people and machines to cross crevasses. Aircraft land on ice runways in Antarctica. Once, in the Napoleonic wars, a squadron of English cavalry captured a warship locked in ice.
Last summer, in Antarctica, a drill rig was erected on sea ice floating on the Ross Sea by a six-nation team including New Zealanders from Victoria University.
Although the project had to be cut short when the ice threatened to break up, some interesting finds were made. At a depth corresponding to 1.5 million years ago, the drill went through a metre-thick layer of sea shells, implying that the Antarctic Icesheet was much reduced at that time and temperatures much warmer than today.
Such warm intervals during the 2.5 million years of the Ice Ages may have been crucial to human evolution, as fossil evidence indicates that our brain size increased fourfold during this period.
The boom times that follow each retreat of the ice, when plant life and animals become more abundant in temperate latitudes such as Europe, could have propelled human evolution in several ways. First, the abundance of food would have allowed a greater variety of human races to survive. In times of plenty, survival of the fittest is replaced temporarily by the survival of most.
Boom times would also favour the breeding success of those individuals who reach sexual maturity earlier. Within a few generations, their descendants would outnumber those of late maturers. More than that, there is evidence that times of abundance actually induce early sexual maturity in many species. For example, during the 1982-83 El Nino, persistent rains in the Galapagos Islands caused trees to go through seven seeding cycles in one season. The finches that feed on these seeds began breeding at the age of three months instead of the normal two years.
Similarly, the ever-decreasing age of sexual maturity in girls in many Western societies has been attributed to plentiful food supplies.Early onset of sexual maturity also goes some way to explaining the changes that have occurred in human body shape over the period of the Ice Ages. These include larger head-to-body ratio, flatter faces and smaller teeth, all of which are characteristics of juveniles in many species.
When an animal reaches sexual maturity, body development slows down dramatically. If, due to environmental factors such as abundant food supply, that stage occurs in a young animal, its juvenile features are retained into adulthood.
It may very well be that the comings and goings of the ice have made us what we are today! However, if all this seems a bit much to take in as you rattle the ice cubes in the bottom of your glass, perhaps it’s time for another drink.