is no doubt that the size of the Arctic cap decreases in summer and extends again in winter. In April 2007, the decrease in size, as seen from orbiting satellites, was almost equal to the surface area of the USA. By October, the annual freeze was under way and the size of the ice cap was fully restored by November (see Figure 5.1). There was little if any overall melt. The icebergs that form during the spring as the ice cap melts provide environmentalists with photo opportunities. They have also provided a shipping hazard for hundreds of years; one of which caused the Titanic to sink.
Figure 5.1: Sea ice extent in the Arctic (a) April 2007, (b) August 2007, (c) November 2007 (source: Bremen, ERSP1 and 2)
The melting ice is not causing the extinction of the majority of the populations of polar bears. These animals are carnivorous and will follow the food supply, not the ice. They are extremely good swimmers and photos that purport to show them isolated on a raft of ice may have been artificially manipulated; they are unlikely to be a result of a natural occurrence. In spite of about 50–100 bears being shot each year, there has actually been a net increase from an estimated 500 in 1950 to 1,500–2,000 in recent times. Their survival depends on the availability of food, rather than ice. They live happily in the warmer climbs of Regent’s Park Zoo in London. The threat to wildlife comes from man’s encroachment on their environment, not from changes in temperature of less than one degree in a hundred years. Similarly, although some Antarctic penguin colonies, especially those near human bases, have decreased in numbers, the overall numbers of penguins are steady or increasing. The colony in the Coulman Islands and Cape Washington have an estimated 20,000 fledgling or breeding pairs. This has not prevented the Worldwide Fund for Nature (WWF), an aggressive environmental body, from claiming they face extinction.
The actual reduction in the size of the Arctic ice really depends less on absolute temperature changes than on the relative lengths of winter and summer and on the amount of rain that falls in the winter. The effect of global warming has tended to reduce the duration of the winters rather than to warm up the summers. Evidence from satellite photographs indicates that, in the past decade or so, ice has often been more rapidly lost in summer than it has been replaced in winter (a process reversed in 2005–08, when the ice cap increased in size).
The summer of 2008 was the coldest in Anchorage, Alaska, for 40 years. However, the absolute significance of any change in the Arctic temperature will be known only after 50–100 years. It is recorded that the temperature in this region was higher than today for a short time in 1998 and this had little if any long-term effect.
The temperature of the southern hemisphere, measured by NASA satellites, suggests that, over the past 100 years, the surface temperature has increased by about 0.05ºC (see Figure 5.2).
Figure 5.2: Temperatures in the southern hemisphere (source: NASA)
Nevertheless there does seem to be some decrease in the overall size of the Arctic ice cap in the past hundred years. As four-fifths of the ice is under water, this melt has not produced a noticeable rise in sea level. Even if it melted entirely – a very unlikely proposition because it is a gigantic iceberg – it will not cause more than a few millimetres’ rise in sea level as all but its visible surface is already under water.
There would be a potential problem if cold water from the melting ice cap and Greenland pushed the Gulf Stream further south. It has been suggested that this may cool the seas around the British Isles, causing colder, dryer weather. There have been dire predictions, from the environmentalists, that northern Europe will become a frozen wasteland. There is no evidence to support this view; in fact, the Gulf Stream is not getting weaker and it is getting warmer, not colder. Buoys in the Atlantic do not indicate any change in the strength or direction of the Gulf Stream.
There is rather more cause for concern about the land-based ice covering most of Greenland. Although neither the land nor the sea temperature has changed dramatically in the past 100 years and land-based ice is probably increasing slowly, there is evidence of a loss of ice from edges of this landmass and an increase in the rate at which the ice melt from glaciers is being added to the seas around the coast. This is believed to be the effect of water seeping through cracks in the glaciers, called moulins, and acting as a lubricating slide under large masses of ice, easing their passage downhill towards the sea.
An alternative explanation suggests that the landmass under the ice is being warmed by geophysical heat from below the Earth’s thin crust, heat that cannot easily be dissipated due to the insulating effect of the overlying ice. It is known that Iceland sits on a tectonic ridge, which releases huge amounts of geothermal energy. It is a land of earthquakes and geysers. The significance of this ‘glacier shortening’ can be seen in perspective when one considers the evidence from isotopic studies of land-based organisms present in the Arctic seas. Although scientifically not as reliable as other studies, they do indicate that the increase in the rate at which glaciers have been melting started over 200 years ago, long before a rise in CO2 levels (see Figure 5.3). The ultimate significance of this effect will depend upon the balance between new ice formation and the rate at which the ice is lost into the sea. Present evidence suggests that this is not a new phenomenon and that it is not specifically linked to anthropogenic global warming.
Figure 5.3: The rate of glacier shortening has been constant since about 1800 (after Robinson et al. (2007), Journal of American Physicians and Surgeons 12, p. 79)
What about the Antarctic?
Unlike the Arctic, the Antarctic ice is mainly heaped up on land. It is not possible to traverse the Antarctic by submarine as it is in the Arctic. Frozen sea surrounds the landmass of the Antarctic. The extent of the frozen sea depends on the seasons, as the migrating penguins know well. Like any iceberg, this ice ledge is vulnerable to an increase in sea temperature and there is some secondary evidence that the sea in this region is getting slightly warmer. Over the past hundred years a large finger-like iceberg projecting north into the Southern Ocean, the Larsen B ice shelf, has progressively eroded and partially collapsed. Like the Larsen B shelf, the Wilkins shelf, which collapsed in 2008, was a gigantic iceberg jutting out from the Antarctic peninsular; it was believed to have been less than a thousand years old. These ice shelves are not land-based and were always in danger of disintegrating during warmer years as they extended further north from the coldest region around the South Pole, and are subject to the strong currents eroding their underwater base. Like all icebergs, they did not cause any recordable change in sea level when they collapsed and melted.
It is difficult to be certain of the balance between new ice formation during the Antarctic winter and its loss during the warmer summer months, but satellite observations suggest that much of the Antarctic ice, especially that covering mountains in the eastern Antarctic, is getting more extensive. In this region the ice is now up to 3.2 kilometres (2 miles) thick. This is probably due more to an increase in rainfall than to any change in temperature.
The story is more complex in the western Antarctic, where coastal ice appears to be subject to the same sliding motion as in Greenland. It is believed that this is an effect of the formation of rivers and lakes under the ice. The loss of ice from these coastal regions is slightly greater than the formation of new surface ice further inland, resulting in a small reduction in the total amount of ice in this region. It is probable that this part of Antarctica is relatively new and that it has been subject to changes in the past. Evidence from fossils found in ice-bore samples in this region suggests that much of Western Antarctica is geologically less than half a million years old, compared with the 20–30-million-year history of
