|
|
|
The Future... The polar ice caps have melted, drowning all of the world's lands under the ocean. The few remaining humans live their whole life at sea, in search of the legendary "dryland"... This is the premise for the recent hit film "Water World" staring Kevin Kostner. The scenario tells us a lot about the environmental concerns and fears of today's society, but could it really happen? Henry Ritson reports. As we explained in the November AA&ES, global temperature has been rising throughout the past century, so is the same true of sea level? The answer appears to yes, but it has been rather difficult to gather the data to prove this. Tidal height records have been kept in some areas of the world for much of this century, but the data are often sporadic and only cover certain areas of the globe. This is problematic because it is important to separate local trends in relative sea level caused by movements in the level of the land relative to the sea (isostatic changes), from global changes in true sea level (eustatic changes). For example, tide gauge records in the south of England show that the sea level has been rising quite rapidly, hence the need to build the Thames Barrier to prevent flooding. However, the main reason for this is that the whole south of England is currently slowly sinking as a delayed response to the changes in pressure on the land when the glaciers which used to weigh down the north of the UK melted 10000 years ago. It does not necessarily mean that the sea level is rising over the whole world. That said, a study published by the Inter-governmental Panel for Climate Change (IPCC) in 1992 concluded that sea level had in fact been rising at a rate of about 1 to 2 mm per year during the past century and that this trend does not appear to be slowing.
We don't stand a chance of predicting the future situation without first understanding in great depth the way that a global temperature change would interact with environmental systems, in particular the oceans and ice caps. The problem is that all to often these relationships are rather complex and, indeed, even counter-intuitive.For example, the media often represent the threat of sea level rise as follows:
"If the global temperature rises, the polar ice caps will melt more. This water will flow into the sea resulting in a sea level rise, flooding the land."However, a deeper investigation shows that this is rather intriguingly not the case. Firstly, it is important to note that the north and south poles are very different. Antarctica in the south is a true continent of land, largely covered in ice which extends in some areas out into the sea where it forms ice shelves, some areas of which are grounded on the sea bed and some areas of which are floating. In essence though, the south pole is land with ice on it. By contrast the northern polar region mostly consists of the arctic ocean, which has an ice cover several metres thick of frozen sea water (the extent of which varies seasonally) Therefore with the notable exception of the Greenland ice cap, the bulk of the north pole ice is floating in the sea.Now, if the global temperature rose, as a general pattern how would these northern and southern areas respond? The first point to note is that if the ice cover on the arctic ocean melts, it does not really affect sea level much. This is because the ice is already in the ocean and so, through displacing the water it is in, has effectively already made the sea rise as much as it can. If the ice melts it will change from a solid to a liquid but not have that much effect on the sea level. On the other hand if the southern Antarctic ice cap were to melt, the effects on sea level would be huge since water which was formerly stored on the land as ice would flow into the sea and so increase the total volume of water stored in the worlds oceans. However, there are certain characteristics of the Antarctic environmental system which make it highly unlikely that the ice would melt, starting with the fact that it is very, very cold indeed...
Contrary to popular belief it actually hardly ever snows in most of Antarctica. It is so far below zero degrees for such a large area that the atmosphere cannot hold much moisture, the air is very dry, and precipitation does not occur very often (hence the phrase "its too cold to snow"). In fact under some classifications based on average annual precipitation , most of Antarctica is technically desert! The only reason that there is so much snow around is that it is so persistently cold that any snow that does fall effectively never melts, so it just keeps building up.
Now, since most of Antarctica is permanently a few tens of degrees Celsius below zero, the temperature can effectively rise as much as it likes and Antarctica will still be sub-zero (at least in as much as we'd probably all die of the heat before it started to effect sea level!). Certainly even the most extreme suggestions of possible temperature rises would not seem to be enough to melt the Antarctic. However, what could well happen is that an increase in temperature in the southern oceans could result in greater evaporation, more moisture in the atmosphere and ultimately more snow falling on the Antarctic. Therefore we are left with a situation in which a temperature rise would actually take water out of the oceans and put it on the land, thus contributing to a sea level decrease!
So if the melting of the Arctic Ocean would have little effect on sea level and a temperature increase in the Antarctic would actually tend to lower sea level, are we saying that the sea level is not going to rise? Well, not exactly - the point of this was to show that matters are not as simple as "Hotter = Melting = Sea Level Rise". There are other reasons why we are probably going to have to face the prospect of an increasingly watery world.
Firstly there is the issue of water held in glaciers and small ice caps. As shown last month, valley glaciers have been steadily retreating during the 20th Century and, though rather less water is held in them than in large areas like the Antarctic, they can still contribute to sea level rise as they melt. For example, Meier has calculated that from 1900 to 1961 glacier retreat has contributed 2.8cm to sea level rise. It seems likely that glacier melting would contribute positively to sea level in the event of a continued warming. Also the relatively large Greenland ice-cap could start to melt more significantly, thus raising sea level. However, one of the most important factors is likely to be the thermal expansion of the oceans. When a fluid is heated it expands and becomes less dense. This is why warm air rises and cool air sinks. The main reason that a global temperature rise would induce a sea level rise is therefore that the water in the oceans will expand.
In summary, the IPPC concluded in 1992 that
"Most of the contribution [to sea level rise] is estimated to derive from thermal expansion of oceans and the increased melting of mountain glaciers and small ice caps... On the decadal time scale the role of the polar ice sheets is expected to be minor, but they contribute substantially to the uncertainty. Antarctica is expected to contribute negatively to sea level due to increased accumulation."It is apparent that there are mechanisms which mean that any further global warming would raise sea levels, but by how much? The truth is that the scope of the system is so large, with so many possible feedbacks and interactions that it as way beyond the scope of the human brain to work this out. The answer lies, at least partly with computers.These bold issues of environmental futurology can only reasonably be even attempted with the assistance of massive computing power to integrate all the possible factors. One therefore usually constructs a virtual model of the environment within a powerful computer and set it off, calculating the result.
However, one must always be cautious of the results obtained: All too often the results of a computer model are treated with an unreasonable amount of faith on the basis that "computers are very accurate so their results must be accurate". Unfortunately a computer can also highly accurately produce entirely the wrong result if it is programmed to do the wrong thing. There is a principal which very elegantly describes this. It is GIGO - i.e. Garbage In, Garbage Out! Ultimately, a computer never actually 'knows' any more than what it is told by the people who programme it - it is merely able to do big sums using that information. If the data being fed into the model are inaccurate, or if the researchers are unaware of some important aspect of the system then the results of the model can be rendered fairly irrelevant. That said, some of the more comprehensive models produced recently seem to provide the best estimate we yet have for potential sea level rise. As a result of this type of work the IPPC stated that they envisaged that by the year 2030 global sea level would be 8 to 29cm higher with a best estimate of 18cm. Furthermore, they stated that by 2070 sea level would be 21 to 71cm higher with a best estimate of 44cm. The models suggested that an increase of over 1m seemed highly unlikely during the next century.
These predictions were based on a 'business as usual' scenario which assumes that anthropogenic Carbon Dioxide emission levels will simply remain at today's levels. Rather disturbingly, it has also been calculated that even if all human CO2 emissions were stopped today we would already be committed to 18.5cm sea level rise by 2100 due to lags in the system. It would appear that there is nothing much we can do to avoid this much change. Of course in actual fact greenhouse gas emissions are probably more likely to rise than fall.
So, with sea level rises of significantly under a metre on the cards, the 'Water World' scenario of total global flooding obviously lies well into the realms of fantasy. Indeed, even if all the world's ice melted we would still not have enough water to cover all the world's land. But this does not mean that we are not in any trouble. For many areas any notable rapid rise will be very big trouble indeed.
For example, The Maldives, and other low island states, are very worried about the predicted increases. The Maldives are coral islands in the Indian Ocean which are effectively entirely at sea level since the coral that forms them grows up to sea level and not above. Since coral can grow at 3mm per year or more, the islands should be able to effectively grow up with the sea level rise. However pollution and mining of the reef has left most of the coral dead, thus removing the land's natural protection from the rising waters. The capital, Male, is already suffering severe flooding during storms and there is a strong possibility that that entire nation could sink beneath the waves within this generation.
As well as island states, countries with large low lying delta areas are also particularly vulnerable. Bangladesh for example has vast tracts of land at or below: Milliman et al (1989) and Broadus (1992) postulated an inundation of up to one third of the country due to 1m and 3m rises in relative sea level. However, again we are faced with major difficulties in prediction since the coast responds dynamically to sea level change with more sediment accreting, raising the natural coastal defenses.
Over large time scales, sea level fluctuations, even of the order of hundreds of meters are perfectly normal ( we are currently at the higher end of these natural cycles and fluctuations ) so does it matter at all that sea level may rise during the next century? Well, what is different about the current situation is firstly that any changes will have major effects on human populations, so are of rather more concern to us, and secondly that the changes are probably being driven by human agency. This means both that we are in some way responsible for the effects, and that, most importantly, we have the potential power to control the situation. A Water World of perpetual ocean may be in the realms of fantasy, but an environmental crises of unprecedented proportions is unnervingly possible during the next century. The more we learn, the more we find out how little we know, but one can only hope that we are soon able enough to understand the way our environment acts to anticipate, prevent and adapt to the changes that may lie ahead.
© Copyright Henry Ritson. 1995
First published in Amateur Astronomy & Earth Sciences, December 1995.
