Background Information: The Environmental Impacts of Agents for Change
What Has Been Happening Over the Past Century?
Here we can refer to the Second Assessment Report of the Intergoverrunental Panel on Climate Change approved after detailed scrutiny at its Rome Meeting in December 1995 and published a few months later by the Cambridge University Press in the United Kingdom.
Despite significant variations from year to year:
- Global mean surface temperature has increased through 0.3 to 0.6 degrees Celsius.
- Recent years comprise the warmest spells since 1960 and this is so despite the fact that the period includes the 1991 Mount Pinatubo volcanic event which caused a short period cooling effect.
- Nighttime temperatures over land have tended to increase more than daytime temperatures.
- The recent warming has been greatest over the mid-latitude continents particularly in winter and spring. The North Atlantic has shown a tendency to escape the general trend and has experienced some cooling, and precipitation has increased over land in the high latitudes of the northern hemisphere during winter.
- Global sea level has risen between 10 and 25 centimeters in the past century and the rise can be related to the increase in mean temperature world wide.
- The last decade has shown more persistent and unusual warm phases of the El Niño Southern Oscillation [ENSO] associated with droughts and also floods.
- The general consensus suggests that, on balance, there is a discernible influence on climate arising from human activity.
- Climate is expected to continue to change through ensuing years.
What is expected to happen in the near future?
The best estimate of global warming is that relative to 1990 conditions, global mean surface temperatures are likely to increase by approximately 2 degrees Celsius by the year 2100, slightly down on earlier estimates. Uncertainty of climate sensitivity, however, sees this figure as a best estimate. A low value of climate sensitivity gives a projected increase of 1 degree Celsius by 2100 while for the highest scenario suggested by the IPCC, the projection gives a warming of 3.5 degrees Celsius. These values may seem to be quite small but nevertheless they indicate a rate of warming probably greater than any seen in the last 10 000 years.
The climate models expect sea level to rise by approximately 50 centimeters from the present to 2100, again 25 percent lower than earlier 'best estimates', but this figure ties between 15 centimeters for estimates based upon low sensitivities, to 95 centimeters for high sensitivities.
Warmer temperatures are expected to lead to more vigorous atmospheric conditions. There is likely to be an increase in extremely hot days and a decrease in exceptionally cold days, while the prospects for extreme conditions of flood and drought may be greater.
The explanation of the range of estimates from low to high sensitivity, of course, can be found fundamentally in the difficulty of establishing climate models which can with certainty reproduce the complex interactions of the atmosphere/ocean and the land masses. However a very important factor is the role of aerosols introduced into the atmosphere by human activity. The role of the aerosols is especially difficult to model and their contribution to the climate can be remarkably different regionally, even different in sign.
The ocean is slow to take up or release heat and so lags behind the changes in the atmosphere, a property which is given the name 'inertia.' In consequence, due mainly to this inertia of the ocean, the global response to the accumulation of greenhouse gases between the present day and 2100 may be only 50 to 90 percent of the change which has already been triggered. Even if concentrations of the gases were to be stabilized by that date, 2100, temperature, and sea level will continue to rise.
Sea Level Rise
Of all the impacts of global warming, it is the threat of sea level rise which has focused the attention of the Pacific Island communities. As President Gayoom pointed out, if the land surface of an entire country has a maximum elevation of only two meters above the level of the ocean, at a time when the scientists are suggesting that the ocean is likely to rise through 50 centimeters and then continue to rise in the long term future, then this is a matter of great anxiety. In the context of this module, there is ample justification of a detailed look at the problem of sea level rise.
The concept is certainly not new. It has been known for half a century or more that this can and generally, is taking place. On a global basis it has been estimated that sea level may have already risen by about 15 centimeters during the last century, but not uniformly over the entire globe, so that 1 to 2 millimeters per year represents an average value. Then there is the complication that the land surface may be moving also as the crystal plates of the earth's slide against each other and as crystal regions which suffered badly in the last ice age, some 20 000 years ago, continue to recover from the release of the ice load so long afterwards. The classic case is to be found in northern Europe where at the head of the Baltic Sea in the Gulf of Bothnia, the land is rising so noticeably due to this relaxation of load that sea level is seen to be falling with respect to the land by as much as one centimeter per year. Northern Canada represents another such case. Fortunately in the case of the Pacific Islan
There is another feature of sea level trends which is worthy of note. When the scientists refer to a trend, say, of 2 millimeters per year, it is natural to consider this to be a uniform world wide effect. This may not be true. Already attention has been drawn to the' effect of crystal plate movements and earthquake events which may produce regional and local anomalies by moving the land with respect to the ocean level, however there is another important process to consider. If sea level rise takes place solely due to the warming*g and consequent expansion of the upper layers of the ocean, then, since there has been no change to the distribution of load on the ocean bed, there has been no warping as a result, and it would be expected that sea level trends will not significantly differ in a geographical sense. If sea level were to rise in response to the addition of mass by adding melt water and precipitation directly to the ocean, then, since the oceans are arranged randomly about the earth and of vastly
While the current climate models are suggesting that in the next century or so sea level is likely to rise by about 50 centimeters we have to bear in mind that the world has already adjusted during the last century to half this amount, and this has taken place without causing major concern. The question now is: how will society and the environment manage to adjust to twice this rate of rise?
Effects on Coastal Process
Tides, wind waves, and ocean currents cause ocean water to be continuously in motion. Waves tend to wear away coastal materials. Boulders are broken off from rocky shores by the force of water dashed against the coast. During storms, waves pound the shore zone with tremendous force. Water may enter cracks and crevices and enlarge the openings into caves. Waves have abrasive or cutting action as they roll rock fragments back and forth across the shore zone, Abrasion causes most coastal destruction. Also when tides and stormy waves move in the same direction, surges of water pile up on shore and do great damage. These processes are happening everyday on our coasts and shorelines. Although they are regarded as a natural phenomena, they have already caused damage to some areas.
As climate changes take place it is likely that stronger and more frequent tropical cyclones which may be experienced will produce stronger and bigger waves. This will result in more damage and coastal erosion. In consequence, this poses a big threat to our low lying islands in the South Pacific, which have little natural protection.
As the sea level rises, more of our coastal areas development will be exposed to these natural forces. Rising sea level will also increase the mean depth of water and so the speed of waves and the energy they transmit across the reef and against the shore. This will cause changes in coastal sediment, erosion and deposition. In general, wave action and sea level rise lead to the destruction of our beautiful tourist beaches, cultural and historical sites and other areas of special value on the coasts or beaches.
Total protection of our shorelines may be unrealistic. However, we can do something to minimize any damage that might be caused by the natural forces of wind and oceans or sea level rise. Sea walls can be built along the shoreline to prevent erosion and other damage due to wave action. We must also limit any activities that might encourage erosion on our sea front such as sand.
Cyclone Frequency
The damage caused by floods, storms, and tropical cyclones might become worse. Coastal areas could experience more frequent flooding during extreme high tides and in particular during storm surges.
Many South Pacific countries have experienced very strong cyclones which destroyed houses, vegetation and other structures. Among many some examples of such destructive cyclones are Cyclone Nam which affected the Solomon Islands in 1986, Cyclone Uma which hit Vanuatu in 1987 and Cyclone Ofa which caused much destruction in Samoa and Niue in 1990. A memento of Tropical Cyclone Ofa maintained at Niue Hotel in Niue. According to the Hotel Manager, a big boulder was thrown into the swimming pool, but of course that had to be removed and could not be keep as a souvenir.
Among many other cyclones in recent years, a very severe Tropical Cyclone Martin hit the Cook Islands in November 1997. It was one of the most severe cyclones experienced in the region in recent time. Pukapuka in the northern Cook Islands was the first victim, On Manihiki atoll alone, 20 people were left dead. In order to salute the surviving people of Manihiki, an aerial view of Manihiki atoll is illustrated on the front cover of this booklet. In general most of the tropical islands in the Pacific face the possibility of being hit by a cyclone on an annual basis. The cyclone season extends from November 1st to April 30th. It is possible that in association with climate change the cyclone zones may move in latitude. The cyclone hazard may in future reach locations previously inexperienced.
Effects on Water Resources
Even without climate change, humanity faces increasingly serious problems with water supply. In our islands some areas already have limited water resources. This being so any threat of climatic change could well be a sensitive issue. It is very likely that such climate change might alter regional precipitation and evaporation patterns. Precipitation would increase in some areas and decline in others. This will mean that where there is higher rainfall there could be a danger of flooding and where there is lower rainfall ground water reserves will be reduced.
The scenarios for the consequences to groundwater of climatic change are numerous, both because of local variations in island groundwater resources and because of uncertainties in the exact patterns and magnitudes of climate change. However, basic hydrologic principles in combination with our general knowledge of the directions of climate change permit us to sketch the major issues that need attention.
Small islands, and particularly atolls, provide a special case of exposure to degradation of fresh water sources. Take the typical atoll composed of coral sands which are permeable. In this way, the saline ocean water continues at depth through the island. Rainfall captured by the island percolates through the surface layers, and because it is relatively fresh and so of lower density it tends to float above the saline ocean water in what is referred to as the fresh water lens. As the name suggests, the fresh water pocket is very thin at the coastline and can be found at sea level, leaking out to the ocean through precipitate 'beach rock'. Inland, given homogeneous conditions, the pocket becomes deeper and shows a vertical profile consistent with the topography of the island, thinning again as it approaches the opposing coast. The water body then fits the descriptive name, lens.
A fundamental feature of the system is that the lens is controlled by the mean level of the ocean. Whereas sea level rise may cause an horizontal advance of the shoreline thereby reducing the habitable area of the island to a small degree, so far as the water reservoir is concerned, its base will also have been raised and this has the effect of a much greater proportional reduction of its volume. This process presents perhaps the major impact of climate change on the Pacific Islands in the shorter time scales.
Sea Level Rise and Salt Water Intrusion
Sensitive physical processes control the sustainability of island resources, among which the fresh water supply is perhaps the most critical. In many cases the Pacific islands consists of porous coral sands and the inhabitants rely upon an underground water reservoir fed by rainfall from the surface, as indicated above.
Due to the work of two scientists, Ghyben and Herzberg, in the late nineteenth century, the dimensions of such a reservoir have been identified. Rainfall percolating through the surface collects underground in a fresh water lens of considerable depth, floating above intruded saline ocean water which continues through the island structure below the lens. The upper surface of the lens is to be found near sea level at the coast line, and rising above that level inland in the form of a subdued copy of the island topography. However, the lower surface of the fresh water lens dives deep below the island. For the interface between the fresh and saline waters to be maintained, its gradient must be steep. In fact where the water table rises, say, one meter above sea level at a particular point on land, the bottom of the reservoir will be found some 40 meters below sea level, and this ratio between elevation of the water table and the reservoir depth remains constant where's condition of hydrostatic equilibrium is
There is a catch however. The same argument suggests that the picture is very sensitive to small changes in sea level. Consider an atoll where the habitable land rises just two to three meters above sea level, and this implies that in a condition of abundant rainfall, the upper surface of the fresh water reservoir rises somewhat less than that across the island. Now consider the prospects of sea level rise due to climate change, and also consider that sea level rise may be of the order of half a meter above present levels. That part of the reservoir above sea level will feel the restriction of the new significantly lower topography and will no longer rise to the same level above sea level as before. The Ghyben-Herzberg relationship then suggests that for every centimeter of shortfall in the height of the upper surface of the fresh water lens above the new sea level, the lower surface will rise by forty centimeters, i.e., the size of the reservoir will be significantly reduced as a response to small chang
What is more important is the tendency at this stage for problems to multiply:
If the reduced fresh water reservoir becomes stressed in usage, then the condition of hydrostatic equilibrium will be violated so that the Ghyben-Herzberg principles will no longer apply probably causing a further reduction in the volume of the reservoir. Then there is likely to be a growing concern over water quality superimposed upon the original concern over quantity. If the local technology is based upon tube wells or boreholes, an early sign of trouble will be in the salinity of the supply since the tube base may now be penetrating the lower boundary of the fresh water and found to be accessing water of ocean origin. The temptation then will be to abandon the deep wells in favor of shallow wells retained within the fresh water lens. Flow volume may then be an issue. In fact of course, tube wells are not the informed and technically-preferred system for the access of good quality water in such an environment. If not already in place there will be an urgent need to invest in slit trenches, taking the
Recharge
Changing rainfall and temperature patterns will alter the amount of water available for recharge of the reserves. Higher temperatures will cause more evaporation, whether rainfall will increase enough to compensate for this is not currently predictable for specific locations. If recharge or rainfall decrease, islands, that currently have limited potable portions of their underground water will be particularly susceptible; a decrease in recharge could mean a threat to potable water available for withdrawal after natural losses. This may cause salt to leach from soil and raise soil salinity between ground level and the underlying water table. A lowered water table in coastal areas will draw sea water into areas previously occupied by fresh ground water.
If rainfall patterns were to change so that precipitation, regardless of quantity, were to fall during less reliable or shorter rainy seasons, leaving longer periods without rain, then a crisis could occur as both demand and pumpage increased during the dry season and excessive withdrawals were made from a depleting lens. Increased seasonal swings in water lens size would be likely to promote additional mixing of fresh and saline water, thus further limiting and degrading the resource.
The coastal strip with its traditional vegetation in particular, so essential to the image of the Pacific islands is most at risk from salinization as the fresh water lens is threatened.
Island Size
Over longer time scales of decades or more, rising sea level may be expected to erode coastlines and flood low-lying portions of islands. It is unclear to what extent island accretion will be able to keep up with sea level increases. Decreased island size will mean less surface area for infiltration and potentially a smaller volume of aquifer available for the storage of fresh water. If the population is not reduced in proportion to the changing area, then a greater proportion of the island might also then be covered with impermeable surfaces.
The rise of sea level due to global warming will definitely threaten the viability of fresh water aquifers and other sources of fresh ground water. Ground water in low lying atolls will become more saline. Intrusions of salt water into fresh water sources will make ground water unfit for household and agriculture use. Vulnerability to climate change will be greatest for islands that are particularly low in elevation or small in area.
Water Quality
Decreasing water lens size due to the effects discussed above would tend also to decrease water quality by making any groundwater development more likely to produce some intrusion of saline water. In addition, if the climate were to change so that severe storms became more frequent, then over-topping of the island with salt water by storm surge waves could contaminate the water lens more often. While in some cases recovery can progress fairly quickly, storm contaminated groundwater may be unusable for a period of months or longer. Greater population density (from decreased area as well as from population growth) may intensify the amount of hardship.
Water Management
Improved water management is needed to minimize the impact of climate change. Growing population and rising living standards are increasing humanity's I consumption of water. If climate change does reduce rainfall and cause sea level to rise there is bound to be real problems with our water resources. To respond to this threat, there is a need to upgrade existing infrastructure for water storage. People will need to be more sensitive to the problems of water wastage and to introduce policies that would cut waste and constrain demand. Activities such as careless logging activities will spoil many of our fresh water resources. Such activities should be carefully monitored to avoid unnecessary pollution of water sources.
Impacts on Coastal Area Development
Half of the world's population inhabits coastal regions and the flood prone delta regions of Egypt, India, Bangladesh, and China. Present plans for coastal development and the design of buildings is based on the assumption that past climate is a good guidance for future climate.
For example, to fight against the possible sea level rise, some major works would be necessary to protect low-lying areas and especially waterfront buildings, marinas, ports, and industries sited near coastline for cooling water or ease of freight handling. An extension of the cyclone belt would mean more buildings could be at risk from high wind speeds, intense rainfall and coastal flooding.
Effects on Farming
With a changing climate, farmers and agricultural scientists cannot took to past records to predict the frequency of good and bad years or the extremes of rainfall, temperature, frost and so on.
For food plants like cereals, a rise in temperatures would mean that they grow faster, mature earlier and so have less time available for accumulating food reserves in seeds. Increasing temperatures, changes in rainfall, soil loss from rain in some areas and new varieties of plants could mean major changes in areas where crops are grown. While frost is not a concern of most of the Pacific islands we should note that some plants would benefit from longer frost-free growing periods due to higher temperatures but frequent frosts and low temperatures are needed by other plants in order to stimulate flowering.
An increase in carbon dioxide has been shown to act like a fertilizer on most plants while many have a reduced need for water in such circumstances. So for wheat, an ability to grow with less water could allow it to be grown in more arid regions. For forest plantation, any decrease in rain could reduce productivity.
Changes in preferred breeds of sheep and cattle could be expected. Maybe cattle replace sheep in areas receiving more rain. The amount of nutrients in plants may alter with changes in their growing seasons. Heat stress in animals could increase.
Distribution of fish species may change with changes in sea water temperatures and ocean currents. Mangroves and river estuaries, important spawning grounds for fish, would be, affected by coastal flooding and erosion.
Impacts on General Environment
The speed of climate change is a critical factor in determining whether native animals and plants adapt and whether their abundance and distribution suffer. The fate of a particular animal or plant is determined by the combined effect of changes on climate, food, soil, nutrients, predators, competitors, available habitats, diseases, changes in distribution of other species, and so on.
Even species which could relocate so as to keep pace with the climate changes might find that there are cities, agricultural areas and other developments acting as barriers and in some cases there may not be another suitable habitat.
In regions of increased rainfall, water birds may be favored by the more extensive and reliable wetlands. Where rainfall is reduced, water birds may suffer. Rising sea level and changing storminess may reduce the tidal flats around islands, affecting wading birds, those that use salt marshes and trans-equatorial migrants.
Alpine environments could shrink or disappear because of shorter, milder winters. All the alpine organisms would have a much reduced habitat within which to survive. These habitats would also be isolated and making species less capable of coping with climate change or competition from specious colonizing the habitat.
Changes in the Boundaries of Vegetation Zones
Projected changes in global temperature in the range 1.5 to 4.5 degrees Celsius together with changes in precipitation will result in the movement of the boundaries of vegetation zones, and will impact on their floristic composition and associated animal species. Boundaries, temperate forests, grasslands, etc., are expected to shift perhaps over several hundreds of kilometers over the next 50 to 100 years. Real rates of the movement of species, however, will be restricted by limits on their ability to disperse.
Both coniferous and broad-leaved tree species of warm environments will find favorable environments much further poleward than their current limits. In the northern parts of Asian Russia, it is forecast that the boundary of the zone may move northward from 40' to 50' latitude. The tundra zone is expected to disappear from the north of Eurasia.
Expected changes in precipitation will allow species to extend their boundaries equatorward. As a result, broad-leaved species range may expand and these ecosystems most likely will be more maritime in terms of species composition. The forest steppe subzone in the European Russia will change while in southern portions of western Siberia, it is suggested that the forest steppe boundary could move up to 200 kilometers.
In the semi-arid, and and hyper-arid ecoclimatic zones of the Mediterranean, greenhouse-gasinduced climate change should reduce plant productivity and result in desertification of the North African and Near Eastern steppes owing to increased evapotranspiration. The upper limit of the deserts would migrate under the influence of climate change and most likely extend into the area that currently corresponds to the lower limits of the Semiarid Zone.
The impact of climate changes on the present tropical and temperate rainforest is uncertain. For example, almost all of Tasmania [Australia] is expected to become, at best, climatically marginal in terms of temperate rainforests, largely owing to a rise in winter teftiperatures suggested by climate scenarios. This increase in temperature is unlikely to have a direct effect on the forest, but may facilitate the invasion of less frost-tolerant species.
Possible Effects of Rising Sea Level on Reef Landforms
One relevant question in any consideration of the effect of a continually rising sea level is the effect on coral growth. Coral reef growth is primarily a response to light availability and is greatest just below the surface water layers. According to past records, it was noted that coral growth kept pace with sea level rise during the most rapid rises in the early Holocene when rates of accretion were measured at 5-8 millimeters per year.
There have been a variety of estimates of average and maximum rates of both coral growth and vertical reef accretion, and the question of maximum rate of accretion is vital to any projection of coastal process and morphological changes which occur in response to rising sea levels. Based upon scientific records, on average individual coral colonies grow upwards and outwards at about 25 millimeters per year. Reefs grow more slowly due to the compounded effects of storm damage and recovery, plus the interaction between the growth of coral, algal mantles and the rate of clastic sediment production to fill the interstices of the reef platform. Normal rates of reef growth may be taken as 5-20 millimeters per year.
It is to be noted that predictions of reef growth rates must take into account the structure of the reef community, and the likely occurrence of local events such as cyclones or predator attacks, which would temporarily inhibit reef growth. In addition, growth is unlikely to be sustained if water temperatures reach 30 degrees Celsius, which may occur across much of the tropical Pacific with global warming. A major and noteworthy point then is that although rising sea levels generally favor reef growth, this growth may not keep pace with predicted sea level rises due to the changed conditions of global warming.
It may be possible that in many situations coral growth will be able to keep pace with even rapidly rising sea levels, perhaps with changed reef composition and form, if such rises occur no more rapidly than 30 millimeters per year and such rates of rise are not certainly contemplated. However, the growth rate depends on a number of biological factors. In such a situation the removal of surface sediments may be followed by the upward growth of coral, and the development of submerged atoll structures.
Then of course there are the questions, how many of island reefs are healthy enough to respond? Have we already caused too much damage to the natural environment to limit its potential for recovery?
Concluding Comment
This text is by no means a comprehensive survey of the impacts of climate change. It has been produced with a main focus on, and a dedication to, the Pacific Island peoples and has addressed, in particular the main source of their concern, namely the threat of rising sea level. There still remains the need to attempt to put the many points raised into perspective.
The text has been based upon the work of many reputable scientists who have studied difficult elements, aspects, and interactions of the atmosphere/ocean/land surface that make up the total environmental model. Due note has been taken of the considered view of the international scientific community in this matter, condensed into a consensus of opinion incorporated into the Second Assessment Report, 1995-1996, of the Intergovernmental Panel on Climate Change. The brief survey here attempts to set the Panel's assessments against a background of long period oscillations in climate which are known to occur naturally, without any intrusion of human activity. The point has been made that we really cannot be certain as to what the future holds for us in climate matters. The position is that no one can be sure that the variation of climate that has been experienced in recent time is, in fact, due to the processes contained in the greenhouse models and that they are the result of recent human activity.
The climate models contain many terms and, as time proceeds, more are discovered and will be incorporated. Nevertheless, we have followed the consensus view that, on balance, the available evidence does suggest a link with the greenhouse gases and human practices. This step has been taken since the authors are firn-fly of the view that the risks taken, should the model outputs be ignored until proven, are far too serious and, in this, we have in mind that, due to major lags in the system, any climate variation set in motion now may well set in train impacts which will grow and continue into the filter - even if gas emissions are stabilized from this moment onwards. The decision has been made that it would be imprudent not to accept the likelihood that global warming has indeed been initiated.
Meanwhile, the authors firmly believe that the most helpful action that can be taken now is to establish carefully planned high resolution monitoring programs which, given time, will eventually identify what is actually taking place. Such programs will help the models to solve the problems of climate variation by providing real input values and also help to provide a check on intermediate output. At the same time, there is welcome assistance to the world communities at all levels to understand the nature of climate issues, to appreciate the difficulties of interpretation, and to indicate actions that might help to limit environmentally unfriendly practices in the interests of the common future. Again, an example of this policy is the initiative of the Government of Australia in establishing the South Pacific Sea Level and Climate Monitoring Project, and the Information and Training programs that are incorporated within it.
As the end of this module approaches, we should, in passing, note that the consensus on climate change does not represent a unanimous agreement. There are scientists who recommend caution in accepting the present greenhouse philosophy, and point to the fact that two to three decades ago, the main concern of the scientific community was over the 'probability' of global cooling - not global warming. In fact, a current issue of a reputable scientific journal contains two references to the probability of the earth entering into a cooling phase within the next 50 years, and the suggestion is made that we may well need the influence of the greenhouse gases to moderate the hazards of global cooling that is likely to come.
