Climate change and sea levels in the South Pacific Student Activities Case Study Teacher's Notes Draw an annotated diagram which clearly explains the difference between the greenhouse effect and the enhanced greenhouse effect? Make a list of the greenhouse gases and where they come from. The trend rates for sea level changes from Australian stations are given in the table below. Table 2: The estimated relative sea level trends for tide gauge locations around Australia which have at least 25 years of hourly data on the National Tidal Facility archive. Location Years of data Estimated Trend (mm per year) Darwin 34.9 -0.02 Wyndham 26.4 -0.59 Port Hedland 27.7 -1.32 Geraldton 31.5 -0.95 Fremantle 90.6 +1.38 Bunbury 30.2 +0.04 Albany 31.2 -0.86 Esperance 31.2 -0.45 Thevenard +0.02 31.0 Port Lincoln 32.3 +0.63 Port Pirie 63.2 -0.19 Port Adelaide - Inner 41.0 +2.06 Port Adelaide - Outer 55.1 +2.08 Victor Harbor 30.8 +0.47 Hobart 29.3 +0.58 Georgetown 28.8 +0.30 Williamstown 31.8 +0.26 Geelong 25.0 +0.97 Point Lonsdale 34.4 -0.63 Fort Denison 81.8 +0.86 Newcastle 31.6 +1.18 Bundaberg 30.2 -0.03 Townsville 38.3 +1.12 Select a station near where you live. Draw a map which shows the proximity of the Australian sea level monitoring station to your home. Describe the location of the station. Describe the likely impact of this sea level change on the region with reference to: The physical environment (landform) The surrounding built / human environment (houses, roads, industry, recreational facilities etc) The economic activity in the area / region Plants and animals in the area / region On your map and in words, describe the likely future impacts in 20 and 50 years time? Suggest 5 ways in which local planners could reduce the impact of sea level changes in the region. Draw up a map of the South Pacific region and mark the locations of the SEAFRAME monitoring stations. Mark in the equator and determine how far the stations are from it. Does this proximity to the equator pose any problems for the region? On the map, indicate the recent sea level trends for each station, and state any particular problems each island may have if sea levels change. From Table 1 calculate the sea level change which has occurred at each station since its establishment. Table 1 also includes the change in the trend from the previous month. If this change continues, what is the likely sea level change in the future? Sea level changes are very variable, but the changes around the Pacific appear to be about 1 mm per year. This means that the instruments used to measure sea level change over a long period need to have a resolution of approaching one millimetre. The term 'resolution' here means that two sea level recordings which differ by one millimetre can be recognised as different measurements. In past years, sea level measurements were made using a float in a tube in the water. The float was connected to a recording pen through a series of pulleys and counterweights. These instruments were subject to a number of problems. These included that the sea level in the tube was not always the same as the outside sea level during turbulent periods. There were also problems with the pulleys and the connections, and with the recording pens themselves. Intensive care and maintenance was required to obtain reasonable records. The SEAFRAME monitoring stations use an Aquatrak sensor which operates acoustically. A pulse of sound is fired down a tube to the water surface. This pulse is reflected back from the surface of the water. Knowing the speed of sound and measuring the time taken for the sound to travel from the sensor and back from the sea surface allows the sea level to be determined. The speed of sound varies with temperature, humidity and air pressure and the Aquatrak sensor automatically compensates for these variations which are also measured by the SEAFRAME station. The principle used in the Aquatrak sensor is similar to the one we can use during a lightning storm to determine how far away the storm is. If after you see a lightning flash, you start counting slowly until you hear the thunder, you can estimate how far away the storm is. The speed of light is so fast (approximately 300,000 kilometres per second) so that the time the light from the lightning flash takes to reach us is negligible. The distance away of the storm is the time taken for the sound to arrive times the speed of sound (approximately 330 metres per second). If the time for the sound to arrive was 10 seconds, then the lightning storm was 3300 metres away. If the storm moves 660 metres closer, the time between the lightning flash and the thunder would be 8 seconds. You could determine this movement if you had a watch with a second hand allowing you to measure individual seconds. However, if the storm had moved only 495 metres, the time interval would be 8.5 seconds, and you would need a watch which measures to at least half a second to accurately determine (or resolve) how close the storm had moved. Now consider the Aquatrak system. If the sea surface is 8 metres below the sensor, how accurately do you need to measure time to distinguish between two sea level measurements which differ by one millimetre? Why is the sea level rising? Sea level is rising and we are not sure about the rate of the rise. Another question we are unsure about is: "Where does the extra water come from?" Fifty years or so ago it was thought that a warming phase of climate was responsible for partial melting of the polar ice caps and a consequent addition of mass and volume to the waters of the ocean. As a result of recent research, models have been proposed which suggest that rather more than half of the sea level rise which has occurred in recent geological time is more likely to be due to the warming of the upper layers of the ocean and a subsequent increase in the volume of the waters. The same models also suggested that slightly less than half of the rise was due to the addition of fresh meltwater from land-based ice in the form of glaciers and the icefields associated with mountainous (Alpine) areas in the mid to sub-polar latitudes. Note that these models ignore the possible contributions from the polar ice caps. So what about them? Melting of the northern ice caps, notably the Greenland ice sheet, contributes about 12% of the total sea level rise. However, in Antarctica, warming of climate will initially increase precipitation as snow which will contribute to the ice caps rather than release water into the ocean. This increase has been estimated at a rate of 13% of the total sea level rise. Consequently, the two polar ice caps virtually cancel each other in so far as sea level rise is concerned for perhaps 100 years, then the Antarctic ice is expected to melt significantly. Why is it important that we know the source of the sea level rise? If the sea level trends are due solely to the warming of the ocean, the surface water of the ocean would expand and increase in volume, but its density is reduced. Consequently, there would be no change to the pressure applied to the ocean bed, and Earth's crust would not be subject to distortion by the uneven load imposed by an ocean confined within the irregular boundaries of the continents. We might then assume that the sea level trend discovered at one tide gauge site would apply to all locations. If the sea level rise is due to melting of ice, then the mass of water in the oceans would increase and apply extra pressure on Earth's crust. Let us consider the change in volume of a water body, ?V, due to a temperature change, ?T. This is often referred to as thermal expansion. ?V = ß Vo ?T Where Vo = the initial volume and ß = the coefficient of thermal expansion. The coefficient of thermal expansion of standard sea water under normal conditions is approximately 300 x 10-6 oC-1. Now consider an upper layer of ocean, 50 metres thick over an area of 1000 m2. Calculate Vo, and then the change in volume if there is a temperature increase of 1 oC. Given that the surface area of the patch of ocean is unchanged, what is the increase in the thickness of this layer of ocean? What is the change in sea level? A simple experiment can demonstrate this concept of thermal expansion. Materials required: 500 ml conical flask bunsen burner with stand and gauze mat two-hole stopper hollow glass tube thermometer ruler graph paper (Diagram of set up, The South Pacific Sea Level and Climate Change Newsletter, Vol. 4, No. 4, page14) Procedure: Fill the flask to the top with water. Place the hollow glass tube and thermometer in the stopper and gently press the stopper into the flask Mount the ruler so that the water level in the glass tube can be measured. Heat the water slowly and record the water level at 20C intervals. Record the temperature and water level at each of ten or more stages in the warming process. Plot the results on graph paper (temperature versus water level) to show volume expansion. What is the effect of ice melting? This involves a transfer of mass from the ice sheets to the oceans and a change in the pressure applied to Earth's crust. Below the crust, the molten layer called the 'mantle' can flow. The major climate changes of the past which produced the ice ages created depressions of the crust under the increased continental ice load as the mantle below was squeezed and extruded horizontally. Today, in the higher latitudes, recovery is taking place as the load has long since melted away and the plastic mantle is squeezing back to its former locations. In Scandinavia, for example, particularly around the coasts of the Baltic Sea, tide gauges show that the land is rising with respect to sea level. Over a large region this land rise is of the order of one centimetre per year, much larger than the accepted mean sea level trend elsewhere. Knowledge of changes such as these in different parts of the world are necessary to ensure the accuracy of the analysis of the tidal gauge data. This process of equalisation of pressures in the crust and mantle is called isotasy. A couple of simple experiments can demonstrate the effects of ice melting. Materials required: large, rectangular container to hold water rectangular piece of wood, about 20 cm x I0 cm and 3 to 4 cm thick. The wood should be of low density for it to float readily on water. supply of ice cubes and water Experiment A Procedure: Partly fill the container with water. Cover three quarters of the water surface with ice cubes, and ensure that the free water surface is at least 2 cm below the container rim. Carefully measure the height of the free water surface below the rim of the container. Measure and record the temperature of the free water, and also its distance below the rim of the container at intervals of five minutes until all the ice melts. Plot the temperature of the free water against time. Explain the shape of the plot. Why is this so? Experiment B Procedure: On the surface of the piece of wood, mark the points of the compass (N, E, S, W). From N to S across the surface of the wood draw E-W lines at 1 cm intervals. Along the N and S edge mark lines at 2 mm intervals. Part-fill the container with water and float the wood in the water. Place one or two ice cubes on the N edge of the floating wood. As the ice melts, watch and note the level of the water in the container and on the N and S edges of the wood Imagine that the wood block represents a continent 'floating' on the mantle which is able to 'flow'. The ice cubes represent an ice sheet or glacier field overlying Earth's surface. Questions: What happens to the water level in the container as the ice melts and why? What happens to the N and S edges of the block of wood as the ice melts? Does this experiment help to understand whether melting glaciers and other land-based ice masses will make sea level rise? Under what circumstances will the meltwater submerge the continents on which the ice used to be? Since many of the world's tide gauges are sited on the edge of continents which in the recent geological past carried the load of massive ice sheets, what difficulties arise for the determination of sea level trends? Is it possible to obtain a good measure of trends in the world's sea levels considering that the conventional tide gauge measures sea level with respect to local land levels? How important is an associated program which can independently measure vertical land movements? Isostasy implies equalisation of pressure. Is this an appropriate word to describe the above principles? Mangrove forests occupy the boundary between the sea and the land. They thrive on the edge of the sea in intertidal zones and estuaries. Most of the tropical and subtropical coastline is dominated by mangroves which are estimated to cover an area of 22 million hectares. Mangrove forests have a high level of biodiversity as they consist of a large number of species of plants and animals which are adapted to the conditions of the coastline. However, over the recent years the area of mangroves has been decreasing at an accelerating rate. As mangroves occupy the coastline, they are very susceptible to changes in sea level. Mangrove ecosystems have traditionally been managed sustainably by local populations in the South Pacific for the production of food, medicines, tannins, fuel wood and construction materials. For millions of indigenous coastal residents, mangrove forests offer dependable, basic livelihoods and sustain their traditional cultures. The protective mangrove buffer zone also helps minimise damage of property and losses of life from hurricanes and storms. In regions where these coastal fringe forests have been cleared, tremendous problems of erosion and siltation have arisen, and sometimes terrible losses to human life and property have occurred due to destructive storms. Mangroves are said to contribute to biodiversity on Earth. What is biodiversity? Why do we need to preserve biodiversity? What ecological processes occur in mangrove forests and what is their wider environmental importance? Organisms living in mangrove forests are well adapted to their environment. What are the environmental conditions on the coastline to which these organisms need to be adapted? What are some of the adaptations organisms possess for these conditions? Rises in sea levels can lead to a reduction in mangroves. What are some of the other processes which may threaten mangrove forests? Mangroves are found not just in tropical areas. They also extend to the southern parts of mainland Australia, but not Tasmania. In Victoria, there a number of mangrove forests including those around Westernport Bay and Barwon Heads. Are there any mangrove forests near you? In terms of species composition and biodiversity, how do these compare to mangroves in other parts of the world? What is the environmental significance of these mangroves? Is the area of mangroves decreasing in your area? If so, what are the threatening processes? How can these mangroves be preserved? What can you do to preserve mangroves? Sketch a profile of a mangrove forest showing how it changes from the seaward edge to the landward edge. If possible, visit a mangrove forest and identify the flora and fauna present. What are some of the adaptations of these organisms to their environment? What other types of ecosystems are likely to be threatened by sea level changes? Reference Undrwood, A. J. and Chapman, M. G. (1995). Coastal Marine Ecology of Temperate Australia. (University of New South Wales Press: Sydney.) Robertson, A. I. And Algoni, D. M. (2001). Mangrove Ecosystems in Australia: structure, function and Status. In State of the Marine Environment Report. Technical Annexe: 1. http://www.environment.gov.au/marine/information/ reports/somer/somer_annex1/som_ann11.html   Top © Commonwealth of Australia