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New South Wales Government
The term "greenhouse effect" is presently being used to describe a postulated warming of the earth due to the accumulation of certain gases in the atmosphere. In particular, the increase in levels of carbon dioxide (CO2) resulting from the burning of fossil fuels is of interest.
The current consensus of scientific opinion is that such changes could result in a global warming of 1.5o to 4.5oC over the next 30 to 50 years. Such a warming could lead to a number of changes in climate, weather and sea levels. These in turn could cause significant changes to coastal processes, e.g. increased severity and frequency of storms resulting in increased wave heights and erosion potential.
The atmosphere plays a crucial moderating role in the heat balance of the Earth. The principal gases of the atmosphere are nitrogen (78%) and oxygen (21%). However, their ability to absorb heat is low and they play little part in the heat balance. In contrast, carbon dioxide, nitrous oxide, methane and water vapour, which in total amount to less than 1% of the atmosphere, have high heat capacities and play a major role in the heat balance.
Relatively small changes in the concentrations of these gases may result in significant changes to the heat balance and to atmospheric temperatures. Hence the concern over CO2 levels.
Radiocarbon dating and the analysis of small air bubbles trapped deep in antarctic ice has made it possible to reconstruct some of the past history of the world's climate. Figure B12.1 shows the variation of CO2 levels, atmospheric temperature and sea level over the past 160,000 years. CO2 levels were determined from air in the ice; temperatures at the time of ice formation were estimated from the relative concentrations of the isotopes oxygen-16 and deuterium (Fifield, 1988; Barnola et. al., 1987). The sea level changes shown in Figure B12.1 are taken from Chapman et. al., 1982, and are the same as those shown in Figure B2.5. The variations of Figure B12.1 indicate two ice ages (150,000 and 20,000 years Before Present) and two warm periods (125,000 years ago and present time).
The data shows a good correlation between the variation of CO2 levels in the atmosphere and change in surface temperature. The correlations between sea level and surface temperature and between sea level and CO2 concentration appear reasonably good. These correlations are not as distinct as that between surface temperature and CO2 concentration because of the relatively inferior accuracy and density of the sea level data set.
Carbon dioxide levels in the atmosphere are thought to have increased by about 50 ppmv since the industrial revolution. This has been attributed to the burning of fossil fuels but a variety of other factors, often surprising in their nature, make significant contributions (bovine flatulence, paddy fields, etc.). Figure B12.2 shows the increase in CO2 levels in Hawaii between 1958 and 1980. Over this period of time, the mean monthly level increased from 315 to 340 ppmv.
To summarize: To date, the "hard" evidence in support of a "greenhouse" increase in temperatures is limited to the observed increase in CO2 levels from 1958 onwards and global temperature trends over the last 100 years.. Historical evidence suggests that CO2 levels have varied between 190 and 340 ppmv over the last 160,000 years. Again, historical evidence suggests that atmospheric temperature changes follow changes in CO2 levels. The relationships between CO2 levels, temperature and sea level are reasonably good.
Scientific opinion is divided regarding the timing and the likely degree of "greenhouse" warming. There is however a general consensus that warming will occur. If warming occurs it is generally agreed that sea level will rise.
It is believed that any "greenhouse" warming will not be uniform over the earth's surface or throughout the year. Warming is expected to be greater at higher latitudes and in winter time. In Australia, it is postulated for example that surface temperatures may increase by 2oC in the northern tropics and by 3 to 4oC in southern latitudes.
In terms of average climatic conditions, the greenhouse effect may only produce rather small changes. However, its effect on extreme events may be much more marked. For example, a rise of 2 to 3oC in the surface temperature of the Tasman Sea may enable cyclones along the east Australian coast to move an additional 200 to 300 km southwards. Thus, the North and Mid-North Coast sectors of the New South Wales coastline will be more prone to cyclonic events. Additional details are given in Appendix C9.
In addition to climate change, global warming may also produce a worldwide sea level rise (eustatic rise). In the first instance this will be caused by the thermal expansion of the upper ocean layers. Sea level rise of between 0.5 and 1.5m by the year 2100 has been adopted by one technical committee (NAS, 1987). Sea level rise to the year 2050 of between 7 and 67cm (best estimate range 24 to 38cm) has been projected by others using box diffusion models. (Warrick, 1990, Pers. Comm.) (See Appendix C9.)
Any sea level rise is expected to lag behind global warming by some one to two decades because of the slow nature of heat transfer from the atmosphere to the oceans. Further, the rate of sea level rise will not be uniform, but will accelerate with time (see Appendix C9).
There are a number of uncertainties with respect to the timing and degree of any global warming. There is little evidence to suggest that existing coastal development and/or structures will be directly threatened in the immediate future. However, with the weight of scientific opinion suggesting that climatic change and associated sea level rise may become significant within the next 30 to 50 years, there is a need to incorporate these effects in coastal planning, management, and engineering works of the future.
Barnola, J.M., Raynaud, D., Korotkevich, Y.S., & Lorius, C., (1987). "Vostok Ice Provides 160,000-Year Record of Atmospheric CO2". Nature, Vol. 329, 1 October, 1987, pp. 408-413
Fifield, R., (1988). "Frozen Assets of the Ice Cores". New Scientist, 14 April, 1988, pp. 28-29
Gordon, A.D., (1989) "A Greenhouse Owner's Guide to Ark Construction". National Environmental Engineering Conference, Sydney, March, 1989. I.E.Aust. Publication No. 89/3.
NAS, (1987). "Responding to Changes in Sea Level: Engineering Implications". (National Academy Press, Washington DC: 1987).
NRC, (1983). "Changing Climate". A report by the Carbon Dioxide Assessment Committee, National Research Council. (National Academy Press, Washington DC: 1983).
Warrick, R.A., (1990). Climate Research Unit, University of East Anglia. (Pers. Comm.)