^ffe^lf. If all other things remain equal, temperatures certainly should rise as carbon dioxide builds up.

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1 Carbon dioxide is as essential to life as oxygen so it can hardly be described as a pollutant. But that doesn't mean we can pour ever-increasing amounts of it into the air and be certain nothing will change for the worse. 3 1 ^ffe^lf lllllllh As well as providing the carbon needs of green plants, and hence indirectly of the rest of life, atmospheric carbon dioxide helps warm the earth. It behaves like the glass walls and roof of a greenhouse, trap ping energy from the sun near the earth's surface. Any increase in the carbon dioxide content of the atmosphere will increase this warming effect, with results that are hard to predict. I f average tem peratures rise throughout the world, other changes in climate are inevitable. If all other things remain equal, temperatures certainly should rise as carbon dioxide builds up. The carbon dioxide content of the atmosphere has increased this century from about 290 parts per million in 1900 to about 330 p.p.m. now. Until 1940 it appeared that predictions of temperature rises accompanying the carbon dioxide build-up were being fulfilled; meteoro logists reported an increase of 0.6 C averaged over the Northern Hemisphere between 1880 and But since then temperatures in that Hemisphere have fallen, by an estimated average of 0.4, although carbon dioxide concentrations have continued to rise. I f all other things remain equal, tem peratures certainly should rise as carbon dioxide builds up. This is because the gas absorbs heat radiated from the earth's surface but not the incoming solar radia tion that generates the heat. The more carbon dioxide there is to absorb heat, the Nobody knows what offsetting influences produced the falls in temperature after greater should be the temperature rise in the atmosphere and on the surface. Doubts However, all other things don't remain equal. Nobody knows what offsetting in fluences produced the falls in temperature after 1940; the workings of the atmos phere are extremely complex and only partially understood. Scientists are now raising doubts about whether the North ern Hemisphere really did heat up by 0 6 after Averages are hard to determ ine from the limited number of haphaz ardly distributed measuring stations that have records going back into last century. All these stations are on land, so no read ings taken over the oceans are available. Also, many of the stations are in growing cities, and would be affected by the heat the cities produce. (See An urban heat island, Ecos 1.) The picture for the Southern Hemis phere is even more confused, because it has fewer long-standing measuring stations and much wider expanses of ocean. About all that can be said with confidence is that the cooling trend re corded north of the equator since 1940 has not occurred in the south. Some evidence exists of a continuing slow rise. Scientists use mathematical models of atmospheric processes to try to work out 13

2 Why temperatures rise under atmospheric carbon dioxide, as they do inside a greenhouse. what effects increased carbon dioxide concentrations should have on surface temperatures. The range of answers they come up with is as clear a demonstration as any of the need for much more knowledge. Predictions of the average temperature rise that would result if the carbon dioxide content of the air doubled range from 0-3 C to 3 C. Predictions of how or even whether the production of growing amounts of carbon dioxide will affect future atmospheric concentrations are just as hard to make. If all the carbon dioxide injected into the atmosphere by burning oil, coal, and gas in 1972 had remained there, the concentration would have risen by about 2-6 p.p.m.; in fact it increased by about 1-2 p.p.m. Between 1960 and 1970 production of the gas rose by half, but no corresponding increase occurred in the rate of build-up in the atmosphere. The need to know There can be little doubt about the importance of working out how carbon dioxide output, atmospheric concentrations of the gas, and temperatures are linked. Fossil-fuel burning, on the basis of current trends and knowledge of reserves, seems likely to quadruple by the year 2000 and then go on rising steeply for perhaps another century. The amount of carbon dioxide produced will follow the same pattern. Only small global temperature variations are needed to trigger off major climatic changes; the rise or fall is much greater at the Poles than at the equator when world-wide changes occur. A fall averaging about 6 C would bring on another Ice Age, and a rise averaging perhaps only 4 C could melt the polar ice caps, raising sea levels by about 60 metres and drowning most of the world's seaboard cities. From the evidence available to date, the possibility of carbon dioxide emissions producing such a change appears small, but it can't be ruled out. Scientists are now making regular, precise measurements of atmospheric carbon dioxide in many parts of the world. In the Southern Hemisphere, permanent measuring stations are operating in New Zealand and at the South Pole. Since 1972 the CSIRO Division of Atmospheric Physics, in a wide-ranging data-gathering program, has been measuring the carbon dioxide content of air gathered over the Installing an airborne sampling unit. Antarctica. Its ice may throw light on An ice core emerges from a drill at past carbon dioxide concentrations. Casey, Antarctica. 14

3 Great Australian Bight, Bass Strait, the Tasman Sea, and the Southern Ocean as far south as Antarctica. If the aim was only to keep a watch on global trends in concentrations of the gas, a much smaller measuring effort would suffice. This is because carbon dioxide spreads itself out very evenly through the atmosphere; variations from place to place and month to month are quite small. It comes and goes But isolated readings throw no light on the importance of different sources of carbon dioxide in determining atmospheric concentrations, information that is needed if the effects of Man's input are to be understood. Also needed is knowledge of the capacity of vegetation and the oceans to absorb carbon dioxide. The CSIRO team, led by Dr Graeme Pearman, is looking at changes in concentration with altitude, season, and distance from the equator. As the changes are small the measurements have to be precise, and the aim is to have them accurate to within 0.03%. Analysis of the readings gives information on the movement of carbon dioxide through the atmosphere, which enables the scientists to estimate the contributions of different sources and sinks. The team's air samples are collected during regular airline and Department of Transport flights, and are analysed at the Division's laboratories at Aspendale, Melbourne. Sampling units carried in the aircraft collect air at least once every 2 weeks 3-5 km above Bass Strait, 3-13 km above the Tasman Sea, and km above the Great Australian Bight. Occasional samples are collected between New Zealand and the South Pole, and weekly ground-level readings are made at Aspendale. The scientists hope soon to begin using balloons for collections at altitudes up to 21 km, well into the stratosphere. Some samples already come from the stratosphere; sometimes the boundary between the lower atmosphere and stratosphere is just below the altitude reached by the jet aircraft crossing the Tasman Sea and the Great Australian Bight. As there is much less turbulence in the stratosphere than lower down, movements of carbon dioxide are likely to be quite different there. A rising trend Although the measurements only started in 1972, the CSIRO team has already found a rising trend in carbon dioxide concentrations. In 1972 the average was about 327 p.p.m.; now it is p.p.m. The rise parallels that recorded in the Northern Hemisphere. The measurements also show small variations in concentration with season. The yearly maximum, recorded in spring, is typically 1-2 p.p.m. higher than the minimum, which is recorded in early autumn. A similar, but much larger, The CSIRO team has already found a rising trend in carbon dioxide concentrations. oscillation occurs north of the equator, where the maximum is reached in late winter and the minimum in late summer. The difference between the highest and lowest readings is usually about 10 p.p.m. in the Northern Hemisphere. The CSIRO measurements show no sign of this seasonal oscillation in the stratosphere, probably because the rate of flow of air into the stratosphere is slow. But the difference between spring and autumn readings is greatest just below the stratosphere, and decreases towards the ground. The explanation seems to be that the oscillations originate north of the equator; the spring and summer vegetation flush, which is much more pronounced in the north than the south, takes large amounts of carbon dioxide from the atmosphere. Air moves between the hemispheres at altitudes approaching the stratospheric boundary, and the northern carbon dioxide uptake is reflected in the south, about 6 months later. The CSIRO team has also found that the atmosphere's carbon dioxide concentration increases slightly with altitude in winter and decreases slightly with altitude in summer. This is probably due to the fact that cold water can absorb more of the gas than warm water. As the ocean cools in the winter it takes in large quantities of carbon dioxide, and when it warms up with the approach of summer it releases large amounts. The oceans play a major role in determining atmospheric carbon dioxide content; the water itself has a very large capacity to absorb and release the gas, and marine organisms use and produce large quantities. The amount of carbon dioxide moving between the atmosphere and the oceans each year is 20%, or possibly more, of the total in the atmosphere. By contrast, the amount given off throughout the world by fossil-fuel combustion is less than 1% of the total. If the balance tips Of course the oceans are both a source and a sink while the fossil fuels are only a source. But if the balance between ocean intake and output is upset for any reason, this will influence the atmospheric carbon dioxide content. The fact that only about 15

4 A much greater percentage of the atmosphere's carbon dioxide is transferred each year between ocean and air than between land and air. Burning fuels add to the total amount circulating. Growth in fossil fuel use, and hence carbon dioxide output, is likely for at least another century. These curves are based on two estimates of world oil and coal reserves. half the carbon dioxide given off by burning fuels shows up in increases in concentration in the atmosphere suggests strongly that ocean intake at present exceeds output. One possible cause of a big increase in output would be a rise in the average temperature of the oceans. Another would be an increase in 'upwelling' of cold water rich in carbon dioxide from the ocean depths to the surface. The amount of carbon dioxide transferred between the land and the atmosphere each year is about 5% of the atmospheric total, considerably less than between the oceans and the atmosphere. But here also an upset in the balance between intake (by plants) and output (through burning and decay of plants and respiration) would change the carbon dioxide content of the atmosphere. Could the massive clearing of tropical forest that is now going on produce changes? Detailed research remains to be done, but some simple calculations by Dr Pearman indicate that it may well do so. If timber-getters and farmers clear 0-5% of the forest area a year and retain half the wood, letting the rest decay or burn, the additional carbon dioxide in the atmosphere should be enough to increase the concentration by 0 3 p.p.m. each year. That is about one-third of the measured annual rise. Forest clearing influences carbon dioxide exchange in three ways: Wood gives off the gas when it burns or decays. Trees store carbon for decades after they take it from the air, while crops planted to replace forest return it rapidly as carbon dioxide. (If the crops are eaten, people or animals return it.) Forest removal can lead to the depletion of soil organic material, which produces carbon dioxide as it decays. Dr Pearman's calculations are not based on accurate statistics and he doesn't suggest that they represent the real situation. But they do indicate that destruction of forest may contribute in quite a large way to changes in atmospheric carbon dioxide. Fuel burning, and carbon dioxide output, seem likely to quadruple by the year Past concentrations One of the big problems in assessing the significance of today's rising concentrations is that scientists have very little information on the composition of the atmosphere in the past. Although some measurements of carbon dioxide were made last century, regular precise 16

5 Keeping all stations in line When different groups around the world are recording changing concentrations of some component of the atmosphere, it is important that their readings be equally reliable. The people measuring carbon dioxide have tried to achieve this by all comparing their air samples with standard gas mixtures from one supplier. Unfortunately the system has not worked well in the past; discrepancies as large as 5 p.p.m. have shown up. Dr Graeme Pearman and Dr John Garratt, at the CSIRO Division of Atmospheric Physics, recognized the problem soon after the CSIRO monitoring program began and have played a major role in the search for a solution. instead they operate by comparing the carbon dioxide content of an air sample with that of a reference gas mixture. The gas mixtures are supplied by the Scripps Institution of Oceanography at La Jolla, California. the same reference gas; some give readings that are several parts per million too high and others readings that are too low by a similar margin. Also, the same analysers give different results at different altitudes; this is important, because some measurements are made at mountain stations. All stations except the one atstockholm, Sweden, use mixtures of carbon dioxide and nitrogen as the reference. The gas supplied to the Swedish group also contains the other main ingredients of air, oxygen and argon, in the proportions found in air. A year ago Dr Pearman visited the main monitoring stations around the world. He took along six cylinders of compressed gas mixtures and, with the people at each station, used them to try to work out the size of the measurement errors. The project was largely successful, and most discrepancies have been accounted for. Corrections can now be applied to most readings to bring them into line with Studies by the CSIRO team and scientists at the Swedish station show that the cause of the discrepancies is the use of carbon dioxide in nitrogen rather than The analysing instruments used around carbon dioxide in air as the reference. one another. However some unexplained the world to measure carbon dioxideconcentrations Compounding don't the problem, do the job different directly; discrepancies sometimes as great as types of analyser give different errors with 1 p.p.m. still exist. and rotting vegetation should have a lower ratio than that emitted by the ocean. So should the output of burning oil, coal, and natural gas, because these were once vegetation. measurements did not begin until Have rises extending over decades occurred in the past, or are we seeing something quite new? Members of the CSIRO group recently began two projects that they hope will throw some light on the question. One involves examining the wood of old trees, and should provide information on the relative importance of different sources of atmospheric carbon dioxide over the last 250 years. In the other project, the scientists will soon begin analysing bubbles of air trapped in ancient Antarctic ice. If all goes well, the measurements will give them information on changes in atmospheric carbon dioxide content over the past years or so. For the first experiment, the scientists are now examining portions of two Tasmanian King Billy pines one of them about 150 years old and the other about 250. They count the growth rings to select wood of the age they want. Then they measure isotope ratios in its carbon. The reason why this gives useful information is that plants select the lighter C 12 isotope in preference to C 13 when they take carbon dioxide from the air to build cellulose. The C 13 :C 12 ratio in the wood should vary with, among other factors, the ratio in atmospheric carbon dioxide, and that in turn should vary as the outputs of different sources of the gas change. Because of the preference of plants for C 12, carbon dioxide given off by burning Trees from the north Studies of trees in the Northern Hemisphere show a gradual decrease in the isotope ratio over the last few decades, corresponding with the increasing use of fossil fuels and the rising atmospheric carbon dioxide concentration. But they also show curious fluctuations early this century, which haven't been explained and may be related to variations in climate or atmospheric composition. The CSIRO team has made few measurements so far, but a similar general trend appears to be showing up. The early 1900s fluctuations are there too, but on a much smaller scale. It's early days yet, and many more measurements are needed before detailed interpretation begins. The scientists use a mass spectrometer at the Antarctic Division of the Department of Science in Melbourne for thenisotope ratio measurements. The Antarctic Division is also working with the CSIRO team in the ice bubble project. Research teams overseas have used polar ice to look at the composition of the atmosphere over the centuries, but their measurements haven't been precise enough to show changes in carbon dioxide content. It is a very minor constituent 17

6 after all, making up only about 0 03% of the atmosphere's volume. The general conclusion from previous experiments is that air has not changed in composition over thousands of years, within the rough limits of measurement that have been achieved. The CSIRO and Antarctic Division scientists are confident that their ice cores will yield more accurate carbon dioxide measurements. Their method will be to extract all the gas from an ice block weighing about half a kilogram by grinding or melting it. Then they will measure the carbon isotope ratio in the carbon dioxide. Next, a precisely measured quantity of carbon dioxide with a different isotope ratio will be added. The isotope ratio of the mixture will tell them, much more accurately than direct measurement could, how much carbon dioxide there was in the ice block's air. Isotope ratio measurements may also be able to help the scientists work out details of the origins and movements of the carbon dioxide in the air now; for example they could confirm or disprove that the annual dip in average concentration has a biological cause. Dr Pearman and his colleagues have recently begun measuring the ratio in some of their air samples. Big problems k e Much more collection and analysis of data is needed before scientists will know enough about the causes of changes in atmospheric carbon dioxide content to confidently predict future concentrations. The problems of then relating the predictions to climate forecasts are immense. For example, if increased carbon dioxide caused global temperatures to rise, some polar ice would melt. This should produce further heating, because the sea is much less efficient than ice at reflecting solar energy back into space. As the ice caps retreated, more solar radiation would be converted to heat. However, increased temperatures should cause more evaporation from the oceans, which in turn should produce more cloud. Then the increased cloud cover would reflect more solar radiation back into space and hence tend to lower temperatures. Again, increased atmospheric temperatures would lead to higher ocean temperatures, and this would reduce the ocean's capacity to store carbon dioxide. The result should be further increases in atmospheric carbon dioxide. But these increases should encourage photosynthesis, leading to the removal of more carbon dioxide from the air. Those are just a few of the interrelations that scientists are trying to sort out. It is quite easy to identify them, but vastly more difficult to work out the size of each effect. One big problem is that no way has yet been found to accurately measure global cloud cover. As a result, if small changes in cloudiness that could affect surface temperatures occur, they probably will not be detected. More about the topic 'The CSIRO (Australia) Base-line Atmospheric Carbon Dioxide Monitoring Program Progress Report No. 2.' G. I. Pearman and J. R. Garratt. (CSIRO Division of Atmospheric Physics: Aspendale 1973.) 'The CSIRO (Australia) Base-line Atmospheric Carbon Dioxide Monitoring Program Progress Report No. 3.' G. I. Pearman, J. R. Garratt, P. J. Fraser, D. J. Beardsmore, and J. G. O'Toole. (CSIRO Division of Atmospheric Physics: Aspendale 1975.) Large-scale CO2 fluxes in the Southern Hemisphere troposphere. J. R. Garratt and G. I. Pearman. Nature, Physical Science, 1973, 242, Space and time variations of tropospheric carbon dioxide in the Southern Hemisphere. G. I. Pearman and J. R. Garratt. Tellus, 1973, 25, CO2 concentration in the atmospheric boundary-layer over south-east Australia. J. R. Garratt and G. I. Pearman. Atmospheric Environment, 1973, 7, Global aspects of carbon dioxide. G. I. Pearman and J. R. Garratt. Search, 1972,3, Atmospheric carbon dioxide. J. R. Garratt and G. I. Pearman. Proceedings, International Clean Air Conference, Melbourne, Global pollution and climate change. A. J. Dyer. Proceedings, International Clean Air Conference, Melbourne, Climate: is Australia's changing? G. B. Tucker. Search, 1975, 6, Errors in atmospheric CO2 concentration measurements arising from the use of reference gas mixtures different in composition to the sample air. G. I. Pearman and J. R. Garratt. Tellus, 1975, 27,