Global warming. Models for global warming Sand analogy

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1 8.10 Global warming Assessment statements Describe some possible models of global warming State what is meant by the enhanced greenhouse effect Identify the increased combustion of fossil fuels as the likely major cause of the enhanced greenhouse effect Describe the evidence that links global warming to increased levels of greenhouse gases Outline some of the mechanisms that may increase the rate of global warming. We can see from our model of the greenhouse effect that the temperature of the Earth depends on several factors. For example, if the amount of radiation coming from the Sun were to increase then there would be more energy reaching the Earth. This would cause the temperature of the Earth to increase until equilibrium is restored; this is called global warming. Models for global warming The factors affecting the temperature of the planet are very complex and interrelated. Physicists use their knowledge to make mathematical models so that they can predict the outcome of changing variables. However, this problem is rather more difficult than an ideal gas or any other system considered so far in this course, and the equations are equally difficult to solve. To solve these equations, physicists make computer models and program computers to do the millions of calculations required. To show how these are built up, we will consider an analogous situation. Sand analogy It is difficult to imagine what is happening to all the energy flowing in and out of the Earth. To make this easier to visualize, we can consider an analogous situation such as loading sand onto a truck. Imagine you are filling a truck with sand. As you put sand onto the truck someone else takes off a fixed percentage of the complete load. As the amount on the truck increases, they take off more until they take off as much as you put on an equilibrium is reached. 100 kg in 10 kg out leaves 90 kg Figure 8.0 In the sand analogy, if 100 kg are put in each minute and 10% is taken out each minute then the outcome would be as shown. 100 kg in makes 190 kg 19 kg out leaves 171 kg 301

2 8 Energy, power and climate change To find out how the load varies until equilibrium is reached we have to do a lot of calculations, or we can use a spreadsheet to do it for us (See Figure 8.1). Once the data has been entered into a spreadsheet it is a simple matter of copying the formula down in as many rows as you want. If you do this, then make sure you copy row 3 not row, since row only contains zeros. If this is copied down to 100 minutes you will see that equilibrium has been reached when the load contains 900 kg, as every time another 100 kg is added, 100 kg will be taken out. Once you have made this model it is very easy to change the variables to see what would happen. You can try adding more sand by changing column D or taking more out by changing the factor 0.1 in column E. Figure 8.1 shows how the sand analogy would be put into a spreadsheet and how the results can be displayed as a graph of load against time. The amount taken out is *(D3 F) C Time (mins) 3 D In (kg) E Out (kg) F Load (kg) The amount left is what was there what was added what was taken out F D3 E load/kg time/min Global warming is not as simple as the sand model but it s the same principle. To see a simple spreadsheet model for the Earth (without the greenhouse effect), visit enter the express code 6P and click on Weblink 8.9. Here you will also find a lot of information about climate change and details of the other simulations they are running. Modelling global warming The energy flow for the Earth and atmosphere is rather more complicated than the sand analogy. One complication is the greenhouse effect, which would be like putting some of the sand back into the truck. Computer simulations that model the climate of the Earth must take all factors into consideration, and that takes a lot more computer power than even the fastest home computer. One concerned group, Climate prediction, have been doing calculations on thousands of private computers in homes and schools all around the world to gain the power needed to run their computer model. Exercises 3 Try making the spreadsheet model as above. See what happens if you change the variables. 30

3 Causes of global warming By analysing the energy flow diagram in Figure 8.38 we can see that there are several ways that the temperature of the Earth could increase. The radiation from the Sun The radiation from the Sun is not constant. If the amount of radiation incident on the Earth increases, then its temperature would increase. There are several factors that affect this: Solar flares/sunspots Sunspots are black spots on the surface of the Sun that can be seen if you look at the Sun through a sufficiently dark filter. The spots are cool areas. However, when there are a lot of sunspots, the Sun emits more energy due to the increased temperature of the gas surrounding the spot. The number of sunspots varies on an 11-year cycle. Earth s orbit The Earth s orbit is not circular but elliptical; this means that the distance between the Earth and the Sun is not constant. There are also other variations due to the change of angle of the Earth s axis in relation to the Sun. These are called Milankovitch cycles. Enhanced greenhouse effect If the amount of greenhouse gases in the atmosphere is increased, then the amount of radiation absorbed by the atmosphere increases. The amount of energy leaving the Earth is reduced and the temperature of the Earth would rise until equilibrium was restored. The greenhouse gases include water, carbon dioxide, methane and nitrous oxide. Water is the biggest contributor to the greenhouse effect, with carbon dioxide second. Methane has a much bigger effect, but there isn t so much of it in the atmosphere. Ice cores For hundreds of thousands of years the ice of Antarctica has been growing. Each year a new layer is added on top of the old, so that the ice that is there now is made up of thousands of layers, each layer representing a year s growth (like the rings of a tree). By drilling into the ice with a hollow drill it is possible to extract samples (ice cores) that were laid down thousands of years ago. From the concentration of the different isotopes of hydrogen in the water it is possible to determine the temperature of the layers: more heavy isotopes means the temperature was colder. These layers of ice also contain bubbles of air that have been trapped since the ice was laid down; from these bubbles we can find how the composition of the atmosphere has changed since the ice was formed. If we compare the temperature of the Earth with the concentration of CO we get an interesting result, as shown in Figure 8.. Comparing these two graphs we can see that when the concentration of CO increases, so does the temperature. The increase in temperature as a result of many sunspots together leads to solar flares, jets of gas flying out from the Sun s surface like huge flames. Greenhouse gases There are many gases that contribute to the greenhouse effect and the complete picture of their combined contribution is very complicated and out of the scope of this book. To simplify matters we will only deal with CO since this is the major contributor. 303

4 8 Energy, power and climate change Figure 8. Data from the Vostok Antarctic ice core can be explained using the greenhouse gas model, however this piece of evidence alone is not enough to say that temperature depends on CO concentration. If the results from the ice core were a class practical, then you would devise an experiment to test the hypothesis that, when the concentration of CO increases so does the temperature (making sure that the variables were controlled). It s of course not possible to experiment with the amount of CO in the atmosphere so computer simulations may be used instead. The temperature scale is the difference in temperature between the temperature calculated from the hydrogen isotopes in the ice core and the average temperature now. To find data from the ice cores, visit enter the express code 6P and click on the Weblink CO /ppmv temperature/ C More recent data thousands of years ago From measurements of the average temperature of the Earth and the amount of CO in the atmosphere, it can be seen that there could be a relationship between the two. The graphs in Figure 8.3 show both have risen in the past 50 years. Notice there is regular yearly variation in CO, the maximum coming after the northern hemisphere winter; this is because during the summer, plants absorb CO from the atmosphere. Figure 8.3 Graphs to show the temperature anomaly (difference between the measured temperature and the average temperature) and CO concentration measured since temperature anomaly/ C year CO /ppmv year What causes the change in CO? It is widely believed that the increase in CO is due to human activity, mainly because of the burning of fossil fuels which produces the gas, and deforestation which removes plants that would normally absorb the gas. This hypothesis is supported by the increased use of fossil fuels during the past 50 years (see Figure 8.).

5 growth in fossil fuel consumption/year natural gas coal year oil 1985 Figure 8. The exponential increase in fossil fuel consumption has contributed to the increase in CO in the atmosphere What might happen and what can be done? Assessment statements Define coefficient of volume expansion State one possible effect of the enhanced greenhouse effect Outline possible reasons for a predicted rise in mean sea-level Identify climate change as an outcome of the enhanced greenhouse effect Solve problems related to the enhanced greenhouse effect Identify some possible solutions to reduce the enhanced greenhouse effect Discuss international efforts to reduce the enhanced greenhouse effect. The Steigletscher glacier in Switzerland photographed in 199 (top) and 006 (bottom). Glaciers are shrinking like this all round the world. To find out what would happen if the average temperature of the Earth were to increase, we can look at what has happened during the past 50 years, especially global warming in recent years, or we can use computer simulations. 305

6 8 Energy, power and climate change Rise in sea level As the temperature of a liquid increases, it expands. The relationship between the increase in volume ( V) and the temperature change ( T) is given by the formula V 5 V 0 T where 5 coefficient of volume expansion V 0 5 the original volume If this is applied to water, then we can conclude that if the average temperature of the oceans increases then they will expand. This has already been happening; over the past 100 years sea level has risen by 0 cm. Trying to predict what will happen as the sea temperature increases is complicated by the anomalous expansion of water. Unlike a lot of other liquids, water does not expand uniformly, in fact from 0 C to C water actually contracts and then from C upwards it expands. Trying to calculate what happens as different bodies of water expand and contract is very difficult, but most models predict some rise in sea level. Positive feedback Melting ice caps have dual impact. We have discussed the way that the amount of radiation reflected off the Earth is an important factor in determining the Earth s temperature. The ice caps are white and therefore reflect a high amount of radiation (their albedo is high). If they melt, then the amount of reflected radiation would be reduced, causing the temperature to rise further; this is called positive feedback. Another factor that could cause the sea level to rise is the melting of the ice caps. This is the ice that covers the land masses of Antarctica, Greenland and the glaciers in mountainous regions. It doesn t include the Arctic since this ice is floating. Floating ice displaces its own mass of water so when it melts it makes no difference. If the ice caps melt and the water runs into the sea, then it could make the sea level rise so much that some countries could disappear under water. Change in the weather The other obvious consequence of the enhanced greenhouse effect is a change in the weather. What would happen is, again, difficult to predict but most models agree that countries near the equator will get hotter and countries in the northern hemisphere will get wetter. Solutions To reduce the enhanced greenhouse effect, the levels of greenhouse gases must be reduced, or at the very least, the rate at which they are increasing must be slowed down. There are several ways that this can be achieved: 1 Greater efficiency of power production In recent years the efficiency of power plants has been increasing significantly. According to the second law of thermodynamics, they can never be 100% efficient but some of the older less efficient ones could be replaced. This would mean that to produce the same amount of power would require less fuel, resulting in reduced CO emission. Replacing the use of coal and oil with natural gas Gas-fired power stations are more efficient than oil and gas and produce less CO. 3 Use of combined heating and power systems (CHP) Using the excess heat from the power station to heat homes would result in a more efficient use of fuel. 306

7 Increased use of renewable energy sources and nuclear power Replacing fossil fuel burning power stations with alternative forms such as wave power, solar power and wind power would reduce CO emissions. 5 Use of hybrid vehicles A large amount of the oil used today is used for transport, and even without global warming, there will be a problem when the oil runs out. Cars that run on electricity or a combination of electricity and petrol (hybrid) are already in production. Aeroplanes will also have to use a different fuel. 6 Carbon dioxide capture and storage A different way of reducing greenhouse gases is to remove CO from the waste gases of power stations and store it underground. An international problem Global warming is an international problem, and if any solution is going to work then it must be a joint international solution. Before working on the solution the international community had to agree on pinpointing the problem and it was to this end that the Intergovernmental Panel on Climate Change (IPCC) was formed. IPCC In 1988 the World Meteorological Organisation (WMO) and the United Nations Environmental Programme (UNEP) established the IPCC, the panel of which was open to all members of the UN and WMO. Its role was not to carry out research but to assess all the available information relating to human induced climate change. An excerpt from the first report of Working Group I in 1990 states: The experts concluded that they are certain that emissions from human activities are substantially increasing the atmospheric concentrations of greenhouse gases and that this will enhance the greenhouse effect and result in an additional warming of the Earth s surface. Kyoto Protocol In 1997 the Kyoto Protocol was open for signature: countries ratifying this treaty committed to reduce their greenhouse gas emissions by given percentages. By January 009, 181 countries had signed and ratified. Asia-Pacific Partnership on Clean Development and Climate (APPCDC) This is a non-treaty agreement between six countries that account for 50% of the greenhouse emissions. The countries involved agreed to cooperate on the development and transfer of technology with the aim of reducing greenhouse emissions. To access the websites of the IPCC, the UN and the Asia-Pacific Partnership, visit enter the express code 6P and click on Weblinks 8.11, 8.1 and