In recent years, the temperature of the earth's atmosphere has been warmer than at any time since measurements were first taken in 1860. Overall, it is estimated that the average overall temperature increase is between 0.3 and 0.6°C. In April 1997, it was announced that satellite measurements of the northern hemisphere of the globe showed that spring was arriving seven days earlier than ten years before and that the leaf fall of autumn was taking place four days later. These were just two examples of global warming caused by the so-called greenhouse effect.
The way the greenhouse effect is reported in the media, it would seem that it's a bad thing but in fact it's what keeps our planet habitable. If it wasn't for the presence of carbon dioxide and other gases in the atmosphere above the earth, we would be much colder - probably about -18°C! The greenhouse effect is essential to life on earth.
The greenhouse effect is the mechanism whereby heat from the sun reaches the atmosphere, and the layer of gases allows the short-wave radiation to pass through and warm the air and the earth's surface - just like the glass in a greenhouse. Some of the long-wave radiation (the infrared) given off by the warming process returns to space but most is retained because it cannot pass through the gas layer. The process is summarized in Figure 33.
The main gases responsible for causing the greenhouse effect are carbon dioxide, water vapour, methane, nitrous oxide and chlorofluorocarbons (CFCs). The problems arise when the levels of these gases increase in the atmosphere and make the greenhouse effect stronger. It's like putting thicker glass into a greenhouse and making it more efficient at trapping heat.
Of particular concern is the increase in the amount of carbon dioxide. As a result of human activities, the growing population and our increasing use of fossil fuels, the amount of carbon dioxide entering the atmosphere is rising. The concentration of CO2 is 8 per cent greater now than in 1960
Greenhouse gases ("glass")
Figure 33. The effect of greenhouse gases on warming of the earth's surface by the sun's rays Source: Global Warming; the Greenpeace Report, by permission of Oxford University Press
Greenhouse gases ("glass")
Figure 33. The effect of greenhouse gases on warming of the earth's surface by the sun's rays Source: Global Warming; the Greenpeace Report, by permission of Oxford University Press and 25 per cent more than in the early 1800s. The rate of increase is such that, if we carry on as we are, the concentration will double from present-day levels in the next 100 years.1
For thousands of years, there was a balance between the greenhouse gases that were produced naturally (with the exception of CFCs which are artificial substances) and their absorption by oxidation and photoGLOBAL WARMING 97
synthesis. Now it appears that the balance is upset and the gases are being produced faster than they can be utilized. Figure 34 shows the increase in input of carbon dioxide into the atmosphere in the past 100 years.
Although there is a great deal of concern about the warming of the planet, the temperature has been at least as high in the past as it is now. There are various ways of obtaining information on the temperature of the earth and the concentration of carbon dioxide in the atmosphere from thousands of years in the past. For example, Swiss scientists have collected air bubbles trapped in polar ice caps that have been there for centuries and they have analysed their carbon dioxide content.2 The layers of the ice have been dated using radioactive dating techniques based on the ratio of the different oxygen isotopes. This information is then correlated with estimates of the earth's temperature at the time which can be obtained from fossil records or looking at different types of pollen. The results of this type of investigation are shown in Figure 35.
If there is a true correlation between the temperature of the earth and the amount of CO2, then a doubling of the amount of greenhouse gases will give rise to an overall temperature increase of between 1.4 and 4.5°C.
There is still some controversy between scientists about global warming. Although an international panel of experts have assessed all the available data and accepted that global warming is happening, there are others who say that the increase in temperature is nothing more than the natural recovery from the cool of the so-called Little Ice Age which occurred between 1550 and about 1850. It has also been said that the earth will respond to the increase in CO2 because it will promote the photosynthesis of plants. After all, returning to the observation about
160 150 80 40 0
Thousands of years ago Present
Figure 35. Changes of temperature and CO2 over time
160 150 80 40 0
Figure 35. Changes of temperature and CO2 over time earlier springs and later autumns made at the beginning of this chapter, the early arrival of greenery on trees and plants and its prolonged presence means that more of the atmospheric CO2 is being used up by plants. A recent book has put forward an alternative to the 'greenhouse gas theory' for global warming.3 It is based on the interplay between cosmic rays and the fluctuating output from the sun. Cosmic rays help in the formation of the earth's cloud cover which causes a cooling of the earth, whilst when the sun is active solar winds reduce the cosmic rays. The decrease in the cosmic rays reduces cloud cover and the earth warms up. It is proposed that the variations of these two factors explain not only the climate change observed now but also that of past aeons.
Another factor to consider is the movement of the earth around the sun. In the last million years, there have been regular changes in the earth's temperature, with ice ages lasting for about 100,000 years followed by warmer spells lasting about 10-20,000 years. Because of the change in the angle that the sun impinges on the earth there have been ten such cycles in just over one million years and, at the present time, we are nearing the end of a warm spell that began 10,000 years ago. These patterns of temperature change have been studied by astronomers and have been found to be linked to changes in the earth's movement round the sun4 as shown in Figure 36. Although these natural changes of temperature are known about, the scientists studying global warming say that
(f) 19,000-23,000 year cycle
(b) 19,000-23,000 year cycle
(f) 19,000-23,000 year cycle
(b) 19,000-23,000 year cycle
Figure 36. The tilts and wobbles in the earth's circuit round the sun which affect the climate
Figure 36. The tilts and wobbles in the earth's circuit round the sun which affect the climate the rate of temperature increase is faster than would have occurred in the normal course of events.
One of the most important factors to take into account in the relationship between temperature change and the increase in greenhouse gases is the role of the oceans. These cover 70 per cent of the earth's surface and this salt water acts like a giant thermostat, by absorbing heat from the atmosphere when the air is warm and then releasing it when the air gets cold. It also absorbs carbon dioxide as this first dissolves in the water and is then utilized by phytoplankton in their photosynthesis. This exchange between the air and the oceans takes place on a huge scale -involving far greater amounts than those emitted to the atmosphere from human activities, as shown by the following figures:
Carbon dioxide absorbed by the oceans each year = 105 billion tonnes Carbon dioxide released by the oceans each year = 102 billion tonnes Carbon dioxide emitted into the atmosphere from human activities = 6 billion tonnes
From these figures it is clear that the fate of any excess carbon dioxide in the atmosphere is influenced by the number of phytoplankton in the surface layers of the oceans. For example, it has been estimated that the amount of phytoplankton in the oceans at the time of the last ice age was nearly 50 per cent more than today.5 The photosynthetic activity of these organisms reduced the amount of carbon dioxide in the atmosphere and this in turn allowed more infra-red radiation to escape from the earth (the greenhouse 'glass' was thinner). The result of all these interactions was that the temperature of the earth was about 10°C cooler than it is now.
The fate of the increased CO2 being released into the atmosphere as a result of human activities depends a great deal on the number of phyto-plankton in the world's oceans. Not much is known about any changes in their numbers but there is a lot of research taking place. The problem is the lack of information from thousands of years ago when obviously no one was taking systematic measurements. It could be that the increased amount of CO2 will activate the growth rate of the phytoplankton and this will then result in a reduction in the atmospheric CO2. There are other factors to consider, one being the absorption of heat by the oceans. As the earth's temperature increases, the ocean waters also slowly warm up as a result of absorbing this heat and the circulation patterns distribute the heat to different parts of the globe. Another factor is the influence of cloud cover. The increased temperature may give rise to more clouds because of the higher rate of evaporation of sea water. Clouds reflect sunlight and therefore less solar radiation reaches the ground, with the result that not as much heat is absorbed by the earth. On the other hand, water vapour is a 'greenhouse gas' and helps retain heat, although it is estimated that clouds are twice as effective at reflecting sunlight back into the atmosphere as they are at retaining heat.
You will realize from this that there are many arguments about global warming and we still have much to learn. All we have at the moment are predictions and some evidence that the earth is getting warmer, and we have to act on this information.
The best estimates that we have are that the amount of CO2 in the atmosphere will double in the next hundred years and that this will result in an increase in temperature of 4.2°C. What is the likely effect of this in northern Europe? Table 22 sets out some likely scenarios.
These predictions, if correct, will make a profound difference to life in northern Europe. An indication of how great a change may occur can be obtained by looking at historical records of the early Middle Ages (ad 800-1200) when it seems that the average temperature was 1°C higher than at present. At that time, it was possible to grow corn as far north as Iceland, there were trees growing in northern Scandinavia (now it is only mossy tundra) and in southern England, apricots, peaches and grapes were grown outdoors.
Table 22. Climate change in northern Europe predicted from a doubling of atmospheric CO2
Mean annual temperature Mean winter temperature Mean summer temperature Length of growing season Length of winter (temperature
Increased by 4-5°C Increased by 5-6°C Increased by 2-3.5°C Increased by 70-150 days Decreased by 2-4 months below freezing) Length of summer (temperature
Increased by 2-3 months above 10°C) Mean annual rainfall
Obviously with temperature rise of over 4°C, the effects on agriculture will be far more profound and southern Britain would have a climate like the Mediterranean countries today. There would also be much more productive land in northern Scandinavia and Russia. This might seem very desirable, but there is also a marked disadvantage to this predicted global warming. Experts expect that there would be an increase in the rate of desert formation in large areas of the USA, China, North Africa and other countries where agriculture is already difficult because of infrequent rainfall.
Another prediction about global warming is of a general change in our climate. It is expected that, overall, rainfall will increase but summers will be drier. This means that winters will be much wetter and the rain will be accompanied by more ferocious storms which will cause structural damage as well as localized flooding. Some evidence of the change in rainfall in Scotland is obtained by looking at the flow rates of rivers. Figure 37 shows the change in the average flow rate of the River Clyde over the 30-year period, 1966-96.
This steady increase in average flow rate conceals another effect and that is that the low flows of summer have not changed. In other words, the increased average flows have occurred because of more rainfall in winter. Heavy rain in the winters of 1994 and 1996 gave rise to severe and damaging floods in the West of Scotland with some of the highest ever flow rates in the rivers. Since these flow data were published, more measurements have been made of the flow of the Clyde. The upward trend in average flow rate has declined, but this can be attributed to some particularly dry summers, especially those of 1995 and 1996.
River Clyde at Daldowie Average Flows
co to r^ ay co co co oo O) O) o o co to r^ ay co co co oo O) O) o o
Figure 37. Change in the average flow rate of the River Clyde, 1963-95
1. A. Nilsson, Greenhouse Earth, Wiley, Chichester, 1992.
2. H. Oeschger and I.M. Mintzer, 'Lessons from the ice cores: rapid climate change during the last 160,000 years', in M. Mintzer (ed) Confronting Climate Change: Risks, Implications and Responses, Cambridge University Press, Cambridge, 1992.
3. Nigel Calder, The Manic Sun - Weather Theories Confounded, Pilkington, London, 1997.
4. W.R. Peltier, 'Our fragile inheritance', in C. Mungall and D.J. McLaren (eds) Planet Under Stress: The Challenge of Global Change, Oxford University Press, Oxford, 1990.
5. Oceans and the Global Carbon Cycle, Biogeochemical Ocean Flux Study (BOFS), BOFS Office, Plymouth Marine Laboratory, Plymouth, 1989.
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