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Greenhouse gases and global warming

Module by: Free High School Science Texts Project. E-mail the author

Greenhouse gases and global warming

The heating of the atmosphere

As we mentioned earlier, the distance of the earth from the sun is not the only reason that temperatures on earth are within a range that is suitable to support life. The composition of the atmosphere is also critically important.

The earth receives electromagnetic energy from the sun in the visible spectrum. There are also small amounts of infrared and ultraviolet radiation in this incoming solar energy. Most of the radiation is shortwave radiation, and it passes easily through the atmosphere towards the earth's surface, with some being reflected before reaching the surface. At the surface, some of the energy is absorbed, and this heats up the earth's surface. But the situation is a little more complex than this.

A large amount of the sun's energy is re-radiated from the surface back into the atmosphere as infrared radiation, which is invisible. As this radiation passes through the atmosphere, some of it is absorbed by greenhouse gases such as carbon dioxide, water vapour and methane. These gases are very important because they re-emit the energy back towards the surface. By doing this, they help to warm the lower layers of the atmosphere even further. It is this 're-emission' of heat by greenhouse gases, combined with surface heating and other processes (e.g. conduction and convection) that maintain temperatures at exactly the right level to support life. Without the presence of greenhouse gases, most of the sun's energy would be lost and the Earth would be a lot colder than it is! A simplified diagram of the heating of the atmosphere is shown in Figure 1.

Figure 1: The heating of the Earth's atmosphere
Figure 1 (CG11C10_002.png)

The greenhouse gases and global warming

Many of the greenhouse gases occur naturally in small quantities in the atmosphere. However, human activities have greatly increased their concentration, and this has led to a lot of concern about the impact that this could have in increasing global temperatures. This phenomenon is known as global warming. Because the natural concentrations of these gases are low, even a small increase in their concentration as a result of human emissions, could have a big effect on temperature. But before we go on, let's look at where some of these human gas emissions come from.

  • Carbon dioxide (CO22) Carbon dioxide enters the atmosphere through the burning of fossil fuels (oil, natural gas, and coal), solid waste, trees and wood products, and also as a result of other chemical reactions (e.g. the manufacture of cement). Carbon dioxide can also be removed from the atmosphere when it is absorbed by plants during photosynthesis.
  • Methane (CH44) Methane is emitted when coal, natural gas and oil are produced and transported. Methane emissions can also come from livestock and other agricultural practices and from the decay of organic waste.
  • Nitrous oxide (N22O) Nitrous oxide is emitted by agriculture and industry, and when fossil fuels and solid waste are burned.
  • Fluorinated gases (e.g. hydrofluorocarbons, perfluorocarbons, and sulfur hexafluoride) These gases are all synthetic, in other words they are man-made. They are emitted from a variety of industrial processes. Fluorinated gases are sometimes used in the place of other ozone-depleting substances (e.g. CFC's). These are very powerful greenhouse gases, and are sometimes referred to as High Global Warming Potential gases ('High GWP gases').

Overpopulation is a major problem in reducing greenhouse gas emissions, and in slowing down global warming. As populations grow, their demands on resources (e.g. energy) increase, and so does their production of greenhouse gases.

Ice core drilling - Taking a look at earth's past climate

Global warming is a very controversial issue. While many people are convinced that the increase in average global temperatures is directly related to the increase in atmospheric concentrations of carbon dioxide, others argue that the climatic changes we are seeing are part of a natural pattern. One way in which scientists are able to understand what is happening at present, is to understand the earth's past atmosphere, and the factors that affected its temperature.

So how, you may be asking, do we know what the earth's past climate was like? One method that is used is ice core drilling. Antarctica is the coldest continent on earth, and because of this there is very little melting that takes place. Over thousands of years, ice has accumulated in layers and has become more and more compacted as new ice is added. This is partly why Antarctica is also on average one of the highest continents! On average, the ice sheet that covers Antarctica is 2500 m thick, and at its deepest location, is 4700 m thick.

As the snow is deposited on top of the ice sheet each year, it traps different chemicals and impurities which are dissolved in the ice. The ice and impurities hold information about the Earth's environment and climate at the time that the ice was deposited. Drilling an ice core from the surface down, is like taking a journey back in time. The deeper into the ice you venture, the older the layer of ice. By analysing the gases and oxygen isotopes that are present (along with many other techniques) in the ice at various points in the earth's history, scientists can start to piece together a picture of what the earth's climate must have been like.

Figure 2
Figure 2 (CG11C10_003.png)

One of the most well known ice cores was the one drilled at a Russian station called Vostok in central Antarctica. So far, data has been gathered for dates as far back as 160 000 years!

Case Study : Looking at past climatic trends

Make sure that you have read the 'Information box' on ice core drilling before you try this activity.

The values in the table below were extrapolated from data obtained by scientists studying the Vostok ice core. 'Local temperature change' means by how much the temperature at that time was different from what it is today. For example, if the local temperature change 160 000 years ago was -9C, this means that atmospheric temperatures at that time were 9C lower than what they are today. 'ppm' means 'parts per million' and is a unit of measurement for gas concentrations.

Table 1
-textbfYears before present (x 1000) Local temperature change (C) Carbon dioxide (ppm)
160 -9 190
150 -10 205
140 -10 240
130 -3 280
120 +1 278
110 -4 240
100 -8 225
90 -5 230
80 -6 220
70 -8 250
60 -9 190
50 -7 220
40 -8 180
30 -7 225
20 -9 200
10 -2 260
0 (1850) -0.5 280
Present   371

Questions

  1. On the same set of axes, draw graphs to show how temperature and carbon dioxide concentrations have changed over the last 160 000 years. Hint: 'Years before present' will go on the x-axis, and should be given negative values.
  2. Compare the graphs that you have drawn. What do you notice?
  3. Is there a relationship between temperature and the atmospheric concentration of carbon dioxide?
  4. Do these graphs prove that temperature changes are determined by the concentration of gases such as carbon dioxide in the atmosphere? Explain your answer.
  5. What other factors might you need to consider when analysing climatic trends?

The consequences of global warming

Group Discussion : The impacts of global warming

In groups of 3-4, read the following extracts and then answer the questions that follow.

By 2050 Warming to Doom Million Species, Study Says

By 2050, rising temperatures exacerbated by human-induced belches of carbon dioxide and other greenhouse gases could send more than a million of Earth's land-dwelling plants and animals down the road to extinction, according to a recent study. "Climate change now represents at least as great a threat to the number of species surviving on Earth as habitat-destruction and modification," said Chris Thomas, a conservation biologist at the University of Leeds in the United Kingdom.

The researchers worked independently in six biodiversity-rich regions around the world, from Australia to South Africa, plugging field data on species distribution and regional climate into computer models that simulated the ways species' ranges are expected to move in response to temperature and climate changes. According to the researchers' collective results, the predicted range of climate change by 2050 will place 15 to 35 percent of the 1 103 species studied at risk of extinction.

National Geographic News, 12 July 2004

Global Warming May Dry Up Africa's Rivers, Study Suggests

Many climate scientists already predict that less rain will fall annually in parts of Africa within 50 years due to global warming. Now new research suggests that even a small decrease in rainfall on the continent could cause a drastic reduction in river water, the lifeblood for rural populations in Africa.

A decrease in water availability could occur across about 25 percent of the continent, according to the new study. Hardest hit would be areas in northwestern and southern Africa, with some of the most serious effects striking large areas of Botswana and South Africa.

To predict future rainfall, the scientists compared 21 of what they consider to be the best climate change models developed by research teams around the world. On average, the models forecast a 10 to 20% drop in rainfall in northwestern and southern Africa by 2070. With a 20% decrease, Cape Town would be left with just 42% of its river water, and "Botswana would completely dry up," de Wit said. In parts of northern Africa, river water levels would drop below 50%.

Less river water would have serious implications not just for people but for the many animal species whose habitats rely on regular water supplies.

National Geographic News, 3 March 2006

Discussion questions

  1. What is meant by 'biodiversity'?
  2. Explain why global warming is likely to cause a loss of biodiversity.
  3. Why do you think a loss of biodiversity is of such concern to conservationists?
  4. Suggest some plant or animal species in South Africa that you think might be particularly vulnerable to extinction if temperatures were to rise significantly. Explain why you chose these species.
  5. In what way do people, animals and plants rely on river water?
  6. What effect do you think a 50% drop in river water level in some parts of Africa would have on the people living in these countries?
  7. Discuss some of the other likely impacts of global warming that we can expect (e.g. sea level rise, melting of polar ice caps, changes in ocean currents).

Taking action to combat global warming

Global warming is a major concern at present. A number of organisations, panels and research bodies have been working to gather accurate and relevant information so that a true picture of our current situation can be painted. One important orgaisation that you may have heard of is the Intergovernmental Panel on Climate Change (IPCC). The IPCC was established in 1988 by two United Nations organizations, the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP), to evaluate the risk of climate change brought on by humans. You may also have heard of the Kyoto Protocol, which will be discussed a little later.

Group Discussion : World carbon dioxide emissions

The data in the table below shows carbon dioxide emissions from the consumption of fossil fuels (in million metric tons of carbon dioxide).

Table 2
Region or Country 1980 1985 1990 1995 2000 2004
United States 4754 4585 5013 5292 5815 5912
Brazil 186 187 222 288 345 336
France 487 394 368 372 399 405
UK 608 588 598 555 551 579
Saudi Arabia 175 179 207 233 288 365
Botswana 1.26 1.45 2.68 3.44 4.16 3.83
South Africa 234 298 295 344 378 429
India 299 439 588 867 1000 1112
World Total 18333 19412 21426 22033 23851 27043

Questions

  1. Redraw the table and use a coloured pen to highlight those countries that are 'developed' and those that are 'developing'.
  2. Explain why CO22 emissions are so much higher in developed countries than in developing countries.
  3. How does South Africa compare to the other developing countries, and also to the developed countries?

Carbon dioxide emissions are a major problem worldwide. The Kyoto Protocol was signed in Kyoto, Japan in December 1997. Its main objective was to reduce global greenhouse gas emissions by encouraging countries to become signatories to the guidelines that had been laid out in the protocol. These guidelines set targets for the world's major producers to reduce their emissions within a certain time. However, some of the worst contributors to greenhouse gas emissions (e.g. USA) were not prepared to sign the protocol, partly because of the potential effect this would have on the country's economy, which relies on industry and other 'high emission' activities.

Panel discussion

Form groups with 5 people in each. Each person in the group must adopt one of the following roles during the discussion:

  • the owner of a large industry
  • an environmental scientist
  • an economist
  • a politician
  • a chairperson for the discussion

In your group, you are going to discuss some of the economic and environmental implications for a country that decides to sign the Kyoto Protocol. Each person will have the opportunity to express the view of the character they have adopted. You may ask questions of the other people, or challenge their ideas, provided that you ask permission from the chairperson first.

Summary

  • The atmosphere is the layer of gases that the surrounds Earth. These gases are important in sustaining life, regulating temperature and protecting the Earth from harmful radiation.
  • The gases that make up the atmosphere are nitrogen, oxygen, carbon dioxide, argon and others e.g. water vapour, methane.
  • There are four layer in the atmosphere, each with their own characteristics.
  • The troposphere is the lowest layer and here, temperature decreases with an increase in altitude. The troposphere is where weather occurs.
  • The next layer is the stratosphere where temperature increases with an increase in altitude because of the presence of ozone in this layer, and the direct heating from the sun.
  • The depletion of the ozone layer is largely because of CFC's, which break down ozone through a series of chemical reactions.
  • The mesosphere is characterised by very cold temperatures and meteor collisions. The mesosphere contains high concentrations of metal atoms.
  • In the thermosphere, neutral atoms are ionised by UV and X-ray radiation from the sun. Temperature increases with an increase in altitude because of the energy that is released during this ionisation process, which occurs mostly in the upper thermosphere.
  • The thermosphere is also known as the ionosphere, and is the part of the atmosphere where radio waves can be transmitted.
  • The auroras are bright coloured skies that occur when charged particles collide with atoms in the upper atmosphere. Depending on the type of atom, energy is released as light at different wavelengths.
  • The Earth is heated by radiation from the sun. Incoming radiation has a short wavelength and some is absorbed directly by the Earth's surface. However, a large amount of energy is re-radiated as longwave infrared radiation.
  • Greenhouse gases such as carbon dioxide, water vapour and methane absorb infrared radiation and re-emit it back towards the Earth's surface. In this way, the bottom layers of the atmsophere are kept much warmer than they would be if all the infrared radiation was lost.
  • Human activities such as the burning of fossil fuels, increase the concentration of greenhouse gases in the atmosphere and may contribute towards global warming.
  • Some of the impacts of global warming include changing climate patterns, rising sea levels and a loss of biodiversity, to name a few. Interventions are needed to reduce this phenomenon.

Summary Exercise

  1. The atmosphere is a relatively thin layer of gases which support life and provide protection to living organisms. The force of gravity holds the atmosphere against the earth. The diagram below shows the temperatures associated with the various layers that make up the atmosphere and the altitude (height) from the earth's surface.
    Figure 3
    Figure 3 (CG11C10_004.png)
    1. Write down the names of the layers A, B and D of the atmosphere.
    2. In which one of the layers of the atmosphere is ozone found?
    3. Give an explanation for the decrease in temperature as altitude increases in layer A.
    4. In layer B, there is a steady increase in temperature as the altitude increases. Write down an explanation for this trend.
  2. Planet Earth in Danger It is now accepted that greenhouse gases are to blame for Planet Earth getting warmer. The increase in the number of sudden floods in Asia and droughts in Africa; the rising sea level and increasing average temperatures are global concerns. Without natural greenhouse gases,like carbon dioxide and water vapour,life on earth is not possible. However, the increase in levels of carbon dioxide in the atmosphere since the Industrial Revolution is of great concern. Greater disasters are to come, which will create millions of climate refugees. It is our duty to take action for the sake of future generations who will pay dearly for the wait-and-see attitude of the current generation. Urgent action to reduce waste is needed. Global warming is a global challenge and calls for a global response now, not later. (Adapted from a speech by the French President, Jacques Chirac)
    1. How do greenhouse gases, such as carbon dioxide, heat up the earth's surface?
    2. Draw a Lewis structure for the carbon dioxide molecule
    3. The chemical bonds within the carbon dioxide molecule are polar. Support this statement by performing a calculation using the table of electronegativities.
    4. Classify the carbon dioxide molecule as polar or non-polar. Give a reason for your answer.
    5. Suggest ONE way in which YOU can help to reduce the emissions of greenhouse gases.
  3. Plants need carbon dioxide (CO22) to manufacture food. However, the engines of motor vehicles cause too much carbon dioxide to be released into the atmosphere.
    1. State the possible consequence of having too much carbon dioxide in the atmosphere.
    2. Explain two possible effects on humans if the amount of carbon dioxide in the atmosphere becomes too low.
    (DoE Exemplar Paper Grade 11, 2007)

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