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Common Questions about Climate Change
Published in 1997 by the United Nations Environment Programme - World Meteorological Organization




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See: Information from Encyclopædia Britannica about global warming


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What Human Activities Contribute to Climate Change?

The burning of coal, oil, and natural gas, as well as deforestation and various agricultural and industrial practices, are altering the composition of the atmosphere and contributing to climate change. These human activities have led to increased atmospheric concentrations of a number of greenhouse gases, including carbon dioxide, methane, nitrous oxide, chlorofluorocarbons, and ozone in the lower part of the atmosphere. The relative importance of these gases is shown in Figure 4.1.

Figure 4.1

Relative importance of the various greenhouse gases and small particles currently in the atmosphere. Bars extending above the horizontal line indicate a warming effect. Bars extending below the horizontal line indicate a cooling effect. The impacts of tropospheric ozone, stratospheric ozone, and particles are quite uncertain. The range of possible effects for these gases is indicated by the red bar; i.e., the effect is in the range of one end of the red bar to the other.

Carbon dioxide is produced when coal, oil, and natural gas (fossil fuels) are burned to produce energy used for transportation, manufacturing, heating, cooling, electricity generation, and other applications. The use of fossil fuel currently accounts for 80 to 85% of the carbon dioxide being added to the atmosphere.

Land use changes, e.g., clearing land for logging, ranching, and agriculture, also lead to carbon dioxide emissions. Vegetation contains carbon that is released as carbon dioxide when the vegetation decays or burns. Normally, lost vegetation would be replaced by re-growth with little or no net emission of carbon dioxide. However, over the past several hundred years, deforestation and other land use changes in many countries have contributed substantially to atmospheric carbon dioxide increases. Although deforestation is still occurring in some parts of the northern hemisphere, on the whole, re-growth of vegetation in the north appears to be taking some carbon dioxide out of the atmosphere. Most of the net carbon dioxide emissions from deforestation are currently occurring in tropical regions. Land use changes are responsible for 15 to 20% of current carbon dioxide emissions.

Figure 4.2

Carbon dioxide emissions from the burning of coal, oil, and natural gas are shown for the period 1860 to 1992 for three groups of countries.

Methane (natural gas) is the second most important of the greenhouse gases resulting from human activities. It is produced by rice cultivation, cattle and sheep ranching, and by decaying material in landfills. Methane is also emitted during coal mining and oil drilling, and by leaky gas pipelines. Human activities have increased the concentration of methane in the atmosphere by about 145% above what would be present naturally.

Nitrous oxide is produced by various agricultural and industrial practices. Human activities have increased the concentration of nitrous oxide in the atmosphere by about 15% above what would be present naturally.

Chlorofluorocarbons (CFCs) have been used in refrigeration, air conditioning, and as solvents. However, the production of these gases is being eliminated under existing international agreements because they deplete the stratospheric ozone layer. Other fluorocarbons that are also greenhouse gases are being used as substitutes for CFCs in some applications, for example in refrigeration and air conditioning. Although currently very small, their contributions to climate change are expected to rise.

Ozone in the troposphere, that is, in the lower part of the atmosphere, is another important greenhouse gas resulting from industrial activities. It is created naturally and also by reactions in the atmosphere involving gases resulting from human activities, including nitrogen oxides from motor vehicles and power plants. Based on current data, tropospheric ozone is an important contributor to the enhanced greenhouse effect. However, in part because ozone is also produced naturally, and because of its relatively short atmospheric lifetime, the magnitude of this contribution is uncertain.

Contrary to popular perception, the Antarctic ozone hole does not cause global warming. Instead, the global depletion of stratospheric ozone caused by CFCs and other gases has resulted in a small cooling effect as shown in Figure 4.1.

Human activities, such as the burning of fossil fuels and changes in land use, have increased the abundance of small particles in the atmosphere. These particles can change the amount of energy that is absorbed and reflected by the atmosphere. They are also believed to modify the properties of clouds, changing the amount of energy that they absorb and reflect. Intensive studies of the climatic effects of these particles began only recently and the overall effect is uncertain. It is likely that the net effect of these small particles is to cool the climate and to partially offset the warming of increasing concentrations of greenhouse gases.

Go to "How Do We Know that the Atmospheric Build-up of Greenhouse Gases is Due to Human Activity?"

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