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. 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. 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.