The Greenland ice sheet

to damage the ozone layer in the stratosphere and are banned in developed countries.They are used as refrigerants, propellants, and foaming agents for plastics. Perfl uorocarbons (PFCs), GWP 7,000-9,000These chemicals are largely byproducts of aluminum production and semiconductor manufacturing. Sulfur hexafl uoride (SF 6 ), GWP: 23,900This is produced for use as an insulator in various kinds of electrical equipment.This, together with the previous two manmade chemical categories, despite their very high GWP values, contribute roughly 2% of the world's greenhouse gas impact. EMISSIONS AND ECONOMIC LEVELIn agrarian economies with little heavy industry or electrical generation, methane is often the predominant greenhouse gas.In such countries, land-use change and forestry practice (LUCF) such as tropical deforestation account for a large share of emissions.In more industrial countries, electrical generation, transportation, industry, and heating of buildings contribute carbon dioxide, methane, and nitrous oxide, while CO 2 from land-use change is not signifi cant (see Table 1).Concentrations and GWPs for the six gases can be combined to yield one value expressed as million metric tons of CO 2 equivalent to make comparisons possible.On this basis-using data for the year 2000 (the most recent available in 2007 )-the U.S. accounts for roughly 21% of world total, China 15%, and the European Union roughly 14%.The top ten countries account for 75% of emissions, while the top 25 account for 83%.[ See Total Emissions and Emission Intensity, Year 2000, in Appendix .