Carbon dioxide, methane, nitrous oxides and ozone are all greenhouse gases. There is slightly more carbon dioxide in the northern hemisphere than in the southern hemisphere. The difference arises because most of the human activities that produce carbon dioxide are in the north and it takes about a year for northern hemispheric emissions to circulate through the atmosphere and reach southern latitudes. The global average sea level has risen by 10 to 25 cm over the past 100 years. It is likely that much of this rise is related to an increase of 0.3 0.6 C in the lower atmosphere s global average temperature since 1860.
Glaciers are in retreat on every continent.
This loss of ice over the past 100 years has added about 2 to 4 cm to the sea level. Temperature has not increased as much as expected from the observed CO 2 increase. This is related to increases in the amounts of tiny particles in the air arising, for instance, from industrial activities or volcanic eruptions. These block out some sunlight and induce a reversing cooling effect. This projection is based on estimates of future concentrations of greenhouse gases and sulphate particles in the atmosphere.