• These crossroads carry large pollution loads over the Mediterranean, and the negative effects extend far beyond the region. International efforts are called for to reduce these atmospheric environmental stresses and to further investigate the links between Mediterranean and global climate change.

• Atmospheric pollution from major volcanic eruptions can influence the global climate over one to two years.

• Following the most rapid century of warming experienced by modern civilization, the first ten years of the 21st century see an acceleration of atmospheric warming, as average temperatures worldwide rise by .5 degrees Fahrenheit per decade and by as much as 2 degrees Fahrenheit per decade in the harder hit regions.

• Atmospheric aerosols influence the transfer of energy in the atmosphere in two ways: directly through the scattering of sunlight; and indirectly through modifying the optical properties and lifetimes of clouds.

• By comparison, the atmospheric gases that cause the earth 's natural greenhouse effect total less than 1 % of the atmosphere.

• This Section places particular emphasis on current knowledge of past changes in key climate variables: temperature, precipitation and atmospheric moisture, snow cover, extent of land and sea ice, sea level, patterns in atmospheric and oceanic circulation, extreme weather and climate events, and overall features of the climate variability.