These patterns
generally track economic development;
strong negative aerosol
forcing
appeared in eastern North
America
and Europe during the early
20th century, extending to Asia, South America and central Africa by
1980. Emission controls have since reduced aerosol pollution in North
America and Europe, but not in much of Asia. Ozone forcing increased
throughout the 20th century, with peak positive amplitudes around
15°N to 30°N due to tropospheric pollution but negative values over
Antarctica due to stratospheric loss late in the century. The pattern
and spatial gradients of forcing affect global and regional temperature
responses as well as other aspects of climate response such as the
hydrologic cycle. {8.6.2, Figure 8.25}