can be found in Lu et al., (1998), Lu and McCreary (1995), and McCreary and Lu (1994).

Low clouds have an albedo that is not very different from that of sea ice, but they can form almost instantaneously and are very sensitive to the structure of the tropical boundary layer. Increase in low cloud cover has a cooling effect, whereas dissipation of low clouds has a warming effect. The occurrence of low clouds is not solely, or even primarily, a function of temperature. Cloud properties are influenced by dust and aerosols, and an increase in these can make it easier for water to condense into small droplets, yielding brighter clouds and affecting precipitation. All these effects are subject to considerable uncertainties, and they remain as possibilities for mediating abrupt climate change or amplifying effects of THC fluctuations.

Land Surface

Land-surface processes can participate in abrupt climate change in many ways. The albedo of the land surface can change greatly, with fresh snow or ice sheets reflecting more than 90 percent of the sunlight striking them but dense forests absorbing more than 90 percent. Changes in surface type thus can affect solar heating and feed back strongly on climate. Rainfall used by plants and transpired to the atmosphere contributes substantially to local cooling during evaporation, and it supplies clouds and additional rainfall. Rainfall not used by plants typically runs off in rivers to the ocean, freshening surface waters at outflows but leaving low humidity in air over land. Thus, changes in vegetation have effects well beyond localities where the changes occur.

The land surface is the major source of dust, smoke and soot, and a variety of biogenic emissions. These affect cloud formation and albedo, drop size and rainout, and clear-sky radiation. Again, changes in the land surface can feed back on climate. The roles of these and other land-surface processes in causing, amplifying, or allowing the persistence of abrupt climate change are at best poorly understood. Additional work, especially in land hydrology and dust-cloud processes, is warranted.

External Forcings

A few types of forcings external to the climate system could play roles as pacemakers of abrupt climate change. These forcings vary too slowly to be prime movers of abrupt change, but if the climate system exhibits discontinuous response to continuous variations of some forcing parameters,



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