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Climate
We begin by defining what we mean by climate. I received an e-mail
from a colleague in France and at the bottom of his
signature line was the message: "Climate is what we expect and weather is
what we get." This says it in a nutshell. Weather is day-to-day values
of temperature, rainfall, pressure, winds, etc., and climate is the mean of
these variables over some suitably long period of time.
But climate encompasses more than just weather variables. A more general definition of climate is the average behavior of the land, ocean, atmosphere, cryosphere (ice masses), biosphere system over relatively long periods of time. There is not a rigidly defined period for the averaging process, but for many operational applications a 30-year period is used. This definition acknowledges the interactive role of land, water, and ice in determining atmospheric properties. The cryosphere includes the ice masses of Antarctica and Greenland as well as North Polar sea ice and mountain glaciers. Ice masses are very important because their physical dimensions can change, thereby changing the amount of radiation reflected from the surface of the earth. They also are repositories for enormous amounts of H2O, and so their change in volume influences the amount of liquid and gaseous water in the atmosphere and liquid water in the ocean.
PREVIOUS: Important Climate Change Questions