![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
|||||||
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
||
Time Scales of Change
The difficulty of defining appropriate time scales for the climate
system can be better appreciated by considering the residence time of a
water molecule in various components of the climate system. If we put some
water vapor into the atmosphere and tracked the molecules through the
hydrological cycle, we can examine the different time scales. Water
molecules remain in the atmosphere for about two weeks, on average, before being
precipitated out. If they fall in the ocean, they may remain in the upper
layer (top 100 meters) for a couple of months, but if they are by some
process moved to the deep ocean they may reside there for thousands of
years.
Water molecules that fall as precipitation on land might evaporate within the day or might go into the soil and migrate through the groundwater to a stream or lake and then be re-evaporate after a period of maybe six months to two years. If the water molecule is taken up by a plant, it might stay in the biosphere for the growing season before being returned to the soil or atmosphere. If the atmospheric water molecules are deposited on the Antarctic Ice sheet, they might be locked away for 100,000 years. Clearly, different components of the climate system have different time scales. Those components of the climate system that have very long time scales, such as the Antarctic ice sheet, can be considered as unchanging when we are evaluating changes in atmospheric and oceanic circulation. Others, such as the amount of sea ice in the ocean over the North Pole which has a lifetime of half a year, must be considered as part of the changing component of climate.
In summary, the atmosphere responds quickly to climate changes, but the biosphere, surface ocean, deep ocean and ice masses respond successively slower.
NEXT: Components of the Climate System