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.

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