Slide 21 of 26
Notes:
Because there is relatively little mass in the atmosphere, and soil transmits heat downwards very slowly, it takes the atmosphere and land surface only about a month to come into a new radiative balanceafter a change in atmopsheric greenhouse gases.
The water in the ocean, on the other hand is mixed by the wind even when the water at the surface is warmer than that just below. This well mixed surface layer has a much larger heat capacity than the atmosphere-ocean system and warms much more slowly in response to a given net input of heat. Thus, if the atmospheric CO2 is suddenly increased, it takes longer for the warming to become apparent, though the ultimate increase in surface temperature is the same. Since most of the world is covered by ocean, and the CO2 is increasing gradually, this effect delays somewhat, but does not lessen, the global rise in temperature.
There is also a slow exchange of water between the surface mixed layer and the ocean water deep below. This takes thousands of years to renew the deep water. The heat capacity of this water is is so great that if the exchange were faster it would completely neutralize the additional heating due to carbon dioxide for centuries to come. In practice, this deep water exchange reduces the anticipated rate of global warming over the coming century for a given increase in atmospheric CO2 by some 10-20%. More significant is its impact on the amount of CO2 from fossil fuel that remains in the atmosphere. This aspect will be discussed in a later lecture as part of the Carbon Cycle.