The oceans, atmosphere, land and cryosphere are part of a tightly connected climate system. Due to its huge heat capacity, the oceans provide a ``long term memory'' for the climate system. To understand how this works, consider water mass formation in high latitudes of the North Atlantic. An effect is that there is a net loss of heat from the ocean to the atmosphere, which serves to lower the sea surface temperature causing surface water to sink. To replace the sinking water, warm salty water is imported from low latitudes in the upper limb of the thermohaline circulation. The imported water in turn serves to elevate sea surface temperatures. The high latitude surface water is then transported eastward towards Europe. The process contributes to the relatively mild European winters. For example Madrid and New York are about the same distance from the equator, yet the yearly mean temperature in New York is several degrees colder.
There have been small as well as large climate changes in the
past. During the Little Ice Age which lasted from 1500-1800, it
appears the global mean temperature was less than 1
C colder
than today. In the more distant past, there is substantial
geological evidence for colder and longer glacial periods.
Paleoclimate data [e.g., Keigwin et al., 1991] and climate
models [e.g., Manabe and Stouffer, 1988; Power et al.,
1994] suggest that it is the thermohaline circulation which
maintains the import of warm surface waters to the high latitudes.
The idea is that during glacial climates, the thermohaline
overturning is greatly reduced. The combination of recent
paleoclimate and modeling results has propelled oceanographers into
learning more about the thermohaline circulation. This knowledge
will better equip us to recognize changes, learn where it is
important to monitor the ocean for changes, and enable modelers to
better represent the circulation when used in predictive models
for climate variability.