Low clouds over the ocean are extremely important for the energy
balance of Earth because their albedos are much higher than that of
clear sky over ocean and they have only a small effect on escaping
longwave radiation. This is clearly illustrated in the following
figure, which slows the annual mean abundance of clouds with tops at
or below the 680 mb level (heights less than about 3 km) from ISCCP.
Low clouds are abundant over the oceans and are especially common in
high latitudes and over the eastern margins of the oceans where low
sea surface temperatures and atmospheric subsidence encourage their
presence. Also shown is the annual mean net cloud radiative forcing,
the amount by which the clouds that are present in the current climate
change the net energy balance at the top of the atmosphere. There is a
remarkable similarity between the distribution of the fractional area
coverage by low clouds and the net forcing of the climate by
clouds. This is because the low clouds are relatively abundant where
they occur. They have a strong negative effect on the energy balance
because they reflect a lot of solar radiation and yet their tops are
warm enough that they have only a small effect on the amount of
emitted thermal radiation. 
The amount and optical properties of low
stratiform clouds are sensitive to both atmospheric and oceanic
large-scale properties and to the abundance of cloud condensation
nuclei, which may be affected by human activities such as the burning
of fossil carbon. These sensitivities combined with their strong
effect on the radiation balance give them the potential to play a
major role in climate change.
Long-term trends in low clouds and their relation to SST and other
variables are being derived from long records of surface observations
in order to obtain estimates of how low cloud changes have been
related to natural changes in the climate system. The following
figure shows the change in low cloud abundance and SST over the North
Pacific and North Atlantic oceans during the three decades from 1952
to 1981. During this period rather significant downward trends in SST
over the midlatitude oceans have been accompanied by significant
upward trends in the abundance of low clouds. Some tantalizing
similarities in the structure of the SST and cloudiness trends are
apparent. It is believed that these changes are associated with
natural variability in the coupled ocean-atmosphere system. Cloud
changes are larger in proportion to the SST changes over the Atlantic
than they are over the Pacific Ocean. The reasons for this is not yet
understood. . More recent data from the 1980's suggest that
these downward trends of SST and upward trends of low cloud amount
have reversed and shifted toward warming oceans and decreasing low
cloud amounts.
More detailed global data on low cloud abundance, structure, and optical properties can be derived from instruments such as MODIS, CERES, and MISR within EOS. Improved data on the abundance of tropospheric aerosols will be provided by EOSP and MISR. These data can be used to test new models of stratocumulus clouds being developed for possible incorporation in global climate models. If a long enough record can be taken with these instruments, additional insight into natural variability and long-term trends can be obtained. The decade from 1998 to 2008 may be one in which anthropogenic climate change becomes measurable (0.1 - 0.2 C/decade), and it will be important to look at these changes with the EOS measurement system.