The globally and seasonally averaged effect of clouds on the energy
budget today is a cooling by 17.3 W/m
[ Ramanathan et al.,
1989; Harrison et al., 1990], but this figure for net forcing obscures
much larger regional effects and a competition between longwave
and shortwave forcing. Clouds reflect sunlight, cooling the Earth, but
they trap infrared radiation, which warms the ground.
Spacecraft measurements in the Earth Radiation Budget Experiment
[ Ramanathan et al., 1989; Harrison et al., 1990] reveal an average
shortwave forcing or -48.4 W/m
and an average longwave forcing of
31.1 W/m
. The effects of clouds on the components of the energy
budget are considerably larger than the net. Indeed, they are about
the same as the greenhouse effect of all gases other than water vapor
[ Raval and Ramanathan, 1989]. The latitudinal and seasonal distribution
of cloud forcing [ Harrison et al., 1990] reveals even larger local
effects. At high latitudes during summer, negative forcing by clouds
can exceed 100 W/m
, particularly over the oceans. Figure 1
shows zonally-averaged cloud forcing in southern and northern
summers [ Harrison et al., 1990]. Clouds cool low latitudes modestly at
all seasons, warm high latitudes modestly during the winter, and cool
high latitudes very strongly during summer. A negative forcing of
100 W/m
must make high latitude summers a lot colder than they
would be if there were no clouds and no other changes in the
climate system.
There is no great mystery about the large negative forcing at high latitudes. Figure 2 shows that cloud cover over the high-latitude ocean exceeds 80% and varies little with season [ Warren et al., 1986, 1988]. The cloud type is principally low-altitude stratus. In summer, when the sun is shining, the shortwave cooling effect of these clouds far exceeds the longwave forcing. The clouds are still there in the winter, but there is little sunlight for them to reflect.
Why have clouds received so little attention in paleoclimatic research when observed cloud forcing is so large? Because they have left no geological record, and even the instrumental record is unreliable. Because we do not understand cloud processes well enough to calculate cloud properties in different atmospheres. Indeed, we know that the parameterization of clouds is the largest identifiable source of error (disagreement) in global climate models [ Cess et al., 1989]. These reasons, although compelling, do not justify the neglect of what may be a dominant influence on climate evolution. In what follows I shall suggest how clouds may help to resolve three important paleoclimatic problems: the irregular increase of global average temperature during the industrial revolution, the amplification of ice age climate fluctuations, and the progressive cooling of high latitudes during the Cenozoic. My suggestions are no more than speculations. It is hard to see how we can be sure of the role of clouds in climate change until our simulations include much more convincing representations of cloud-climate interactions or some clever person discovers a paleocloud record. I offer these suggestions to stimulate thinking along these lines.