Jökull - 01.01.2005, Side 4

Wallace S. Broecker
Figure 2. A division of the carbon “pie” based on population. – Sé miðað við fólksfjölda, eiga fátækari ríki
heims rétt á 80% hugsanlegs kolefniskvóta en iðnríkin munu klára sín 20% kvótans á 24 árum.
CLOUDS AND AEROSOLS
Another hotly debated aspect of computer simulations
is cloudiness. Clouds play a huge role in the Earth’s
radiation budget for their water droplets intercept both
incoming sunlight and outgoing earth light. Small
changes in the extent, the location or the elevation of
clouds have a significant impact on the model’s cli-
mate (Slingo, 1990). A further complication stems
from the fact that the reflectivity of clouds depends
on the mean size of its droplets. Smaller droplets
are more reflective than larger ones. The mean size
of cloud droplets depends on the availability of so-
called cloud condensation nuclei; the more nuclei,
the greater number of droplets (Penner, 2004). The
greater the number of droplets, the smaller must be
their average size. By generating aerosols, we have
increased the number of available nuclei and thereby
presumably caused clouds to brighten (see Figure 3).
Second in importance to issues related to water va-
por are issues related to aerosols themselves. As do
greenhouse gases, aerosols capture and re-radiate out-
going infrared light (Hansen et al., 2005). Also, as
do rain drops, they scatter incoming sunlight. Unlike
greenhouse gases, their impacts are not readily mod-
eled. One reason is that rather than being uniformly
distributed across the globe, they are patchy and ever
shifting. Evenmore important, while the light-colored
sulfuric acid aerosols (i.e., those created by the oxi-
dation of the SO2 gas released during coal burning)
primarily reflect incoming sunlight, the dark-colored
aerosols (created by both coal and vegetation burning)
primarily absorb outgoing earth light. Making the sit-
uation even more complex, these aerosols collide with
one another and merge to form complex composites.
Further, all of these aerosols serve as cloud conden-
sation nuclei. The bottom line is that, while there
is no doubt that man’s aerosols are altering climate’s
course, it is not even clear whether the net result is
to counter or enhance the warming created by green-
house gases.
Could it be that the presence of aerosols in the
atmosphere will serve on the long term to counter
the continuing increase of CO2? Despite the uncer-
tainty with regard to their current impact, it can be
stated with some confidence that the importance of
aerosols will diminish as time passes. One reason is
that aerosols remain airborne for only days to weeks
before they are removed by impact with rain drops,
4 JÖKULL No. 55, 2005