Geoscience Reference
In-Depth Information
sphere than in the southern hemisphere.
The Intergovernmental Panel on Climate
Change (ipcc) estimates that perhaps aero-
sols can reduce the warming effect of CO
2
by as much as one-third (which is why in-
jection of aerosols into the atmosphere has
been suggested as a means of artificially
cooling the Earth, as discussed in chapter
9). Industrial polluters have in effect been
injecting aerosols into the atmosphere for
years, and thus there may already be a cool-
ing effect. One characteristic of aerosols is
that they have a shorter residence time in
the atmosphere than greenhouse gases do.
A recent study led by Nadine Unger of
nasa's Goddard Institute for Space Studies
revealed the complex interaction of green-
house gases and aerosols. Rather than fo-
cus solely on the volume of gas emissions in
global warming, the study took into account
the combined atmospheric heating effect of
the gas and the cooling effect of the aero-
sols produced at the same time by various
industrial sectors. They concluded that on-
road transportation (cars, buses, trucks) is
the greatest net contributor to global warm-
ing, followed by burning biomass for cook-
ing foods and raising animals for food (par-
ticularly methane-producing cattle).
The most important of these is deforesta-
tion. Removing natural forests reduces the
amount of CO
2
that will be absorbed by
the plants, thus increasing the overall CO
2
concentration in the atmosphere. It also ex-
poses soil to decomposition, further adding
to the emissions.
In the past century the causes of defores-
tation have shifted. Subsistence activities
and government development projects have
given way to mining and large-scale ranch-
ing and farming. There is no agreement on
how much rainforest has disappeared, and
of course some of it has been and is being
replanted after deforestation. The most
extreme deforestation is probably in Haiti,
where 1% of their forest remains. Just 10%
of West Africa's coastal rainforests and per-
haps 12% of the rainforests of South Asia
remain. Brazil has declared deforestation to
be a national emergency.
In the typical process of deforestation,
greenhouse gases are released by the burn-
ing of the felled forest. This burning, of
course, also releases aerosols, which will
have a cooling effect, as discussed above.
Finally, land use change, particularly defor-
estation and agriculture, decreases the al-
bedo of the Earth's surface (the ratio of the
light reflected by a planet to that absorbed
by it). This causes more heat to be taken up
by the Earth.
The role of Land use Changes
Approximately one-third of CO
2
emis-
sions over the past two centuries has come
about because of global land use changes.
