Environmental Engineering Reference
In-Depth Information
removal times on GWP especially for methane when time horizon is increase from
20 to 100 years, and the very high GWP of nitrous oxides and halocarbons for both
time horizons considered. On the other hand, though water vapor is very signifi-
cant in terms of infrared radiation absorbance (its contribution to the natural
greenhouse effect is preponderant, in the order of 60-80% of the total natural
greenhouse gases), climate forcing and GWP values are not reported for this
molecule. This is due to an important distinction which has to be taken into
account between climate forcing agents, able to create an initial change in the
climate, and feedback agents, responsible for amplifying the initial forcing. This
difference is connected to the removal times of chemical species from the atmo-
sphere, which results to be too short for water vapor (in the order of some days),
with respect to removal times of several years for the other species. Due to its short
atmospheric lifetime water vapor cannot force an initial climate change, but its
concentration in atmosphere can increase as a consequence of temperature increase
caused by long-lived climate forcing agents (other compounds in Table
1.4
).
Additional water evaporation due to this temperature increase contributes to the
greenhouse effect generating further global warming, then acts as amplifying agent
of initial forcings, in a loop called positive feedback of water vapor.
Considering only the species able to act as climate forcing, since the contri-
bution to the greenhouse effect by a specific gas is affected not only by the
characteristics of the gas (time removal, climate forcing capability and GWP) but
also by its abundance, among gases reported in Table
1.4
carbon dioxide is
regarded as the most important greenhouse agent produced by human activities
because of high removal times ([100 years), significant climate forcing value, and
elevated concentration levels in the atmosphere (at least three order of magnitude
higher than methane and nitrous oxide, and six order of magnitude higher with
respect to halocarbons). Even if natural sources of carbon dioxide are about 20
times higher than those anthropogenic, over long periods (from years up to cen-
turies) natural sources enter into the natural carbon cycle (absorption by oceans,
photosynthesis, weathering of rocks). Then the problem of carbon dioxide as
regard climate change issues is connected to its additional release derived by
human activities, which is essentially constituted by fossil fuels burnings, and
made worse by deforestation. For this reason in recent years, the attention of
climate experts and intergovernmental organisms has been paid to the problem of
atmospheric CO
2
concentration increase and to possible effects on earth's global
warming.
As mentioned before the link between greenhouse gases increase and global
warming is cause of strong disagreement in the scientific debate, in particular the
large uncertainty level regarding natural variability and time history of the most
important climate forcing agents makes that linkage very difficult to be
unequivocally established. In fact, the observation that the magnitude of the global
warming registered in the last decades is too large when compared to natural
variability can only suggest the validity of the relationship between greenhouse gas
concentration increase and climate change, but does not constitute a scientific
demonstration.
