Sources
of other greenhouse gases – the human connection
Methane
is generated during the breakdown of organic matter
by bacteria that thrive in anaerobic (i.e. oxygen-free)
environments – principally in waterlogged soils
(bogs, swamps and other wetlands, whence methane's
common name of ‘marsh gas’) and in the
guts of termites and grazing animals. But today, only
some 30% of global CH4 emissions
come from natural sources, with natural wetlands accounting
for about two-thirds of the total. Rice paddies, effectively
artificial marshes, contribute a further 11%, and
an astonishing 16% is due to the flatulence of grazing
livestock (cattle, sheep, etc.)! While such sources
are undoubtedly biogenic in origin, they also clearly
have an anthropogenic element –
closely linked to human food production, in this case.
Waste management (e.g. organic matter rotting in landfill
sites) adds a further anthropogenic source of CH4
(around 17% of global emissions). And since natural
gas is mainly methane, so too does leakage from natural
gas pipelines and the common practice of venting the
gas to the atmosphere at oil production sites and
from coal mines (a further 19%). Finally, burning
vegetation can also generate CH4, depending on the
way it burns (i.e. smouldering as opposed to flaming).
Nitrous oxide is part of the natural
nitrogen cycle; it is produced by the activities of
micro-organisms in soils and sediments. Again, the
increase in its atmospheric concentration is thought
to result mainly from agricultural activities, such
as the application of nitrogenous fertilisers to boost
crop yields; some of the nitrogen ends up in the air
as N2O. In addition, the high-temperature combustion
of fossil fuels (or indeed, any kind of vegetation)
in air produces some N2O (through reaction between
N2 and O2 in the air), along with other nitrogen oxides
(notably nitric oxide, NO).
Ozone
is also a natural component of the lower atmosphere
(due in part to transport down from the stratosphere),
but the normal background level is low. However, enhanced
concentrations of tropospheric ozone are now found
in many polluted environments, especially over densely
populated industrialised regions. Here, ozone
is generated close to the surface by the action of
sunlight on the mix of gaseous pollutants that is
typically found in vehicle exhaust fumes: un-burnt
hydrocarbons, carbon monoxide (CO) and nitric oxide
(NO). Ozone is one of the more noxious components
of ‘photochemical smog’,
since exposure to enhanced levels of the gas is harmful
to both human health and plant growth.
Unfortunately, gains made in reducing vehicle emissions
of the key ozone ‘precursors’ (by fitting
catalytic converters) are being outweighed by the
worldwide growth in car usage. And
there are many other anthropogenic sources of these
pollutants as well – including power stations,
industrial processes, and the burning of vegetation. |
