Geoscience Reference
In-Depth Information
There are two reasons for the different warming contributions each gas makes.
First, the concentrations and human additions to the atmosphere of each gas are
different. Second, because of the physicochemical properties of each gas, each has a
different warming potential.
With regards to post-18th-century changes to the concentrations of the various
gases, they were attributable to the post-Industrial Revolution anthropogenic increases
in each gas: human influences on the global atmosphere were very different before
the Industrial Revolution. The changes in the concentration of these key greenhouse
gases have each largely arisen from different sets of human actions. For instance, part
of the increase in carbon dioxide comes from the burning of fossil fuels and part from
deforestation and changes in land use. Some of the increase in methane comes from
paddy fields, whereas part of the rest comes from the fossil fuel industry and biomass
(which includes rotting dead plants and animals, and fermentation in animals). We
shall examine this in more detail in the next section when looking at the carbon cycle,
but other methane increases (or, in the prehistoric past, decreases) are due to more
complex factors such as the climate itself, which can serve to globally increase or
decrease the area of methane-generating wetlands.
Both carbon dioxide and methane are part of the global carbon cycle (see the
following section). Nitrous oxide (N 2 O) forms part of the nitrogen cycle and, like
carbon dioxide and methane, has both natural and human origins. Naturally, nitrous
oxide is given off by the decomposition of organic matter in soils, in particular by
tropical forest soils that have high nutrient-cycling activity, as well as by oceans.
Human sources include biomass burning and the use of fertilisers. The principal
agent removing nitrous oxide from the atmosphere is photolysis - removal by the
action of sunlight - ultimately resulting in nitrogen (N 2 ) and oxygen (O 2 ).
As to the second factor determining the different warming contribution that each
gas makes, each has different physicochemical properties. These are quantified for
each gas in what is called its global warming potential (GWP). GWP is a comparative
index for a unit mass of a gas measured against the warming potential of a unit mass of
carbon dioxide over a specific period of time . Carbon dioxide has, therefore, a defined
warming potential of 1. A complicating factor is that because different greenhouse
gases have different atmospheric residence times (see Table 1.1) GWPs must relate
to a specific time frame. A GWP expressed without a time frame is nonsense. This
can be understood by considering methane, which only has an average atmospheric
residence time of a dozen years. Nearly all of a kilogram of methane will still be in
the atmosphere after a year. Roughly half of it will be in the atmosphere after 12 years
and, assuming exponential decay, a quarter or less after 24 years. This means that the
average life time of a typical molecule will be around 12 years. 2 Conversely, nitrous
oxide has an average residence time of more than a century. So, clearly, comparing the
GWPs of nitrous oxide and methane over a decade will give different warming figures
compared with the same comparison over a century. Finally, because of uncertainties,
not least with carbon dioxide's own atmospheric residence times, different researchers
2
Residence times are both estimates and also can alter in different atmospheric conditions such as gas
concentration and temperature. So, do not be surprised if you see slightly different figures in the academic
literature. Sound advice is to use the most recent as well as authoritative estimates.
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