An introduction to climate change - Climate Change: Biological and Human Aspects - page 8

Table1.2 Globalwarmingpotentials(GWPs)forsomeoftheprincipalgreenhousegasesover
threetimeframes(IPCC,2001a)

GWP

Gas Atmospheric lifetime (years) 20 years 100 years 500 years

Carbon dioxide 50-200 1 1 1

Methane 12 62 23 7

Nitrous oxide 114 275 296 156

have different GWP estimates. This can be especially frustrating, as estimates
'improve' with time and - as different theories about the dominating effect of, for
example, an aspect of the carbon cycle, come into vogue - it means that GWPs often
vary with both research team and time. Even the IPCC's GWP estimates vary a little
from report to report. Furthermore, because the IPCC is science by committee -
where uncertainty is resolved through consensus of opinion - one cannot simply
dismiss one research team's estimates as being completely out of hand. Instead, when
looking at a research team's climatic model, you need to see what GWP estimates are
used as well as the model itself and then make your own judgement on its results com-
pared to those of another team. Table 1.2 summarises the IPCC's 2001a estimates for
GWPs for carbon dioxide, methane and nitrous oxide. Chlorofluorocarbons (CFCs),
hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs) are not included
because there are so many different ones. However, typically most have GWPs of a few
thousand (compared to carbon dioxide's GWP of 1) for time horizons up to 500 years.
Fortunately because of their low atmospheric concentration, human-made chemic-
als such as CFCs and HFCs contribute less than a quarter of current warming (see
Figure 1.2).
It is therefore possible to use GWPs to standardise each greenhouse gas into a
'CO 2 equivalent' (CO 2 -eq). And so CO 2 -equivalent emission is the amount of carbon
dioxide emission that would cause the same warming (radiative forcing), over a
given time horizon, as an emitted amount of another greenhouse or a mixture of
greenhouse gases. CO 2 -equivalent emission is obtained by multiplying the emission
of a greenhouse gas by its GWP for the given time horizon. For a mix of greenhouse
gases it is obtained by summing the CO 2 -equivalents of each gas. CO 2 -equivalent
emission is a standard and useful metric for comparing emissions of different mixes
of greenhouse gases, and in policy discussions about reducing warming irrespective
of which gas emissions are being reduced. For example, in the latter half of the 2000s,
Russia greatly reduced methane leakage from its natural gas industrial infrastructure.
Methane is a far stronger greenhouse gas than carbon dioxide: it has a higher GWP
even on the 100-year timescale. So the reduction would count for far more than if a
similar volume of carbon dioxide emission had been reduced, and the reduction can
be calculated in CO 2 -equivalents.
There is one other important greenhouse gas that has only briefly been mentioned so
far, and that is water vapour. Water vapour is a powerful greenhouse gas, contributing
a significant proportion of the natural (as opposed to the human-induced) greenhouse
effect. There is sufficient water vapour above the troposphere for it to absorb much of