Agriculture Reference
In-Depth Information
Another third of the FAO's 18 per cent - representing 5.5 per cent of all the world's
GHGs - derives from nitrous oxide emissions. Nitrous oxide, or N
2
O, is a potent green-
house gas which leaks out of the agricultural nitrogen cycle at various stages and finds its
way into the atmosphere, where it remains for a disturbingly long time. When nitrogen fer-
tilizers are applied to crops about one per cent of the nitrogen ends up as N
2
O, and this
rate is assumed by the IPCC to be the same for synthetic fertilizers, organic manure and
green manure. However the storage of manure and the grazing of animals also leads to N
2
O
emissions; and there are further 'indirect' N
2
O emissions as nitrogen leaches or spreads in-
to waterways and the wider environment.
Table 6 shows how the FAO have derived their N
2
O emissions from these various
sources, using IPCC 1997 guidelines. There are some impressive looking equations in these
guidelines, up to seven lines long, though on scrutiny these are not nearly as daunting to the
non-scientist as they first appear. They have since been updated in 2006, and some default
'emission factors', key to the FAO's calculation, have been lowered significantly, suggest-
FAO's figures are based on IPCC emission factors that are continually being reassessed as
scientific opinion shifts about matters that are still poorly understood.
But there are other reasons to suspect that the total of 4.6 million tonnes of N
2
O emitted
by, through or on behalf of animals, equivalent to 5.5 per cent of the world's GHGs, may
not be an accurate reflection of the toll exacted on the world's climate by livestock. We also
have to examine the 'rebound' emissions, or what economists call 'opportunity costs'. If
we got rid of all our livestock, would the 4.6 million tonnes of N
2
O disappear?
In Table 6 the figures given for both direct and indirect N
2
O emissions from fertilizer ap-
plication refer only to synthetic fertilizers applied to animal feed crops, representing about
20 per cent of all synthetic nitrogen. If the human race were to get rid of its domestic anim-
als, or even just those which are fed on chemically grown feed crops, then those emissions
would be eliminated.
But the much larger figures given for 'manure application and deposition' and 'indirect
manure emission' refer to manure applied not only to feed crops but also, and probably
mainly, to human food crops. If we dispensed with livestock we would have to find another
source of nitrogen for these food crops, either synthetic nitrogen or green manures, which
would (according to the IPCC methodology) be emitting equivalent amounts of N
2
O both
directly and indirectly.
The same will be true of many of the leguminous feed crops which, in the case of lucerne
and clover, produce a surplus of nitrogen in their roots and residues available for subse-
quent food crops. And it will be true of a proportion of the grazing animals, about two
thirds of which are reared in mixed farming systems, where pasturing builds up fertility
