Agriculture Reference
In-Depth Information
By convention, CO
2
has a GWP of 1; the GWPs of all other gases are expressed
relative to this. Because GHGs have different atmospheric lifetimes, their GWPs
change differentially after emission—for example, the GWP for a quantity of N
2
O
emitted today is higher than it will be a century from now, when less of it will
remain in the atmosphere. Methane, with its briefer atmospheric lifetime (~12
years vs. 114 years for N
2
O), will have a correspondingly smaller impact a century
after emission. To provide a common means for comparison, the IPCC has identi-
fied 100 years as an appropriate standard time horizon for comparing mitigation
options (Forster et al. 2007). Methane has a 100-year GWP of 23 and N
2
O, 298.
Manufactured halocarbons with atmospheric lifetimes of millennia can have 100-
year GWPs greater than 10,000 (Prinn 2004).
We use the term GWI to refer to the effect of a given activity or group of activi-
ties on the atmosphere's heat-trapping capacity. Both GWP and GWI are measured
in CO
2
equivalents (CO
2
e). By way of example, a cropping practice that releases 1
g m
−2
of CO
2
has a GWI of 1 g CO
2
e m
−2
, and a practice that releases 1 g m
−2
of N
2
O
has a GWI of 298 g CO
2
e m
−2
; the GWI of both practices combined would be 299
g CO
2
e m
−2
. Thus, management practices that affect N
2
O fluxes can disproportion-
ately influence climate forcing relative to practices that affect fluxes of CO
2
.
GWI in Practice
The literature is rich with estimates for GWIs of individual cropping activities.
These include the effects of tillage on soil C sequestration (e.g., no-till management
increases soil organic C; Paul et al. 2015, Chapter 5 in this volume); the amount
of CO
2
emitted by the manufacture, transport, and application of agrochemicals;
and the amount of N
2
O emitted from fertilized fields as a function of the rate, tim-
ing, and formulation of N fertilizer (Millar and Robertson 2015, Chapter 9 in this
volume). Still rare, however, are full-cost accountings of entire cropping systems
or farms, in which GWIs from all significant sources are tallied to provide a sys-
temwide net GWI.
Cropping systems with a net positive GWI are net emitters of GHGs and thus
drivers of anthropogenic climate change, whereas systems with a net negative GWI
mitigate climate change. Important to realize, however, is that any system or prac-
tice with a GWI lower than that which is currently the norm will represent mitiga-
tion relative to business as usual—even if the GWI of the new system or practice
remains positive. Equally important is the notion that only by placing GWIs for
different practices in an ecosystem context can the net benefits of any change be
assessed. No-till practices, for example, will save fuel and store more soil C rela-
tive to conventional tillage, but the need for additional herbicide use has a C cost
that will offset some of the fuel savings and soil C gain, and in some soils no-till
practices may increase N
2
O emissions (van Kessel et al. 2013).
Results from a full-cost analysis of GWI in the MCSE (Table 12.2) illustrate
both tradeoffs and synergies. In one of the first whole-system analyses of the contri-
bution of different GHGs to agriculture's GWI, Robertson et al. (2000) showed that
the GWI of MCSE cropping systems differed markedly—and for different reasons.
