Potential for Abating NonCO2 Greenhouse Gas Sources

Non-CO2 greenhouse gases and black carbon are responsible for about 45 percent of total anthropogenic radiative forcing (Houghton et al. 2001), a magnitude that provides substantial opportunities for abatement. These gases include CH4, N2O, tropospheric O3, and halocarbons (Prinn, Chapter 9, and Robertson, Chapter 29, both this volume).

The total anthropogenic flux of CH4 is 344 Tg CH4 y-1, equivalent to 2.1 PgCequiv y-1 (based on a 100-year GWP time horizon). About half of this flux (1.1 PgCequh) is of agricultural origin; the remainder is from energy extraction and production, industrial combustion, and landfills (Houghton et al. 2001; Robertson, Chapter 29, Table 29.3). There are multiple options for reducing the total CH4 flux; for example, industrial sources including landfills have declined in the United States by 7 percent since 1990 largely owing to the economic value of methane recovery. If these efficiencies were applied more broadly, we might expect to achieve a 10 percent global flux reduction in the next 20 years and a 25 percent reduction in the years following.

The large agricultural CH4 flux is mainly from enteric fermentation, rice cultivation, biomass burning, and livestock waste treatment. Current technology is available to abate 10—30 percent of emissions from confined animals via nutritional supplements, as is now common in U.S. feedlots and dairies. With adoption of best management practices, methane in rice can be reduced substantially; recent results suggest a potential for 50—80 percent abatement based on irrigation management and management for high yields (Robertson, Chapter 29). Likewise, technology is available now to abate most CH4 from animal waste by storing waste in lagoons and generating power from the captured methane; such use has a net negative carbon cost. Modest abatement of these fluxes in the next 20 years and in some cases more aggressive abatement afterward, together with industrial savings, could generate a total abatement of 75 PgCequiv over 100 years.

N2O fluxes can also be mitigated. The total anthropogenic flux of N2O is 8.1 TgN2O-N y-1, equivalent to 1.0 PgCequiv y-1. More than 80 percent of this flux (0.9 PgC ) is from agriculture; most of the rest is from the industrial production of

& equiv & r adipic and nitric acids and combustion. Agricultural soils emit annually about half of the entire anthropogenic flux; waste handling in feedlots and dairies generates about 25 percent of the flux, and biomass burning and industry generates the remainder (Table 5.4).

Technology is now available to abate most of the industrial sources of N2O, and N2O from waste handling could be largely comitigated with CH4 waste management abatement (as described above). Agricultural soils are more problematic; soils emit N2O largely as a function of available soil N, and although it is easy to reduce soil N, it is difficult to do so without affecting yields. Better nitrogen placement and timing could reduce current fertilizer needs and therefore N2O flux by perhaps 20 percent using today's technology, which is the basis for our 20-year abatement estimate (Table 5.4); further mitigation using site-specific farming technologies, varietal improvements for plant nitrogen-use efficiency, and new forms of fertilizer and nitrification inhibitors could lead to 80 percent mitigation in 20-50 years. If so, total 100-year N2O abatement could reach 75 PgC .

& equiv

Other non-CO2 greenhouse gases, notably the halocarbons, the ozone precursors CO and NOx, and black carbon, are also abatable. Although GWPs for the ozone precursors are at present uncertain and for black carbon unknown, best estimates suggest

120 | I. CROSSCUTTING ISSUES Table 5.4. Potential for non-CO2 greenhouse gas abatement and biosphere carbon storage

(TgCqy1)

Feasible abatement rate

TCeqy-1)

0-20 y 20-100y >100 y

Total abatement (PgCq/100y)

ch4

Industry including landfills

1,016

100

250

250

22

Agriculture

Enteric fermentation

590

60

300

300

25

Rice cultivation

251

160

160

160

16

Biomass burning

213

20

50

50

4

Animal waste treatment

88

75

75

75

8

Total agricultural CH4

1,142

53

Total anthropogenic CH4

2,158

75

n2o

Industry, transport

165

80

130

130

12

Agriculture

Soils

533

100

425

425

36

Animal waste treatment

266

200

250

250

24

Biomass burning

63

6

30

30

3

Total agricultural N2O

862

306

705

705

63

Total anthropogenic N2O

1,027

386

835

835

75

Other

Halocarbons

98

10

75

90

6

Ozone precursors (CO, NOx)

1650

150

750

1500

63

Black carbon

nd

nd

Total other gases

1748

160

825

590

69

Biosphere carbon storage

Agricultural soils

0

300

500

0

46

Reforestation/agroforestry

Improved management

0

170

150

0

15

Agroforestry + afforestation

0

200

100

0

12

Cessation of deforestation

0

100

200

0

18

Total forestry

470

450

0

45

Total biosphere C storage

770

950

0

91

Note: Current flux strengths are based on Houghton et Smith, Chapter 28; and Robertson, Chapter 29; all this GWP uncertainty.

al. (2001) and other sources (see Prinn, Chapter 9; volume). nd = currently not determinable because of

a total source strength >1.6 PgCe u¡v y-1, of similar importance to CH4 and N2O (Prinn, Chapter 9, this volume). Mitigation potentials appear to be of a similar magnitude.

0 0

Post a comment