Radiative Forcing of Climate Change - Climate Dynamics - page 162

Table 10.1. Greenhouse gas concentrations and their radiative effects

Direct
radiative
forcing
(W/m 2 )

Preindustrial
tropospheric
concentration

Atmospheric
residence
time

Current
concentration

GWP
(100 yr) Gas

Carbon
dioxide (CO 2 )

280 ppm 390 ppm 1.66 1 ~100 yr

Methane
(CH 4 )

1,820-1,890
ppb

700 ppb 0.48 25 12 yr

Nitrous
oxide (N 2 O)

270 ppb 324 0.16 298 114 yr

Tropospheric
ozone (O 3 )

Hours to
days

25 ppb 34 0.35 N/A

CFC-11 0 239 0.063 4,750 45 yr

CFC-12 0 527 0.17 10,900 100 yr

CF-113 0 77 0.024 6,130 85 yr

CCl 4 0 87 0.012 1,400 26 yr

HCFC-142b 0 20 ppt 0.0031 2,310 17.9 yr

HCFC-22 0 220 ppt 0.033 1,810 12 yr

Halon 1211 0 4.1 ppt 0.001 1,890 16 yr

SF 6 0 7.3 ppt 0.0029 22,800 3200 yr

Source : Table adapted from Recent Greenhouse Gas Concentrations by T. J. Blasing, DOI: 10.3334/
CDIAC/atg.032, http://cdiac.ornl.gov/pns/current_ghg.html.
Note: Values for SF 6 are approximate since concentrations are always changing and the constituent may
not be well mixed. Concentrations listed here are an average of levels reported for Mauna Loa and the South
Pole for May 2011 by NOAA at http://www.esrl.noaa.gov/gmd/dv/hats/cats/cats_conc.html.

to compare one greenhouse gas with another and also with other climate forc-
ing factors such as changes in the solar luminosity or atmospheric aerosol con-
centrations. Figure 10.6 displays direct radiative forcing estimates for various
factors thought to be relevant to contemporary climate change, with error bars
indicated. Carbon dioxide emissions are currently the single most important
cause of positive radiative forcing at 1.6 W/m 2 (see Table 10.1 ). Taken together,
the long-lived greenhouse gases are currently adding an additional 1 W/m 2
of energy into the troposphere compared with the unperturbed radiative bal-
ance. Anthropogenic effects on the surface albedo include decreases (cooling
by increases in surface albedo) due to land use changes, and increases (warm-
ing by a darkening of the surface) due to the settling of black carbon on snow.
The greatest anthropogenic cooling is that associated with aerosols. In Figure
10.6, it appears that the cooling effects of aerosols have the potential to “save
the day,” at least partially, from greenhouse gas-induced warming, and this is
exactly why geoengineering techniques to ameliorate global warming often
concentrate on manipulating atmospheric aerosol concentrations. However, re-
call that aerosol emission is often coupled with greenhouse gas emission. Also,