Forcings from 1750-2000 and 1750-2070 in Wm-2
Source 1750-2000a 1750-2070a
Well mixed GHGs
Carbon dioxide 1.51±0.01b 3.97±0.52b,c
Methane 0.49±0.01 0.78±0.13
Nitrous oxide 0.15±0.00 0.33±0.07
Halocarbons 0.33 0.35
Others
Tropospheric ozone 0.38±0.00 0.73±0.23
Sulfate aerosols -0.40 -0.34±0.18
Stratospheric ozone -0.15 NR
depletion
Biomass burning -0.2 NR
Black carbon aerosols 0.4±0.0 0.66±0.23
Organic carbon aerosols -0.50±0.0 -0.83±0.28
Land use -0.2 NR
Mineral dust aerosols -0.1 NR
Solar constant 0.3 NR
Total, all 2.01 5.65
Total, well mixed 2.48 5.43
GHGs only
Total, others -0.47 0.22
amean ± one standard deviation
bISAM model (reference)
csimilar to IS92a scenario
NR = not reported
The 1750-2070 forcing estimates shown here are based on an average of the projected forcings for 10-12 future emissions scenarios reported by the IPCC and based on other IPCC estimates as well. Somewhat different results will be obtained if other scenarios are used, but we believe this average provides a reasonably relevant estimate on which to evaluate the potential for surface albedo enhancement.
An estimate of future forcing from 2000-2070 is obtained by subtracting the total for well-mixed GHGs in the first column from the second: 5.43-2.48 = 2.95 Wm-2. Because it will be at least 2010 before any large-scale surface mitigation work can begin, a more relevant period is 2010-2070. On average, forcing should increase 0.042 Wm-2 each year from 2000-2070. In reality, the annual forcings should increase more in the decades that follow, i.e., the increases are not linear.
So, to be conservative, the forcing period from 2000-2010 has been assigned a total forcing of 0.2. This means that the total forcing to be offset over the period 2010-2070 is actually 2.95-0.2 or 2.75 Wm-2. To keep the forcing in 2010 constant over the next 60 years, additional forcing must be reduced on average 0.046 Wm-2 each year during that time period. Even though it is expected that the forcing in the last 30 years of this period will be greater than in the first, it is prudent to “front-load” the offsets, since it will be easier to back off later on than increase the coverage.
The total additional watts of heating for the earth due to GHG forcing for the time period 2010-2070 can now be calculated. This will be needed to determine the degree of albedo increase for the covered area, which will of course, be a much smaller area than the total surface area. The total surface area of the earth is 510 million Km2. So, for an increase in forcing of 2.75 Wm-2, the watts to be reduced are (510 x 106 Km2) x (106 m2/Km2) x 2.75 Wm-2 = 1.403 x 1015 watts.
