4.3 Calculation of Coverage Areas to Achieve Desired Level of ForcingOffsets
4.3.1 Input Data
To determine how much land will have to be covered and to what extent its albedo will have to be raised to achieve the necessary albedo enhancement to offset a given amount of radiative forcing, the following information is needed:
(1) Additional radiative forcing from GHGs and related sources for the entire planet over a given time period in Wm-2
(2) Average shortwave radiation reaching the surface of the earth in the area to be covered in Wm-2
(3) Average shortwave radiation reflected by the earth in the area to be covered in Wm-2
(4) Average shortwave radiation absorbed by the earth in the area to be covered in Wm-2
(5) Albedo of the earth in the area to be covered prior to coverage. This can be obtained from (2)-(4) above.
(6) Albedo of the earth in the area to be covered after the coverage is applied. This is simply the solar reflectivity of the cover material.
4.3.2 Additional Radiative Forcing for the Entire Planet from 2010-2070 in Wm-2
Estimates of forcings are being refined as better information on climate feedbacks and other factors influencing them are gained. The most current estimates are those from the IPCC’s 2001 Third Assessment Report, Appendix II, Special Report on Emission Scenarios (SRES) tables in which forcings are given for various time periods, including 1750 to 2000 and 1750-2070 (117). The first period covers the time from the beginning of industrial emission of GHGs to near present day. The second period extends this range to 2070. Some 40 different estimates of future forcings are presented based on various assumed futures with regard to economic growth, population growth and emission controls. A broad range of possible future forcings is the result.
These estimates include a breakout by GHG and other sources of forcings, both positive and negative. The confidence is greatest for the well-mixed GHGs: carbon dioxide, methane, nitrous oxide and halocarbons and much less so for the others (55). Because of this uncertainty and because these other sources tend to cancel themselves out in the current models, it is possible to just use the well-mixed GHGs and achieve nearly the same outcome. Estimated forcings over these two time periods are shown below:
