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deep, narrow reservoirs in less populated and sensitive areas.Strategic environmental assessment provides a vehiclefor optimizing siting, as in the case of the one undertakenfor the Nile Equatorial Lakes region.Depending on location, reservoirs intropical forest areas can have disparatelevels of GHG emissions.Hydropower reservoirs can produce methane, a powerfulGHG, if drowned forests or vegetation inflows decay anaerobically.Complex biophysical processes determine the rate ofmethane production, which is thought to be highest in shallowtropical reservoirs (Metz and others, 2007). A study of9 Brazilian hydropower plants (dos Santos and others 2006)found 3 that produced less than 3 percent of the GHGs of acomparable combined-cycle gas plant and 5 that produced100–400 percent as much. The study did not measure outgassingat the turbines, which may be significant.The fall and rise of WBG involvement in hydropower1990–2008Hydropower comprises the largest share of the currentWBG renewable energy portfolio and has the longest recordwithin the WBG. Hydropower has been supported byevery unit of the WBG.Figure 2.9 documents the decline and rise of hydropowerin the WBG portfolio. During the 1990s, criticism of theenvironmental and social impacts of dams and hydropowerled to the convening of the World Commission onDams and a slowdown in related WBG commitments. TheWBG endorsed most of the World Commission’s recommendations,and commitments rebounded after 2000. TheWBG recently released a document outlining a vision ofincreased investment in hydropower, especially in Africa(World Bank 2009).Table 2.6 shows the breakdown of recent investments bysize and presence of storage. Large hydropower with storage(the most environmentally and socially sensitive category)constitutes about one-third of these commitmentsin volume; some of this is rehabilitation.Regionally, Sub-Saharan Africa accounts for one-third ofcommitments overall, a quarter of the large hydropowerprojects with reservoirs, and about half of the micro andpico (ultra small) hydropower projects. Half of large hydrowith reservoirs is in Europe and Central Asia.Large hydropower with storage constitutesabout one-third of World Bank hydropowercommitments.Figure 2.9The Fall and Rise of WBG Hydropower Commitments, 1990–20081,4001,2001,000Commitment ($ millions)80060040020001991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2000 2002 2003 2004 2005 2006 2007 2008YearHydro > 10 MW Hydro < 10 MW and other renewable energy Hydro < 10 MWSources: IEG portfolio review and WBG renewable energy and energy efficiency progress reports, 1990–2008.Note: WBG = World Bank Group; MW = megawatts.24 | Climate Change and the World Bank Group