on Plant Genetic Resources for Food and Agriculture (FAO, 2002a) specifically focuses on agrobiodiversity conservation and sustainable use. The imperative to address current threats to genetic diversity was recognized by the Conference of the Parties (www.cbd.int/2010-target) to the CBD 2010 Biodiversity Target, which committed the parties "to achieve by 2010 a significant reduction of the current rate of biodiversity loss at the global, regional and national level as a contribution to poverty alleviation and to the benefit of all life on earth." Thus it is recognized that the international, regional and national level conservation and sustainable use of agrobiodiversity is fundamental for future wealth creation and food security.

3.2.2.2.3 Global warming potential, carbon sequestration and the impacts of climate change

The combustion of fossil fuels, land use change, and agricultural activities constitute the dominant sources of radiatively- active gas emissions (i.e., greenhouse gases-GHG) since the advent of the industrial revolution. Expressed in CO2 equivalents (i.e., global warming potential-GWP), agriculture now accounts for approximately 10-12% of net GWP emissions to the atmosphere from anthropogenic sources (IPCC, 2007; Smith et al., 2007), excluding emissions from the manufacture of agrochemicals and fuel use for farm practices. The IPCC also reports that nearly equal amounts of CO2 are assimilated and released by agricultural systems, resulting in an annual flux that is roughly in balance on a global basis. In contrast, agriculture is a significant net source of the important greenhouse gases methane (CH4) and nitrous oxide (N2O), contributing approximately 58% and 47% of all emissions, respectively (Smith et al., 2007).

      Agriculture affects the radiative forcing potential of the atmosphere (Global Warming Potential-GWP) in various ways, including: (1) heat emission from burnings of forests, crop residues and pastureland (Fearnside, 2000); (2) carbon dioxide emissions from the energy-intensive processes required to produce agricultural amendments like nitrogen fertilizers, pesticides, etc. (USEPA, 2006); (3) greater sensible heat fluxes from bare soils (Foley et al., 2003); (4) infrared radiation from bare soil (Schmetz et al., 2005) and reduced evapotranspiration from soils without vegetative cover; (5) decreased surface albedo (i.e., sunlight reflectance) when plant residues are burned (Randerson et al., 2006); (6) soil organic matter oxidation promoted by tillage (Reicosky, 1997); (7) methane emissions from ruminant livestock (Johnson and Johnson, 1995) and wetland rice cultivation (Minami and Neue, 1994); and (8) nitrous oxide emissions (Smith et al., 1997) from poorly drained soils, especially under conditions where N fertilizers are misused. In aggregate, agriculture is responsible for approximately 15% of anthropogenic CO2 emissions, 58% of methane (CH4) emissions and 47% of N2O (Smith et al., 2007).

Agroecosystems can also be net sinks for atmospheric GWP. Best agricultural practices help to minimize emissions of greenhouse gases.

In addition to being a source of greenhouse gas emissions, certain agricultural practices found to increase the "sink" value of agroecosystems include (1) maintaining good aeration and drainage of soils to reduce CH4 and N2O emissions, (2) maximizing the efficiency of N fertilizer use to limit N2O emissions (Dixon, 1995) and to reduce the amount of CO2 released in the energy-intensive process of its manufacture, (3) minimizing residue burning to reduce CO2 and O3 emissions, and (4) improving forage quality to reduce CH4 and N2O emissions from ruminant digestion (Nicholson et al., 2001), (5) maximizing woody biomass and (6) avoiding burning that promotes ozone formation which is photochemically active with OH radicals; OH radicals remove atmospheric CH4 (Crutzen and Zimmerman, 1991; Chatfield, 2004).

Recent studies on wheat, soybean and rice in Free-Air Concentration Enrichment (FACE) field experiments suggest that yield increases due to enhanced CO2 are approximately half that previously predicted.

Free-Air Concentration Enrichment (FACE) experiments fumigate plants with enhanced CO2 concentrations in open air field conditions (Ainsworth and Long, 2005). Yield stimulation of major C3 crops in elevated [CO2] is approximately half of what was predicted by early experiments in enclosed chambers (Kimball et al., 1983; Long et al., 2006), casting doubt on the current assumption that elevated carbon dioxide concentration ([CO2]) will offset the negative effects of rising temperature and drought, and sustain global food supply (Gitay et al., 2001). Notably the temperate FACE experiments indicate that: (1) the CO2 fertilization effect may be small without additions of N fertilizers (Ainsworth and Long, 2005), and (2) harvest index is lower at elevated [CO2] in soybean (Morgan et al., 2005) and rice (Kim et al., 2003).

Crop responses to elevated to CO2 vary depending on the photosynthetic pathway the species uses.

Wheat, rice and soybean are crops in which photosynthesis is directly stimulated by elevated CO2 (Long et al., 2004). When grown at 550 ppm CO2 (the concentration projected for 2050), yields increased by 13, 9 and 19% for wheat, rice and soybean, respectively (Long et al., 2006). In contrast, photosynthetic pathways in maize and sorghum are not directly stimulated by elevated CO2; these crops do not show an increase in yield when grown with adequate water supply in the field at elevated CO2 (Ottman et al., 2001; Wall et al., 2001; Leakey et al., 2004, 2006). At elevated CO2, there is an amelioration of drought stress due to reduced water use, hence yields of maize, sorghum and similar crops might benefit from elevated CO2 under drought stress.