Sustainable Land Management for Mitigating Climate Change

XIII. Desertification Control

77. Desertification is the persistent degradation of dryland ecosystems by variations in climate and human activities (UNEP, 1977, 1990; UNCED, 1992; www.unccd.int; MEA, 2005). Desertification is caused by various social, political, economic, and natural factors which vary from place to place and in time (Mortimore, 1994; Mainguet and da Silva, 1998). In drylands, more people depend on ecosystem services for their basic needs than in any other ecosystem. Yet land degradation in these drylands diminishes biological productivity and directly affects ecosystem services (e.g., food and water, climate regulation, soil conservation, recreation) provided to millions of people. The process of desertification has been studied with regards to its impact on production, income and well being of people (Mendoza, 1990; Blaikie, 1989; Verstaete, 1986; MEA, 2005). The effect of global CC on desertification is complex and not fully understood. However various approaches for combating desertification and mitigating CC are directly linked in many ways thus necessitating synergistic implementation of the United Nations conventions on desertification, climate change and biological diversity (MEA, 2005). It is important to point out that various SLM technologies and practices offer crucial, and in many cases the only, options for combating desertification in many parts of the world. Such practices also provide the best practical options for effective local actions to address climate change.

78. However, the process of desertification is not confined to the drylands of the tropics or economically developing regions alone. It also occurs in developed countries (e.g., USA), high latitude humid ecoregion (e.g., Iceland) and even humid regions (tropical rainforest). It results mainly from land misuse and soil mismanagement. The 2005 Millenium Ecosystem Assessment confirmed that desertification is potentially the most threatening ecosystem change impacting livelihoods of the poor. Land degradation and desertification undermines the long-term development of many nations. The scale of the land degradation challenge calls for sustained and collective action by the international community. But for these efforts to bear sustainable results, the implementation of of measures to combat land degradation and desertification (i.e., UNCCD) must be guided and defined at the country level.

79. Drylands cover 47% of Earth’s land area (Table 15). Because of the harsh climate, these regions are prone to desertification. The problem is exacerbated by marked rainfall seasonality and variability, which determine the pulse of biological activity and NPP. Estimates of the extent of desertification range widely and are highly subjective. UNEP estimated 3.97 Bha in 1977, 3.48 Bha in 1984 and 3.59 Bha in 1992 (UNEP, 1977, 1984a; b, 1992). The land area affected by desertification was estimated at 3.25 Bha by Dregne (1983) and 2.0 Bha by Mabbutt (1984). According to the GLASOD methodology (Oldeman and Van Lynden, 1998), the land area affected by desertification is estimated at 1.14 Bha (Table 16). These estimates are similar to those by UNEP (1991) with reference to degradation of soil and vegetation. In addition, UNEP's (1991) estimates include 2.58 Bha of degraded vegetation on rangelands. Estimates of the current rates of desertification also vary widely, and the annual rate is estimated at 5.8 Mha or 0.13% of drylands in the mid latitudes (Table 17). Eswaran et al. (2001) estimated the land area vulnerable to land degradation and the number of impacted people (Table 18). The area vulnerable to degradation is 4324 Mha (33% of Earth’s land area) and the impacted population is 2.65 billion (46% of the world’s population in 2000). Eswaran and colleagues also estimated land area affected in regions with different population density. Total land area vulnerable to desertification is 1990 Mha for population density of <10 person/km², 1160 Mha for 11-40 persons/km² and 1170 Mha for population density of >41 persons/km² (Table 18). Bai et al. (2008) estimated the global extent of land degradation on the basis of decline in NPP, and reported that 3506 Mha are degraded (23.5% of the land area), and about 1.54 billion (23.9%) people are affected globally (Table 19).

Table 15. The extent of global drylands (recalculated from UNEP, 1992). The climatic classification is based on FAO (1993).
Classification	Land Area (Bha)
	D sub-humid	Semi-arid	Arid	Hyper-arid	Total	% of Global Area
Köppen (1931)	-	1.91	1.61	-	3.52	26.3
Thornthwaite (1948)	-	2.05	2.05	-	4.10	30.6
Meigs (1953)	-	2.11	2.17	0.58	4.86	36.3
Shantz (1956)	-	0.70	3.32	0.63	4.65	34.8
UN (1977)	-	1.78	1.83	0.78	4.39	32.8
UNEP (1992)	1.32	2.37	1.62	1.00	6.31	47.2
Hyper-arid = <200 mm precipitation annually.
Arid = <200 mm of winter rainfall or <400 mm of summer rainfall.
Semi-arid = 200 – 500 mm of winter rainfall or 400 to 600 mm of summer rainfall.
Dry sub-humid = 500 to 700 mm of winter rainfall or 600 to 800 mm of summer rainfall.
Bha = 10^9ha.

Table 16. Comparison between Glasod estimates of desertification in dry areas with that of UNEP methodology (Lal, Hassan and Dumanski, 1999).
UNEP (1991)	Area (10^6 km^2)	Oldeman and Van Lynden (1998)	Area (10^6 km^2)
Degraded irrigated land	0.43	Water erosion	4.78
Degraded rainfed cropland	2.16	Wind erosion	5.13
Degraded rangeland (Soil & vegetation)	7.57	Chemical degradation	1.11
Sub-total	10.16	Physical degradation	0.35
Degraded rangeland (vegetation only)	25.76	Total	11.37
Grand total	35.92	Light	4.89
Total arid land area	51.72	Moderate	5.09
% degraded	69.5	Severe and extreme	1.39
		Total	11.37
		These estimated refer to soil degradation only

Table 17. Estimate of annual rate of land degradation in mid latitude drylands (calculated from Mainguet, 1991; UNEP, 1991).
Land use	Total land area (Mha)	Rate of desertification
		Mha/yr	% of total/yr
Irrigated land	131	0.125	0.095
Rangeland	3700	3.200	0.086
Rainfed cropland	570	2.500	0.439
Total	4410	5.825	0.132
Mha = 10^6 ha

Table 18. Estimates of land area under different vulnerability classes of desertification and the number of impacted people (Eswaran et al., 2001).
Vulnerability Class	Area Affected			Population
	10^6 km^2	% of Global Land Area		10^6 People	% of Global Population
Low	14.60	11.2		1,085	18.9
Moderate	13.61	10.5		915	15.9
High	7.12	5.5		393	6.8
Very High	7.91	6.1		255	4.4
Total	43.24	33.3		2,648	46.0

Table 19. Estimates of area affected by land degradation (Bai et al., 2008).
Parameter	Value
Area affected (10^6km^2)	35.06
Percent of land area	23.54
Total NPP loss (Mt C/yr)	955
Percent of total population	23.9
Total population affected (billion)	1.54

80. Two principal biophysical processes leading to desertification are erosion and salinization. These processes are also affected by overgrazing and fire. Accelerated soil erosion by wind and water are severe in semi-arid and arid regions (Balba, 1995; Baird, 1997), especially those in Mediterranean climates (Brandt and Thornes, 1996; Conacher and Sala, 1998a, b). In the dry Chaco forest of Argentina, Abril et al. (2005) observed that overgrazing was a major cause of decline in soil quality and that overgrazing had a more adverse effect than fire. Burned but ungrazed land recovered sooner than chronically overgrazed land. Desertification control through SLM options involves measures such as conserving, harvesting and recycling water, establishing vegetation cover, creating positive C and nutrient budgets, and reclaiming salt-affected soils. The SCS through desertification control can provide another income stream for farmers and land managers. Furthermore, the residence time of C in drylands is much longer and the decomposition rate much slower than that in humid environments (Gifford et al., 1992). The technical potential of SCS to create an income stream for farmers and land managers through desertification control is discussed next.