Estimating Soil Degradation on a Global Scale

Limitations of Methodology

The scientific relationships between different soil-degradation processes and soil productivity have been established mainly on field and plot level.19,111 Although reliable data are available for a wide range of different agro-ecologies and farming systems, they cannot give a precise picture of degradation-dependent soil productivity for larger land units. The increasing complexity of the determining factors and processes on larger land units, the endless array of location-specific conditions, and the lack of adequate data make upscaling extremely difficult for country and regional levels. Estimates of the extent and severity of soil degradation—and their effects on soil productivity—on a country, regional, continental, and global level therefore bear a substantial degree of uncertainty. Most estimates use the methodology developed by the Global Assessment of Human-Induced Soil Degradation (GLASOD) Project.15,61 The GLASOD estimates are based on expert assessment and are therefore largely subjective. However, the methodology has been widely adopted, and GLASOD estimates of soil degradation are accepted as the best estimates available. Global estimates are, therefore, only rough approximations and should not be taken literally.

Effects on Soil Productivity and Food Production

Table 26.3 shows estimates of degradation and cumulative losses in soil productivity on a continental level for different land-use types. The figures show that, overall, Africa and Central America are the most severely affected continents, both in terms of soil degradation and reduction of productivity. Sixty-five percent of African and 75% of Central American cropland are affected by soil degradation. The overall loss in productivity in these two regions over the last 50 years is estimated between 25% and 37%. On a continental basis, Africa still has land reserves, but large areas of the continent are marginal for crop production, and their food production is therefore most seriously affected by degradation. Europe also shows relatively high trends in degradation, but mainly of pastures and forests. Productivity losses generally are low, as for North America and Oceania (i.e., mainly Australia), and these can most probably be compensated for by improvements in technology and input supply.

A more detailed picture of trends in food productivity for the main food cereals in selected countries with high population growth rates and agriculture-based economies is given in Figure 26.1. The overall past trends in the different countries show a consolidation of per-capita food production. However, the projections until 2025 indicate clear decreases. This suggests that long-term food security in these countries is at stake.

TABLE 26.3 Estimates of Degradation and Losses in Soil Productivity for Different Land-Use Types in the Major Regions

Region

Degraded Areas

Loss in Productivity*

Agricultural

Land

Pasture Land

Forests

Total

Degraded

area

(Mha)

(%)

(Mha)

(%)

(Mha)

(%)

(Mha)

(%)

Cropland (%)

Pasture (%)

Africa

121

65

243

31

130

19

494

30

25

6.6

Asia

206

38

197

20

344

27

747

27

12.8

3.6

South America

64

45

68

14

112

13

244

16

13.9

2.2

Central America

28

74

10

11

25

38

63

32

36.8

3.3

North America

63

26

29

11

4

1

96

9

8.8

1.8

Europe

72

25

54

35

92

26

218

27

7.9

5.6

Oceania

8

16

84

19

12

8

104

17

3.2

1.1

World

562

38

685

21

719

18

1,966

23

12.7

3.8

“Cumulative loss in productivity since 1945. Adjusted figures according to type and degree of degradation. Source: Adapted from Scherr,151 van Lynden & Oldeman,161 and Oldeman.1'31

Per-capita production of selected crops in countries with high population pressure. Source

FIGURE 26.1 Per-capita production of selected crops in countries with high population pressure. Source: Adapted from Engelman & I.eRoy,11 and FAOJ121

 
Source
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