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Desertification and Global Change Author(s): M. M. Verstraete and S. A. Schwartz Source: Vegetatio, Vol. 91, No. 1/2, Vegetation and Climate Interactions in Semi-arid Regions (Jan. 31, 1991), pp. 3-13 Published by: Springer Stable URL: http://www.jstor.org/stable/20038708 Accessed: 28-01-2017 15:43 UTC JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at http://about.jstor.org/terms Springer is collaborating with JSTOR to digitize, preserve and extend access to Vegetatio This content downloaded from 192.206.10.53 on Sat, 28 Jan 2017 15:43:35 UTC All use subject to http://about.jstor.org/terms Vegetation: 3-13, 1991. A. Henderson-Sellers and A. J. Pitman (eds). 3 Vegetation and climate interactions in semi-arid regions. ? 1991 Kluwer Academic Publishers. Printed in Belgium. D?sertification and global change M. M. Verstraete1 & S. A. Schwartz2 1 Institute for Remote Sensing Applications, CEC Joint Research Centre, Ispra Establishment, TP 440, 1-21020 Ispra (V?rese), Italy; 2 Department of Atmospheric, Oceanic and Space Sciences, The University of Michigan, Ann Arbor, MI 48109-2143, USA Accepted 24.8.1990 Abstract Arid and semiarid regions cover one third of the continental areas on Earth. These regions are very sensitive to a variety of physical, chemical and biological degradation processes collectively called desertification. Although interest in desertification has varied widely in time, there is a renewed concern about the evolution of dryland ecosystems because (1) a significant fraction of existing drylands already suffers from miscellaneous degradation processes, (2) increasing populations will inevitably result in further over-utilization of the remaining productive areas, (3) climatic changes expected from the greenhouse warming might result in drier continental interiors, and (4) some of the desertification processes themselves may amplify local or regional climatic changes. This paper reviews some of the many aspects of this issue in the context of the Global Change research program. Introduction This degradation now affects many or most ecosystems on this planet, and there is a progres Humanity has had along association with arid and sive awareness by the public and the decision semiarid regions: the first great civilizations (in makers of the many aspects of this land abuse Egypt and Mesopotamia) developed at the end of (destruction of tropical rain forests, various forms the climatic optimum some 3000 years B.C., at a of pollution, including toxic and nuclear wastes, time when the Sahara appears to have been vege sewage, acid deposition, etc., the so-called tated as parts of the Sahel are today (Butzer 1966; 'ozone-hole', oils spills and chemical or nuclear Lamb 1977). By the time the Great Pyramids were accidents, to name but a few). The release into the erected in Egypt (around 2700 B.C.), the climate atmosphere of large quantities of carbon dioxide, of Northeast Africa and the Middle East was in methane and other pollutants by industrial and a drying phase that resulted in the arid landscapes agricultural activities is now so large that the com we have known for much of the last 5000 years position of the atmosphere is affected, and that in (El-Baz 1983). Superimposed on this long term turn is expected to affect the climate of the Earth climatic evolution, however, is the increasingly (Henderson-Sellers & Blong 1989; Schneider large impact of growing human populations. The 1989). expansion into new territories, and exploitation of In response to this increased awareness, the the natural resources of these drylands beyond scientific community has started to design and their carrying capacity has resulted in rapidly implement a coordinated research effort geared at deteriorating environmental conditions. documenting the current state and probable evo This content downloaded from 192.206.10.53 on Sat, 28 Jan 2017 15:43:35 UTC All use subject to http://about.jstor.org/terms 4 lution of the global system. The International The concept of desertification Geosphere Biosphere Program (IGBP) is an interdisciplinary research effort 'to describe Muchand time and effort has been spent trying to understand the interactive physical, chemical, define the concept of desertification. Such a task and biological processes that regulate the total is difficult, if not impossible, because of the num Earth system, the unique environment that it berpro and complexity of the issues involved, the vides for life, the changes that are occurring interdisciplinary in nature of the problem, and the this system, and the manner in which they rangeare of spatial and temporal scales over which influenced by human actions' (NAS 1988, p. this2). concept is applied (Verstraete 1983, 1986). This ambitious project is coordinated by the Inter For the present purpose, we shall define desertifi national Council of Scientific Unions (ICSU). cation as the set of all environmental degradation The expression 'Global Change' is often used processes as in hot drylands (hyperarid, arid, semi a synonym to IGBP research program, it also arid and subhumid regions), as a result of either designates the US component of the IGBP. climatic In stress or human mismanagement, or this paper, we review the nature, extent and sever both. Desertification will also include the causes ity of desertification, and discuss possible inter (to the extent these can be identified) and the actions between this form of land degradation impactand of degradation on natural and managed the expected climate and environmental changes. ecosystems. Clearly, environmental degradation The specific contributions of in situ observations can occur in all biomes, but the fragile nature, and satellite remote sensing systems in the overall harsh climate and expanding area of these dry strategy to monitor global environmental delands have made it a primary focus of attention. gradation are described in a companion paper Desertification usually has severe long term con (Verstraete & Pinty 1990). sequences for the productivity of the land, and therefore for the populations that inhabit these regions. The word 'desertification' was first introduced by the French forester Aubr?ville in his book Table 1. Extent of drylands by geographical region. [km2, %] Total Hyperarid Arid Semiarid Subhumid Dryland Africa 30,321,130 6,094,094 6,169,507 5,129,749 4,051,032 21,444,382 20.10 20.35 16.92 13.36 70.72 America 42,567,895 164,582 2,099,881 4,679,068 4,327,944 11,271,475 0.39 4.93 10.99 10.17 26.48 Middle East 6,139,098 1,125,997 3,052,996 985,490 769,062 5,933,545 18.34 49.73 16.05 12.53 96.65 Asia 38,120,322 382,439 4,008,463 5,312,779 3,925,704 13,629,385 1.00 10.52 13.94 10.30 35.75 Australia 7,686,884 3,766,572 1,537,377 1,229,900 6,533,850 49.00 20.00 16.00 85.00 Europe 10,507,630 10,096 239,179 267,503 516,778 0.10 2.28 2.55 4.92 Grand total 135,897,300 7,767,112 19,107,518 17,883,642 14,571,146 59,329,417 5.72 14.06 13.16 10.72 43.66 Sources of raw data: (Rogers 1981) and Hammond (1985). This content downloaded from 192.206.10.53 on Sat, 28 Jan 2017 15:43:35 UTC All use subject to http://about.jstor.org/terms 5 Climats, For?ts, et D?sertification de l'Afrique oases (Meckelein 1980), much of the arid, semi Tropicale (Aubr?ville 1949). He witnessed the arid and subhumid areas are generally considered degradation and disappearance of tropical forests to be a risk of desertification. in many humid and sub-humid parts of Africa, Table 2 shows the distribution of productive and attributed it to a large extent to the slash and drylands, broken down by type of economic activ burn agricultural practices of the local popu ity, as well as the percentage thought to be deserti lations. This destruction of the forest lead to fied as of the early eighties. These data are not savannas with scattered trees, and then further to directly comparable to those of Table 1, but it is soil erosion and a general tendency towards more interesting to note that rangelands are by far the xeric environmental conditions (Dregne 1986, largest areas affected by desertification. p. 6). Aubr?ville had identified climate change as In the late seventies, the United Nations esti a potential factor, but could not estimate its mated the rate of desertification at between 54000 importance for lack of adequate data. It is only and 58000 km2 (UNCOD 1977, p. 9; United later on that the concept became commonly asso Nations, 1978, p. 2). By 1984, Norman Myers ciated with arid and semiarid regions. estimated that some 120000 km2 of agricultural Hot drylands are characterized by high solar and pastoral land were deteriorating beyond use radiation, potential ?vapotranspiration rates, and ful economic use per year (Myers 1984, p. 46). diurnal ranges of temperature, and low precipi These numbers should be compared to estimates tation and atmospheric humidity. The location of deforestation worldwide, which range from and extent of these regions depends strongly on 100000 to 113 000 km2 per year, with the bulk of how they are defined, and many definitions have the destruction occurring in the tropics (Myers been proposed in the literature.