Aeolian Processes and the Biosphere
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AEOLIAN PROCESSES AND THE BIOSPHERE Sujith Ravi,1 Paolo D’Odorico,2 David D. Breshears,3 Jason P. Field,3 Andrew S. Goudie,4 Travis E. Huxman,1 Junran Li,5,6 Gregory S. Okin,6 Robert J. Swap,2 Andrew D. Thomas,7 Scott Van Pelt,8 Jeffrey J. Whicker,9 and Ted M. Zobeck10 Received 3 January 2010; revised 22 February 2011; accepted 18 April 2011; published 3 August 2011. [1] Aeolian processes affect the biosphere in a wide variety at a variety of scales, including large‐scale dust transport of contexts, including landform evolution, biogeochemical and global biogeochemical cycles, climate mediated interac- cycles, regional climate, human health, and desertification. tions between atmospheric dust and ecosystems, impacts on Collectively, research on aeolian processes and the bio- human health, impacts on agriculture, and interactions sphere is developing rapidly in many diverse and specialized between aeolian processes and dryland vegetation. Aeolian areas, but integration of these recent advances is needed to dust emissions are driven largely by, in addition to climate, better address management issues and to set future research a combination of soil properties, soil moisture, vegetation priorities. Here we review recent literature on aeolian pro- and roughness, biological and physical crusts, and distur- cesses and their interactions with the biosphere, focusing bances. Aeolian research methods span laboratory and field on (1) geography of dust emissions, (2) impacts, interactions, techniques, modeling, and remote sensing. Together these and feedbacks, (3) drivers of dust emissions, and (4) method- integrated perspectives on aeolian processes and the bio- ological approaches. Geographically, dust emissions are sphere provide insights into management options and aid highly spatially variable but also provide connectivity at in identifying research priorities, both of which are increas- global scales between sources and effects, with “hot spots” ingly important given that global climate models predict an being of particular concern. Recent research reveals that increase in aridity in many dryland systems of the world. aeolian processes have impacts, interactions, and feedbacks Citation: Ravi, S., et al. (2011), Aeolian processes and the biosphere, Rev. Geophys., 49, RG3001, doi:10.1029/2010RG000328. 1. INTRODUCTION climate, human health, and desertification [e.g., Goudie and Middleton, 2006; Field et al., 2010]. Aeolian landforms are [2] Aeolian processes (the entrainment, transport, and deposition of sediments by wind) affect almost all aspects found in areas where wind is the primary agent of transport of the biosphere and can have important consequences (erosion and deposition) such as in arid and semiarid regions, for landform evolution, biogeochemical cycles, regional while elsewhere the effects of aeolian processes are often masked by the effects of hydrologic processes. Aeolian erosion, the wind‐forced movement of soil particles, is influenced by geological and climatic conditions and human 1B2 Earthscience, Biosphere 2, University of Arizona, Tucson, Arizona, USA. activities [e.g., Shao, 2008]. Perhaps the most recognizable 2Department of Environmental Sciences, University of Virginia, evidences of aeolian activity on the Earth surface are the sand Charlottesville, Virginia, USA. dunes of different forms and sizes observed in desert and 3School of Natural Resources and the Environment, University of Arizona, Tucson, Arizona, USA. coastal environments. There is a growing body of evidence 4St Cross College, University of Oxford, Oxford, UK. showing that aeolian processes interact with ecosystems at 5Jornada Experimental Range, Agricultural Research Service, different scales, contributing to important biophysical feed- USDA, Las Cruces, New Mexico, USA. 6Department of Geography, University of California, Los Angeles, backs between the biotic and abiotic components of the Earth California, USA. system. Even though at the global scale water is considered 7School of Science and the Environment, Manchester Metropolitan to be the dominant erosion agent, wind is a major erosion University, Manchester, UK. 8Wind Erosion Laboratory, Agricultural Research Service, USDA, driver in dryland systems, which comprise almost 40% of the Big Spring, Texas, USA. Earth’s land surface. In these systems aeolian processes, 9Environmental Programs, Los Alamos National Laboratory, alone or in combination with hydrological processes, are Los Alamos, New Mexico, USA. 10Wind Erosion and Water Conservation Research Unit, considered to be major drivers of ecosystem processes [Field Agricultural Research Service, USDA, Lubbock, Texas, USA. et al., 2010; Ravi et al., 2010]. Copyright 2011 by the American Geophysical Union. Reviews of Geophysics, 49, RG3001 / 2011 1of45 8755‐1209/11/2010RG000328 Paper number 2010RG000328 RG3001 RG3001 Ravi et al.: AEOLIAN PROCESSES AND THE BIOSPHERE RG3001 Figure 1. Conceptual diagram showing factors affecting wind erosion and the effects of wind erosion. [3] The consequences of aeolian processes are diverse and et al., 2001] as well as the nucleation and optical proper- important. At the field scale, wind erosion is associated with ties of clouds [Rosenfeld et al., 2001; Kaufman et al., soil losses, while at the landscape and regional scales the 2005a]. These direct and indirect effects of aerosols can entrainment and transport of atmospheric dust is associated also lead to a weaker hydrologic cycle [e.g., Huietal., with the redeposition of soil particles with consequent 2008], with effects on the availability of fresh water in the impacts on global biogeochemical cycles and climate. The biosphere [Ramanathan et al., 2001]. Collectively, there are effects of wind erosion and subsequent dust emissions several causes and effects of wind erosion (Figure 1). include removal and loss of nutrient‐rich topsoil particles [5] Understanding aeolian processes is critically important [Zobeck and Fryrear, 1986; Zobeck et al., 1989], abrasion for managing the world’s drylands. Wind erosion is thought and injury to growing plants [Fryrear, 1986a; Armbrust and to be the major cause of land degradation in arid and semiarid Retta, 2000], air pollution and consequent aggravation of regions of the world. On a global scale, 432.2 million (M) ha respiratory diseases [Pope et al., 1995; Choudhury et al., of drylands are susceptible to wind erosion [Middleton and 1997; Griffin et al., 2001], alteration of global biogeo- Thomas, 1997], with the continent of Africa having the chemical cycles [Duce and Tindale, 1991; Okin et al., 2004; largest wind erosion susceptible area (159.8 M ha) followed Mahowald et al., 2009], and changes in atmospheric radia- by Asia (153.1 M ha), Europe (38.7 M ha), North America tion budgets [Ramanathan et al., 2001; Kaufman et al., (37.8 M ha), South America (26.9 M ha), and Australasia 2002]. In arid and semiarid environments aeolian processes (15.9 M ha). In the United States wind erosion is the dominant redistribute soil particles and nutrients [Schlesinger et al., 1990; problem on about 30 M ha of land area, and the soil loss by Okin and Gillette,2001;Ravi et al., 2007a] and thereby affect wind erosion is estimated to be 840 million tons per year [Soil the soil surface texture and hydrologic properties [Lyles, 1975; Survey Division Staff, 1993]. In particular, wind erosion is a Offer et al.,1998;Li et al., 2009a]. These changes in soil serious problem in the cultivated soils in the Great Plains, characteristics affect the composition, productivity, and spatial where wind erosion rates are comparable or may exceed those patterns of vegetation [Schlesinger et al., 1990; Ravi et al., of water erosion [Hagen and Woodruff, 1973; Ervin and Lee, 2007a, 2008]. 1994; Nordstrom and Hotta, 2004]. In this region, dramatic [4] Atmospheric processes are notably sensitive to aero- soil losses and dust emissions (the “Dust Bowl”) were sols generated by aeolian processes. The entrained soil observed in particular in the 1930s [Bennett, 1938; Worster, particles are transported in the atmosphere by advection and 1979] as a result of poor land use practices in conjunction turbulent diffusion and are finally deposited through dry and with dry climatic conditions. Similar examples of intense wet removal [Shao, 2008]. The finer soil particles are aeolian activity induced by the combined effect of drought preferentially eroded and carried away by wind over large and land use can also be found in other regions of the world, distances [Swap et al., 1996a], and large quantities of these for example, southern and West Africa, South America, and nutrient‐rich particles are deposited in the oceans and on Australia [Gillieson et al., 1996; Buschiazzo et al., 1999; other continents, with implications for global biogeochem- Bielders et al., 2000]. ical cycles [Duce et al., 1991; Okin et al., 2004]. Moreover, [6] Aeolian processes are also recognized as major abiotic the global dust emission resulting from wind erosion is the drivers in the Earth system, and there is a growing interest in largest source of tropospheric aerosols and affects the atmo- the scientific community to quantify and model the bio- spheric radiation balance [Tegen et al., 1996; Ramanathan physical drivers and biogeochemical implications of aeolian 2of45 RG3001 Ravi et al.: AEOLIAN PROCESSES AND THE BIOSPHERE RG3001 processes at different scales [Field et al., 2010; Ravi et al., Barbados, the Bahamas, Florida [Muhs et al., 2007a], and 2010]. Global climate models predict