Possible Implications of Increased Carbon DioxideLevels and Climate Change for Desert EcosystemsE. LIOUBIMTSEVAGeography and Planning DepartmentGrand Valley State UniversityAllendale, Michigan 49401, USAJ. M. ADAMSEarth and Environmental Sciences DepartmentWesleyan UniversityMiddletown, Connecticut 06459, USAABSTRACT / Despite the considerable progress achievedduring recent years in quantifying and modeling climatic andecological processes caused by increasing concentrations ofgreenhouse gases in the atmosphere, there are still major un-certainties regarding the potential effects of increasing con-centrations of either CO2(carbon dioxide) or future climatechange in arid ecosystems. General Circulation Models pre-dict varying patterns of moister or drier conditions in desertsfor the next century, but the results of climatic and ecosystemmodeling in relation to deserts in a future “greenhouse effect”climate are complex and contradictory. Nevertheless, ifdeserts do respond more dramatically to global temperaturechange, as they did during the Holocene and, especially thelast interglacial era (130,000 years ago), they might act as glo-bally significant sinks of carbon into soils and vegetation.Some growth chamber experiments have indicated that in-creased CO2will significantly affect desert shrubs, whereasother chamber and field experiments suggest that rising levelsof atmospheric CO2may not dramatically affect desert eco-systems, although certain individual species may be stronglyfavored. It is difficult to make a firm statement whether thereare any valid analogs between the climate changes of the pastand future climate change induced by greenhouse gases.It has often been suggested that deserts and semi-deserts will be one of the most responsive of the world’secosystem types to the ongoing increase of carbon di-oxide (CO2) in the atmosphere and associated globalclimatic changes (Bazzaz 1990; IPCC 2001; Smith andothers 2000). Yet despite the considerable progressachieved during recent years in quantifying and mod-eling climatic and ecological processes caused by in-creasing concentrations of greenhouse gases in theatmosphere, there are still major uncertainties regard-ing the potential effects of increasing concentrations ofeither CO2or future climate change in arid ecosystems.Evaluation of the role of deserts and semideserts aspotential carbon sinks or sources requires further datacollection, experimental work, modeling, and interdis-ciplinary exchange to improve our understanding ofclimate–ecosystem interactions at various spatial andtemporal scales.Because it has a low organic carbon storage per unitarea, the desert biome is often ignored in the biogeo-chemical literature, and its role in the global system hasattracted relatively little interest from scientists. Yetdeserts and semideserts are the most extensive of theworld’s biome types, and the possibility that their ex-tent may drastically alter under future global change[as has been the case under changes in the recentgeological past (Petit-Maire and Guo 1996; Lioubimt-seva and others 1998)] warrants further considerationof their responsiveness. To the many millions of peoplewho live in desert-margin environments, such questionsare certainly of more than academic interest.The present review has two objectives:1 To summarize and discuss current understandingby several disciplinary areas of the effects of increas-ing concentrations of CO2and accompanying cli-mate changes on temperate and tropical desert andsemidesert ecosystems of the world2 To consider the role that changes in desert extentand desert processes might play in the future car-bon cycleThe World’s Deserts and SemidesertsDeserts are created where climatic, topographic, oroceanographic factors exclude moisture-bearingweather systems. In subtropical desert regions, such asthe Saharan and Arabian deserts, the southwesternNorth American semideserts, and the Australian semi-deserts, aridity is caused by a stable tropical-to-subtrop-ical convection system known as a Hadley Cell. SolarKEY WORDS: Carbon dioxide; Desert; Ecosystem; Precipitation; Fertil-ization; BiomassPublished online January 23, 2004.DOI: 10.1007/s00267-003-9147-9Environmental Management Vol. 33, Supplement 1, pp. S388–S404© 2004 Springer-Verlag New York, LLCheating of the atmosphere is strongest at the equator,causing air to rise and then cool. The condensation ofwater in this cooling air forms clouds and an equatorialrainy belt. The rising equatorial air spreads out at highaltitude toward the poles and descends in the subtrop-ics at around 30° north and south. The now-dry airwarms due to compression as it descends, creating aconsistently warm and dry region in which many of theworld’s desert and semidesert areas exist. These aridlands are surrounded by narrow semiarid regions. Whatlittle rainfall does occur in such regions comes mainlyfrom seasonal monsoons penetrating high- pressurecells from the equator.At higher latitudes beyond the subtropics, air circu-lation is controlled more by the rotation of the Earth,and by the temperature contrast between the equatorand the poles. In general, this high-latitude pattern isnot conducive to creating aridity.The distance from sources of moisture (continental-ity) also encourages aridity; arid regions tend to occurnear the centers of large landmasses. Because the outertropical wind belts known as the Trade Winds blowfrom east to west, deserts and semideserts commonlyextend westward to the coasts because these winds losetheir moisture in traveling across large continents. Thiseffect produces relatively localized arid zones sur-rounded by broad semiarid regions.Mountain ranges prevent moisture-laden air fromentering some regions by forcing the air to rise, whichmakes it cool and causes the water to condense andprecipitate. On the lee side, the descending air iswarmed and dried adiabatically. Classic examples ofarid lands caused by rain shadow effect are semidesertsin the western United States and eastern Australia.Likewise, the rain shadow effect of the Andes producesthe Argentinean drylands.Finally, cold ocean currents flowing from the highlatitudes toward the equator cause low rates of sea-surface evaporation, low precipitation (often as fog anddew due to the lower temperatures), and a low temper-ature range, reinforcing other climatic factors. Thisphenomenon affects the western coastal margins ofSouth America, southern Africa, and Australia. For