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INTERNATIONAL JOURNAL OF ENVIRONMENTAL SCIENCES Volume 1, No 5, 2011 © Copyright 2011 All rights reserved Integrated Publishing Association Research article ISSN 0976 – 4402 Assessing the Impacts of Desert Afforestation on the Spread of Infectious Agents Richard A. Manfready Department of Biology, Massachusetts Institute of Technology Cambridge, MA, USA [email protected] ABSTRACT Afforestation of the Sahara and Australian deserts has been proposed as a geoengineering technique by which to mitigate the effects of greenhouse gases in the Earth's atmosphere. The afforestation proposal entails planting and irrigating eucalyptus forests on a massive scale in the present­day arid Sahara desert—an expensive but potentially effective way to sequester atmospheric CO2. Several unintended consequences have been associated with this technique, to include salt deposition, decreased oceanic fertilization by dust, and locust swarms. However, the effect of desert afforestation on the propagation of disease­carrying avian species has not been studied. It is hypothesized that afforestation of the Sahara will increase the number of avian species carrying disease to European and sub­Saharan regions. To assess this possibility, a field test scheme is presented to measure avian flux through the trans­Saharan region via radar­based monitoring. The test will assess flux for both arid and afforested conditions at several points along major trans­Saharan flyways, with emphasis on zones known to be fatal for Sahara­crossing birds. Results from the field experiments will be input into an in silico model that will extrapolate the findings over the entire Sahara region and incorporate other parameters such as breeding. The preliminary model described here will simulate flux of disease­carrying avian species across the Sahara for a user­defined number of migratory seasons, and will compare changes in species­specific flux, migratory patterns, and cross­infection between arid and afforested scenarios. It is expected that desert afforestation will heighten trans­Saharan flux of disease­carrying avian species. If this prediction is validated by the simulation, then European and sub­Saharan regions may be at greater risk of avian­borne disease if Sahara afforestation is implemented. Desert afforestation as a geoengineering technique must be critically assessed with respect to its potential effects on disease vector propagation before its implementation is considered. The proposed experiments provide an outline with which to effectively estimate such effects, should the need for long­term risk assessment arise. Keywords: Afforestation, avian, desert, disease, field simulation, geoengineering, infection 1. Introduction Reducing fossil fuel burning and trading industrial emission rights are often cited as the most direct strategies to combat the ongoing change in global climate. Yet, a number of edge studies designed to radically reverse climate change have gathered momentum over the last decade. Collectively, these studies refer to “geoengineering” techniques that will counteract large­scale dangerous or unwanted aspects of the biosphere by deliberate manipulation of the environment (Keith, 2000). While some of these techniques aim to enhance the Earth System by reducing the prevalence of natural disasters, others attempt to permit the continual
Received on January, 2011, Published on February 2011 901 Assessing the Impacts of Desert Afforestation on the Spread of Infectious Agents existence of life on earth by reducing or eliminating the impact of unsustainable human practices. One class of geoengineering technologies is concerned with harnessing and amplifying greenhouse gas (GHG) storage techniques, either natural or manmade. The oceans have been proposed as primary stages for these technologies, particularly because of their high potential for carbon fixation and storage. For example, by fertilizing the oceans with iron, stimulated phytoplankton growth might increase the amount of CO2 that sinks and is stored at the ocean’s bottom (Buesseler et al., 2008). Yet, eutrophication caused by ocean iron fertilization might disrupt natural phytoplankton bloom cycles and food webs (Chisholm et al., 2001). Others have proposed separating CO2 from waste gas steam and sequestering it by injection into the deep ocean (Caldeira and Rau, 2000). However, CO2 injection can cause a decrease in ocean water pH, thereby endangering deep­sea animals, many of which have low tolerance for acidic water (Seibel and Walsh, 2001). Although the oceans can serve as a massive sink for anthropogenic CO2, the terrestrial biosphere represents a second, though smaller, reservoir for carbon sequestration. Once considered the “missing sink” (Siegenthaler and Sarimiento, 1993), this reservoir contains much of its carbon in living plant biomass. Consequently, reforestation has been suggested by the National Academy of Science in 1992 as one of four geoengineering strategies to mitigate climate change (Keith, 2000) (NAS, 1992). Replanting forests that have been burned or clear­cut is expected to promote carbon fixation. 2. Desert Afforestation as Geoengineering Instead of replanting preexistent forests by reforestation, a novel (and potentially more effective) strategy is to plant new forests—a process called afforestation. Previously, geoengineers had considered afforestation at temperate high latitudes, but recent models have shown such projects to be counterproductive: the high albedo of barren land has a higher net cooling effect on climate than forests, even when CO2 release by deforestation is accounted for (Bala et al., 2007). In 2009, Leonard Ornstein of the Mount Sinai School of Medicine revived afforestation ideas by proposing large­scale forest planting in the now­arid subtropical regions of the Sahara desert and Australian Outback (Ornstein et al., 2009). Unlike afforestation at high latitudes which has been shown to significantly reduce surface albedo, afforestation in subtropical regions could promote net CO2 sequestration and produce an increase in evapotranspiration, which could lead to an increase in cloud cover and hence atmospheric cooling despite loss of albedo (Bala et al., 2007). Ornstein et al. (2009) suggest that the amount of CO2 sequestered by an afforested Sahara desert and Australian Outback could compensate for all CO2 released by fossil fuel burning. In the Sahara, the afforestation proposal consists of planting 5­8 year old Eucalyptus grandis plantations (100 m canopy height) over a landmass of 9.8 x 10 8 ha at a density of 1,000 trees per ha. The forest would be irrigated with 500 mm of desalinated water per year, and would sequester 6­12 Gt C each year in wood, bark, leaves, and roots; this rate is suspected to last about a century (Ornstein et al, 2009). Irrigation for the massive proposed forests could be accomplished through freshwater provided by reverse osmosis and desalination of seawater (Ornstein et al, 2009). Ornstein
R.A. Manfready International Journal of Environmental Sciences Volume 1 No.5, 2011 902 Assessing the Impacts of Desert Afforestation on the Spread of Infectious Agents calculates that the full irrigation cost could be covered by a $1 USD increase in the per­gallon pump price of gasoline, even assuming no rainfall. Irrigation costs could be further mitigated by profits generated from local communities, which could take part in harvesting and replanting parts of the forest. By capitalizing on the new forest, these communities could create a fruitful biomass market and could help revitalize the forest ecosystem by increasing tree turnover and helping to offset irrigation costs, thus driving the ecological and environmental benefits of carbon sequestration (Figure 1). Such stewardship could be modeled after “social forestry” programs in Nigeria, which have already enabled afforestation in some regions without external funding (Westaneys and Woodley, 1998). Aside from questions concerning the political, economic, and social feasibility and sustainability of large­ scale afforestation projects, Ornstein et al. (2009) speculate that the technology may create an excess of salt output from desalination, an increase in hurricanes, loss of ocean nutrients leading to a lack of oceanic nitrification, and locust swarms. The original study does not address potential changes in avian disease vector propagation. Figure 1: Benefits of desert afforestation as a geoengineering technology Once implemented, desert afforestation, an example of a CO2 capture geoengineering technique in the terrestrial biosphere, must be supported by sustainable maintenance practices. Since maintaining large­scale afforestation would necessarily require high costs, it is suggested that irrigation and forest turnover could be driven by local market interests (Ornstein et al., 2009) and social forestry (Westaneys and Woodley, 1998) which in turn could share the benefits of carbon sequestration and capital. This study will assess the impacts of Sahara desert afforestation on disease­carrying bird migration. Results will be used to predict the impacts of a new forest ecosystem on the propagation of bird­borne disease to countries surrounding the Sahara. 3. Experimental Design/Proposal 3.1 The Scope of Unintended Consequences Associated with Desert Afforestation Despite promises of large­scale CO2 sequestration, proposals to green the world’s deserts may result in several unintended consequences, some of which may fatally destabilize human or environmental processes if unaddressed. Salt leaching, alteration of weather patterns, an increase in natural disasters, and an accelerated spread of disease, all have the potential to increase the risks and costs of afforestation beyond even the highest estimates.
R.A. Manfready International Journal of Environmental Sciences Volume 1 No.5, 2011 903 Assessing the Impacts of Desert Afforestation on the Spread of Infectious Agents Efficient, cost­effective strategies are needed for desalination. Proposals to desalinate seawater via alternative energy sources such as solar power (Sinha, 2002) may effectively address cost concerns associated with desalination, but do not address concerns over storage of extracted salts. If not diluted and disposed in the ocean properly, saline waste from desalination will cause anoxic zones in oceanic basins, and disturb thermohaline circulations (Ornstein et al., 2009), upsetting global climate. Additionally, if irrigation in an afforested region is not sustained, natural dry spells that have eradicated the historical Saharan and Australian forests (Bradstock et al, 2002) may once again eliminate expensive desert vegetation. Elimination of vegetation may lead to declining availability of surface dust, posing a threat to climate. Decreasing the amount of dust normally deposited into the ocean is likely to rid the oceans of an important source of iron (Jickells et al., 2005). Surface dust could also be rendered unavailable by desert irrigation alone. Since dust from the Sahara may have the potential to suppress hurricanes (Lau and Kim, 2007), decreasing dust cover by irrigation might cause more hurricanes than would be eliminated by vegetative carbon sequestration. Ecosystem consequences of desert afforestation have been largely unexplored. Such consequences include unintended alteration of avian migration patterns. Modified migration patterns have the potential to give rise to new routes of disease vector transportation. 3.2 Disease Vector Propagation Hypothesis The proposed study will assess the hypothesis that if a mature forest were to cover the present­day Sahara desert, then more migratory species that potentially carry disease vectors would traverse the trans­Saharan Region. Although most infectious disease vectors are carried by poultry and livestock, migrating waterfowl and other wild birds could also be sources for disease transport. Low pathogenic avian influenza (LPAI), viruses with pathogenic potential, have been found in 26 different families of wild bird species; many of these species winter and breed on opposite sides of the Sahara, providing a possible link between African and Eurasian influenza incidence (Olsen et al., 2006). However, the present­day arid Saharan landmass can be inhospitable for the crossing of migratory birds, especially juveniles (Strandberg et al, 2010). It is predicted that afforestation of the Sahara will increase trans­Saharan avian migration, potentially causing an increase in avian­borne disease in Europe and sub­Saharan regions. 3.3 Experimental Approaches 3.3.1 Migration Initiation Field Assay Given field and in silico representations of arid and afforested scenarios, hypothesis evaluation will be made possible by 1) conducting field tests to determine the effect afforestation might have on trans­Saharan migratory flux, and by 2) using the field data to model changes in large­scale migration dynamics to predict disease propagation risk. Wild birds with the potential to carry influenza A virus migrate over the present­day arid Sahara desert in three broadly­defined patterns: the East Atlantic (EA) flyway, the Black Sea­ Mediterranean (BSM) flyway, and the East Africa­West Africa (EAWA) flyway (Olsen et al, 2006). Birds traversing the Sahara in the southbound direction enter Africa at a rate of 155,376 per kilometer longitude (entering from north or south), yielding an average daily flux
R.A. Manfready International Journal of Environmental Sciences Volume 1 No.5, 2011 904 Assessing the Impacts of Desert Afforestation on the Spread of Infectious Agents of about 2,486 birds per km (Moreau, 1961). Despite its pervasiveness, crossing the desert is perilous. Strandberg et al. (2010) measured the risk of crossing the Sahara by satellite telemetry, and mapped the locations of mortality and retreat for four raptor species. Strandberg observed that the average point of death or retreat is about 700 km (rough graphical estimate) beyond the edge of the desert. To assess the flux of wild birds across the desert in arid conditions (control), six field stations (each 3 km 2 ) will be constructed within the arid region of the Sahara, three in the northern region and three in the southern region. Two stations will be located on the midline of each flyway generalized by Moreau (1961), positioned at a “fatal point” 700 km beyond the desert edge (Figure 2). Four additional stations will be constructed along BSM and EAWA flyways, 700 km away from either “fatal point” station. Over the course of one year, the number of birds that cross through each station will be counted via C­band radar, which can assess density, speed, and direction of the migrating birds (Dokter et al., 2010). Scale and accuracy can be improved by linking multiple radars in a network (Dokter et al., 2010). Measurements will be taken for a full year, but salient data will be taken from biannual migratory periods. If possible, counts of species relevant for disease transmission (listed by Olsen et al. (2006), Moreau (1961), and Jourdain et al. (2007)) will be taken. After assessing migration for the arid case over a number of migratory seasons (> 5 for statistical significance due to year­to­ year variability), the 6 stations will be afforested with 100,000 6­year old Eucalyptus grandis trees at a density of 1,000 trees per ha (3x10 5 trees per 3 km 2 station) and irrigated once per experimental year with 500 mm/yr of desalinated water (Ornstein et al, 2009). Over the course of one year, bird flux (in both north­ and southbound directions) will be measured with the same technique used for the arid case, with focus on the biannual migratory seasons. The differences between fluxes in the arid case and the afforested case will be computed. Measurements will need to be taken several times (> 5) over the course of multiple decades to produce statistical significance. Figure 2: Migratory flux observation stations North­ and southbound avian flux will be measured across six 3 km 2 observation stations (green circles). Each station will be positioned at either the north or south end of a major trans­Saharan flyway, and will be 700 km from the edge of the desert—the distance at which wild raptors entering an arid desert have been shown to either perish or retreat. Additional stations will be added 700 km away from the four stations on BSM and EAWA to account for species that pass the fatal point. After flux through these stations is sufficiently measured in arid conditions, the stations will be afforested, and flux will be reassessed through the mature forests. These experiments will be repeated in replicates (> 5) to achieve statistical reliability. (East Atlantic flyway, EA; Black Sea­Mediterranean flyway, BSM; East Africa­West Africa flyway, EAWA; Degrees north latitude)
R.A. Manfready International Journal of Environmental Sciences Volume 1 No.5, 2011 905 Assessing the Impacts of Desert Afforestation on the Spread of Infectious Agents With the exception of afforestation, all other factors (ie. plot size, observation locations and times) will be held constant between arid (control) and afforested (experimental) scenarios. 3.3.2 Modeling Migratory Fluxes Once the field tests yield changes in trans­Saharan avian flux due to small­scale afforestation, a quantitative model will be used to predict the impacts these changes can have on large­scale migration of disease­carrying species. The model will be built on a digital simulation platform such as a modification of SEARUMS (Rao et al., 2009) that incorporates migratory routes of influenza­carrying avian species. Figure 3: In silico simulation of migratory changes due to afforestation Arid and afforested scenarios will be built from principal input parameters (a) taken from the literature as well as from novel field experiments. For modeling purposes, the African Continent will be divided into three geographical sectors. Flyway, survivability, and breeding parameters (Olsen et al, 2006) (Moreau, 1961) (Jourdain, 2007) will be critical inputs for simulation robustness. Arid and afforested avian fluxes (scaled­up from the field experiments) will be input as initial conditions for each of two scenarios. In the first scenario (b), avian flux data from the arid (control) field experiments will be input along with the other initial conditions. The simulation will continue to simulate flux for 50 years or until a steady state is reached. Avian flux data will then be replaced by data from the afforested (experimental) field experiments (c), and the simulation will run for 50 years or until an equilibrium is reached. Observed changes in species­specific flux, migratory patterns, and breeding­induced cross­infection will be used to estimate risk of disease propagation across an afforested Sahara. (European sector, S­EUR; Saharan interior sector, S­INT; sub­Saharan sector, S­SUB; Fluxes represented by gray arrows; Degrees north latitude) For simplicity, the model will consider the African Continent as three separate sectors, labeled from north to south: the European sector (S­EUR), the Saharan interior sector (S­ INT), and the sub­Saharan sector (S­SUB) (sector size is only relevant for S­INT, which will
R.A. Manfready International Journal of Environmental Sciences Volume 1 No.5, 2011 906 Assessing the Impacts of Desert Afforestation on the Spread of Infectious Agents be entered as 9.8 x 10 8 ha, about the general size of the Sahara desert (Ornstein et al, 2009). The first input to this system will be the flyway patterns described by Olsen et al. (2006). Next, baseline fluxes through each of these flyways (in north­ and southbound directions) will be incorporated from arid (control) field experiment results (average of replicates for each station), extrapolated to the area of each flyway. Initial trans­Saharan survivability will be set at 31% mortality for juveniles and 2% for adults (after Strandberg et al. (2010)). The model will focus on the 111 sub­Saharan bird species that have been shown to disperse pathogens in European areas such as the Camargue in France; interbreeding among members of each species will add further complexity to the simulation (Jourdain et al., 2007). Inputs are summarized in Figure 3a. This model can be validated by comparing it to flux data from Moreau (1961). The resultant simplified model will serve as the Arid (Baseline) Scenario (Figure 3b). After simulating the Arid Scenario until equilibrium is achieved (or for 50 in silico years if equilibrium is not achieved), arid (control) field experiment results will be replaced in the model by afforested (experimental) field experiment results (average of replicates for each station) extrapolated to the area of each flyway. This new model will assume complete afforestation of S­INT (an extrapolation of the field tests with limited afforestation). All other factors will be held constant between the Arid Scenario and this model, which will serve as the Afforested Scenario (Figure 3c). Although the model does not receive irrigation as an input, the afforested scenario will be based on field data taken at 500 mm/yr of irrigation. After simulating the Afforested Scenario until equilibrium or for 50 in silico years, differences in the migration of influenza­carrying birds between the two scenarios will be computed. Specifically, the simulation will measure the difference in the amount of potential disease vectors (infected birds) that complete the journey across S­INT in both directions (S­ EUR ßà S­SUB). If the simulation platform is sufficiently dynamic, then flyway patterns themselves may change with the onset of afforestation. Of particular interest will be the wader species that travel to S­SUB in late spring and summer and return to S­EUR for breeding, as well as vectors traveling to S­EUR that are known to spread West Nile Virus in Europe (Jourdain et al., 2007). 4. Conclusions and recommendations An investigation based on the aforementioned recommendations will assess potential changes in disease vector propagation caused by implementation of desert afforestation. The conclusions from this study will apply most directly to avian disease propagation across an afforested Sahara, but will also indicate whether afforestation of Australian deserts will require a similar assessment. The study proposed here is very preliminary and would be difficult and time­consuming to implement in full. However, it offers an approach to designing long­term infectious risk assessment of afforestation, if necessitated in the future. Results from the suggested field­ simulation coupled analysis will allow for better prediction of threats to human health that might result from implementation of a desert afforestation geoengineering scheme. An increase in species flux between Europe and Africa may promote a higher infection probability for people of both regions. The field experiment proposed here will gauge the change in avian flux across the Sahara in both northbound and southbound directions (S­EUR ßà S­SUB). Baseline fluxes will be measured by the control (arid) field experiment and
R.A. Manfready International Journal of Environmental Sciences Volume 1 No.5, 2011 907 Assessing the Impacts of Desert Afforestation on the Spread of Infectious Agents compared with estimates by Moreau (1961) for reliability. Results from arid and afforested field test scenarios will be extrapolated over the entire Sahara landmass via an in silico simulation, which will impose more detailed parameters to account for breeding and survivability along flyways. After simulating the scenarios over a user­defined number of migratory seasons, the model will reveal changes in migratory dynamics, specifically fluxes and breeding patterns, which may result from the implementation of afforestation. Moreover, the temporal nature of the simulation will reveal when these changes will take place. It is acknowledged that a model constructed by scaling up from small­scale experimental data may not take into consideration biotic, geographical, or behavioral heterogeneity (Underwood et al., 2005) that may dictate migratory responses to actual (complete) afforestation of S­INT. It is further acknowledged that differences in temperature and weather patterns between field observation years may confound the results, further necessitating experimental replicates. Despite these limitations, estimates of how and when the migratory patterns of disease­ carrying birds will change in response in the presence of an afforested desert will have important implications for the overall assessment of afforestation as a geoengineering technology. Afforestation is a costly process that is already accompanied by a number of potential unintended consequences (Ornstein et al, 2009). Provided that future developments obviate problems related to cost, irrigation, and dust cover, as well as overcome social and political roadblocks inhibiting this form of geoengineering, desert afforestation may become viewed as a viable solution to global climate problems. However, even a perfectly­ orchestrated implementation of desert afforestation will leave open the possibility that species distribution and mobility are likely to shift. Little is understood about the effects trans­ continental avian migration might have on the transmission of communicable disease, rendering the prediction of future changes in response to afforestation even more difficult. Nevertheless, strategic coupling of field and simulation experiments will enable problem­ specific data to be extrapolated over time and space in an effort to yield information about post­afforestation migratory patterns. An upregulation of migratory dynamics, for example an increase in S­SUB à S­EUR flux of northern pintail (Anas acuta) in the spring, does not directly signify an absolute increase in disease incidence. However, evidence supporting the spread of influenza from sub­Saharan Africa to the Camargue by the pintail and over 100 other species (Jourdain et al., 2007) suggests that an increase in flux might indeed be associated with an increase in disease incidence in the S­EUR region. The risk is compounded when breeding is taken into account, especially due to the opportunities for nesting in eucalyptus trees. Further work is needed to gauge whether nesting and permanent residence in the afforested region is a sound possibility. It is also recommended that further work address the extent to which the carrying capacity of various bird species will change in response to afforestation. If results from these and other experiments are able to link desert afforestation to a rise in European or sub­Saharan disease incidence, then afforestation may not be a safe geoengineering scheme for implementation. Alternative strategies, however, may involve similar quantities of vegetation for use in GHG sequestration, creating a need to identify other, perhaps non­terrestrial locations to contain the vegetation.
R.A. Manfready International Journal of Environmental Sciences Volume 1 No.5, 2011 908 Assessing the Impacts of Desert Afforestation on the Spread of Infectious Agents 5. Acknowledgments The author would like to thank K. Frois­Moniz, L.A. Roldan, and the Earth System/Ecology faculty of the MIT Department of Civil and Environmental Engineering. 6. REFERENCES 1. Bala G., Caldeira K., Wickett M., Phillips T.J., Lobell D.B., Delire C., and Mirin A., 2007, Combined climate and carbon­cycle effects of large­scale deforestation, PNAS, 104, pp 6550­6555. 2. Bradstock R.A., Williams J.E., and Gill M.A. (2002). Flammable Australia: the fire regimes and biodiversity of a continent. Cambridge University Press. 3. Buesseler K.O., Doney S.C., Karl D.M., Boyd P.W., Caldeira K., Chai F., Coale K.H., Baar H.J., Falkowski P.G., Johnson K.S., Lampitt R.S., Michaels A.F., Naqvi S.W., Smetacek V., and S. Takeda S., 2008, Ocean Iron Fertilization — Moving Forward in a Sea of Uncertainty, Science, 319, p 162. 4. Caldeira K., and Rau G., 2000, Accelerating carbonate dissolution to sequester carbon dioxide in the ocean: Geochemical implications, Geophysical Research Letters, 23, pp 225­228. 5. Chisholm S., Falkowski P., and Cullen J., 2001, Dis­Crediting Ocean Fertilization, Science, 294, pp 309­310. 6. Dokter A.M., Liechti F., Stark H., Delobbe L., Tabary P., and Holleman I., 2010, Bird migration flight altitudes studied by a network of operational weather radars, Journal of the Royal Society, Interface. 7. Jickells T.D., An Z.S., Andersen K.K., Baker A.R., Bergametti G., Brooks N., Cao J.J., Boyd P.W., Duce R.A., Hunter K.A., Kawahata H., Kubilay N., LaRoche J., Liss P.S., Mahowald N., Prospero J.M., Ridgwell A.J., Tegen I., and Torres R., 2005, Global iron connections between desert dust, ocean biogeochemistry, and climate, Science, 308, pp 67­71. 8. Jourdain E., Gauthier­Clerc M., Bicout D.J., and Sabatier P., 2007, Bird migration routes and risk for pathogen dispersion into western Mediterranean wetlands, Emerging Infectious Diseases, 13, pp 365­72. 9. Keith D.W., 2000, Geoengineering the Climate: History and Prospect, Environmental Ethics, pp 245­284. 10. Lau W.K., and Kim K., 2007, How Nature Foiled the 2006 Hurricane Forecasts, Eos, Transactions American Geophysical Union, 88, p 105. 11. Moreau R.E., 1961, Problems of Mediterranean­Saharan Migration, Ibis, 103a, pp 580­623. 12. NAS (1992). Policy implications of greenhouse warming, National Academies Press.
R.A. Manfready International Journal of Environmental Sciences Volume 1 No.5, 2011 909 Assessing the Impacts of Desert Afforestation on the Spread of Infectious Agents 13. Olsen B., Munster V.J., Wallensten A., Waldenström J., Osterhaus A.D., and Fouchier R.A., 2006, Global patterns of influenza a virus in wild birds, Science, 312, pp 384­8. 14. Ornstein L., Aleinov I., and Rind D., 2009, Irrigated afforestation of the Sahara and Australian Outback to end global warming, Climatic Change, 97, pp 409­437. 15. Rao D., Chernyakhovsky A., and Rao V., 2009, Modeling and analysis of global epidemiology of avian influenza, Environmental Modelling & Software, 24, pp. 124­ 134. 16. Seibel B.A., and Walsh P.J., 2001, Potential Impacts of CO2 Injection on Deep­Sea Biota, Science, 294, pp 319­320. 17. Siegenthaler U., and Sarimiento, J., 1993, Atmospheric carbon dioxide and the ocean, Nature, 365, pp 119­125. 18. Sinha S., 2002, Solar desalination of saline soil for afforestation in arid areas: numerical and experimental investigation, Energy Conversion and Management, 43, pp 15­31. 19. Strandberg R., Klaassen R.H., Hake M., and Alerstam T., 2010, How hazardous is the Sahara Desert crossing for migratory birds? Indications from satellite tracking of raptors, Biology letters, 6, pp 297­300. 20. Underwood N., Hambäck P., and Inouye B.D., 2005, Large­scale questions and small­ scale data: empirical and theoretical methods for scaling up in ecology, Oecologia, 145, pp 177­8. 21. Westaneys C., and Woodley E., 1998, Afforestation and Social Forestry in Northern Nigeria: a success story in desertification / land degradation control, International Journal of Climate Change Strategies and Management, pp 1­6.
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