over the last ten years , biomass - based 
 liquid transportation fuels , 
 known collectively as  biofuels ,  have increasingly 
 become part of the world s planned energy mix . in the united 
 states , 36 billion gallons of biofuels are slated to be in use by 
 2022 , which will represent approximately 11% of the country s 
 transportation fuel demand . energy information administration 2010 annual energy outlook 
 reference case projection , see table 2 ( http://www.eia.gov/oiaf/archive/aeo10/aeoref_tab.html ) . this assumes that 36 billion gallons will consist of 31 and 5 
 billion gallons of ethanol and biodiesel , respectively , and adjusts 
 energy content of both fuels relative to gasoline . ) ten percent of 
 the fuel consumed for transportation by 2020 must be from renewable 
 sources in the european union , while brazil retains a long - standing 
 2025% ethanol mandate . biofuels 
 are commonly touted as a dual solution to the problems 
 of dependence on foreign energy sources and climate change , but their 
 impacts are not confined to these two areas . energy production and 
 use is one of the most intensive human enterprises , with numerous 
 economic and societal benefits , as well as societal and environmental 
 drawbacks . any policy that seeks to significantly alter the form and 
 method of energy exploitation raises the specter of unintended or 
 misjudged consequences . the goal of greenhouse gas ( ghg ) savings was 
 challenged after indirect land use change , resulting from displaced 
 food and feed production , was included in the estimation of emissions . though estimates of indirect land use change 
 attributed to biofuels policies continue to be debated in the literature , such attention to the unintended consequences 
 of biofuel production and use has stimulated discussion of other potential 
 economic , social , and environmental impacts , including effects on 
 food security , environmental justice , and biodiversity conservation . many international groups are now proposing to 
 measure , track , and rank forms of renewable energy , including but 
 not limited to biofuels , based on their ability to meet environmental 
 criteria that may not have been previously considered . the capability of decision - makers to evaluate the 
 total impacts 
 of biofuels , including their environmental and economic trade - offs , 
 and the accuracy of those evaluations depend on the availability of 
 robust data and analyses . here , we ask : does the published , peer - reviewed 
 literature present a full portrait of biofuels and their potential 
 impacts and trade - offs ? confidence in incipient policies governing 
 biofuels that aspire to be environmentally , economically , and socially 
 conscious , would be enhanced by an affirmative finding . to answer 
 the above question , we analyzed 1622 biofuels - related 
 articles published between 2000 and 2009 across a range of social , 
 environmental , and technical topics . we also examined geographic trends . 
 finally , we examined the frequency of interdisciplinary research as 
 an index of our understanding of the complex interactions and feedbacks 
 possible under global biofuel production scenarios . such an approach 
 has previously been used to visualize connections between concepts 
 in the scientific literature and to identify logical gaps in study . rather , our primary goal was to ascertain the structure 
 of biofuels research efforts in the past decade in order to stimulate 
 discussion of its utility to a range of stakeholders . we first defined three major areas of research ( termed  themes  
 hereafter ) : environment and human well - being ( eh ) , economics ( ec ) , 
 and production - distribution technology and infrastructure ( pti ) . we also included 
 geography as a fourth theme , to assess the degree to which other topics 
 had been discussed in different regional contexts around the world . number of articles has been adjusted 
 by a relevancy ratio as described in the text . percentages add up 
 to more than 100% because some articles address more than one topic . for each of the topics , we generated an initial set 
 of search terms , 
 or keywords , utilizing both our own knowledge of the biofuels research 
 field and the knowledge of a group of elicited experts . generating 
 search terms a priori rather than deriving them from 
 the literature allowed us to describe patterns in expert - identified , 
 emerging topics in biofuels research in addition to established topics , 
 as well as the relationships between them . we put the keywords through an objective two - step vetting process 
 using isi web of knowledge to ensure their appropriateness / specificity 
 ( defined generally as whether the keyword pertained unambiguously 
 to biofuels in the context of liquid transportation fuels ) , and redundancy 
 ( defined as whether the keyword failed to access a unique subset of 
 the literature ) . biofuel * and keyword  , selected 10 articles at random from the search output , 
 and read the abstracts . we retained the keyword if 2 articles 
 were deemed relevant to the topic with which the keyword was associated . if the combination did not produce more results than either search 
 individually , we only retained the keyword with more results . keywords for geographic regions consisted of the name of the geographic 
 region itself plus the names of the top 20 countries by population 
 ( 1 million people ) in the year 2009 . one exception was africa , where we used zambia instead of niger , 
 because of ambiguity with the microbe aspergillus niger used for biosynthesis of enzymes for biofuel feedstock catalysis . our geographic search sought to describe the regions that were the 
 subject or field location of biofuels research , not the nationality 
 of the authors or funding bodies . we used isi web of knowledge 
 ( including the science and social science citation indexes ) to search 
 peer - reviewed full - length articles and reviews , as well as shorter - form 
 letters , published between 2000 and 2009 . for each topic for example , the search for 
 trade consisted of :  biofuel * and ( trade or import or export 
 or tariff )  . this process resulted in 23 individual topic searches . 
 to examine the connectedness of literature between topics , we searched 
 all pairwise combinations , except pairs of regions . for individual topic searches , we randomly 
 selected 20 abstracts ( or all abstracts if fewer than 20 articles 
 were retrieved ) and read each to evaluate if the article was relevant 
 to the topic . an acceptance rate was determined and multiplied by 
 the raw number of retrieved articles to produce an  adjusted acceptance rates ranged from 0.15 to 0.80 , with 
 an average of 0.52 . for simplicity , when adjusting paired searches , 
 we multiplied the average acceptance rate for all topics by the raw 
 number of articles retrieved to produce the adjusted number of articles . 
 we checked a random subsample of relevance ratios for topic - pairs 
 and found no systematic trends or bias . briefly , 
 we arranged all of our topical areas , except for regions , as nodes 
 on the perimeter of a circle . lines , or edges , connecting nodes were 
 drawn between all topics where we found evidence for literature that 
 addressed both . line thickness was adjusted to categorically reflect 
 the number of articles that addressed a given pair of topics . we found a wide range of 
 research activity on the themes and topics examined in this analysis 
 ( table 1 ) . the number of peer - reviewed articles 
 addressing the economic ( ec ) aspects of biofuels was approximately 
 half that of either environment and human well - being ( eh ) or production - distribution 
 technology and infrastructure ( pti ) articles . the most common topics 
 in the literature were fuel production technologies , ghg emissions , 
 and feedstock production / agronomics , each with over 300 articles . the least studied topics , excluding geographical categories , were 
 human health , trade , and biodiversity , which retrieved 129 articles 
 combined . human health and trade , in particular , were represented 
 by a very limited number of articles ( 15 and 34 , respectively ) . analyses by region revealed substantial heterogeneity in total research 
 effort and in topical emphasis . overall , asia and north / latin america 
 received the most attention in the literature , oceania and africa 
 the least , and south america and europe received intermediate attention 
 ( table 1 , figure 1a ) . we also calculated the relative effort per topic for each geographic 
 area to get a picture of the differing emphases between regions . this 
 was calculated as the ratio of the number of adjusted articles in 
 a particular topic within a region over the average for all topics 
 within that region . ghg s , land use / land change ( lulc ) , and 
 feedstock production had strong representation in the literature across 
 regions , while other topics , including biodiversity and human health , 
 showed consistent lack of literature across regions ( figure 1b ) . the number of water resources articles was equal to 
 the average for asia and north / latin america , and was comparatively 
 underrepresented for the rest of the world , europe in particular . articles on food security were above average for africa and oceania , 
 and below average for all other regions . air quality showed some 
 of the largest differences among regions , overrepresented in asia 
 and africa ( though low in total effort in the latter ) and underrepresented 
 elsewhere . there were few studies in africa , an already understudied 
 region , on the trade and market potential for biofuels . ( b ) ratio of the 
 number of articles per topic for a region to the average number of 
 articles per topic for that region . eh = environment and human well - being , pti = production - distribution 
 technology and infrastructure , ec = economy . the dotted line indicates 
 where topical emphasis for a region is equal to the average for that 
 region . numbers of articles in both panels have been adjusted by a 
 relevancy ratio as described in the text . ( c ) raw number of articles 
 retrieved per year from 2000 to 2010 , unadjusted by a relevancy ratio . our network analysis revealed 
 strong asymmetry in the connectedness of different research topics 
 in the biofuels literature . among the 23 topics we examined , ghg emissions 
 was the most well - connected , sharing at least 50 articles with 11 
 other topics ( figure 2 ) . land use / land cover , 
 feedstock production / agronomics , fuel production technology , and costs 
 of production were also well - connected to other topics , including 
 topics outside of their respective thematic areas . not surprisingly , 
 the same topics with the fewest numbers of articles were also the 
 most isolated . human health , trade , biodiversity , and feedstock logistics 
 tended to be loosely associated or unassociated with articles in other 
 disciplines . noticeably scarce were studies on how economic policy , 
 market factors , and trade might influence how biodiversity is impacted 
 by biofuels . although the lack of connections appeared to be a function 
 of the small number of articles in the case of human health and trade , 
 it was not exclusively so for biodiversity . averaging the proportion 
 of articles connected to each topic , biodiversity articles addressed 
 another topic only 15% of the time versus , for example , 27% of the 
 time for ghg articles . network diagram showing the number of articles in common 
 between 
 all pairs of topics in the analysis except geographic regions . eh = environment and human well - being , 
 pti = production - distribution technology and infrastructure , and ec 
 = economy . numbers of articles have been adjusted by a relevancy ratio 
 as described in the text . our analysis reveals substantial heterogeneity 
 in the amount of 
 research on the numerous critical topics relevant to an industry with 
 as wide - reaching effects as biofuels . this holds for individual topical 
 areas and geographic regions , as well as for cross - topic linkages . it is debatable whether this asymmetry is appropriate and whether 
 the current body of literature is mature enough to effectively support 
 the informed development of this international industry ; or whether 
 the research gaps we have identified indicate substantial risk of 
 unanticipated , unintended consequences . in either case , we argue that 
 there is enough uncertainty to warrant caution , especially in several 
 understudied topics and regions . the paucity of studies addressing 
 human health , trade , and biodiversity 
 has several implications . with the escalation of biofuel production 
 and use locally and globally , a renewed examination of risks and hazards 
 for human populations facing chemical exposures is certainly warranted . 
 the u.s . environmental protection agency s regulatory impact 
 analysis of the 2007 revised renewable fuel standard program found 
 that increasing biofuel production and use would lead to increases 
 in particulate matter and ozone , leading to net negative impacts on 
 several measures of human health . exposure 
 of humans to chemicals via other routes , including surface water and 
 groundwater , were not assessed in the aforementioned analysis . additionally , 
 trade and trade policy will play an increasingly important role in 
 how and where feedstocks and biofuels are produced worldwide . in 2007 , 
 approximately 27% of europe s mandated renewable transportation 
 fuel use was being met by imported biodiesel and ethanol . meanwhile , the u.s . continues to promote domestic 
 production through the use of import tariffs on ethanol from countries 
 like brazil . the elimination of this tariff could increase ethanol 
 imports to the u.s . in 2015 by as much as 87% over projected volumes 
 under current policies . of note , such 
 a change in trade policy coupled with the elimination of the ethanol 
 tax credit passed in the u.s . senate in the summer of 2011 . finally , large - scale production and use of biofuels is likely to 
 modify several of the stressors that already threaten regional biodiversity , 
 including land use change , climate change , air pollution , and biological 
 invasions , e.g. , refs ( 10a10c ) . despite recognition of this in some scientific circles , a lack of research on the effect 
 of biofuels on biodiversity was also identified by a recent report 
 by the national research council . to 
 put this into context , a similar isi search using  biodiversity  
 and  climate change  yielded many more retrieved articles : 
 2657 published between 2000 and 2009 . regionally , the same pattern 
 holds ; adding south america or asia keywords to the above biodiversity - climate 
 change search yielded 95 and 141 articles , respectively  easily 
 surpassing the 5 ( south america ) and 6 ( asia ) articles at the nexus 
 of biofuels , biodiversity , and these regions . though the number of 
 climate change  biodiversity articles was not adjusted per our 
 analysis , they nevertheless exceed those of biofuels by over an order 
 of magnitude . our results strongly imply that empirical data or modeling 
 efforts on the effects of biofuels , explicitly using biodiversity 
 as a dependent variable , are sorely needed . the need may be greatest 
 in countries such as brazil , indonesia , and malaysia , which contain 
 biodiversity hotspots of global importance and are already centers of biofuel production . an under - representation 
 of developing regions in the biofuels literature 
 was found across topic areas . this raises questions about impacts 
 of the industry in places where large - scale development of biofuels 
 could either reap great rewards if done sustainably , or lead to widespread 
 environmental degradation if done irresponsibly . exploring the data set further , we also found a country bias within these regions . for 
 instance , roughly 95% of the asia articles were from china and india , 
 almost 80% of the south america articles exclusively addressed brazil , 
 and roughly 75% of the north america / latin america articles focused 
 on the u.s . thus , vast areas of the world remain virtually unstudied , 
 potentially creating lags to implementation of international monitoring 
 and standards for sustainable biofuel development . furthermore , 
 our analysis of topics according to region revealed 
 some important geographic gaps in research effort . erosion in asia 
 has long been an environmental problem , in particular from the vast 
 agricultural regions of the loess plateau in china covering over 600000 
 km , yet little attention 
 has been paid to examining how biofuels production may impact this 
 sensitive area . surprisingly , market factors and trade were just as 
 underrepresented in asia ( mostly china ) as they were in north / latin 
 america ( mostly u.s . ) . for europe , many of the economic topics were 
 well - represented , perhaps at the expense of important environmental 
 topics such as water resources which have recently been of intense 
 interest for biofuel feedstocks such as giant miscanthus . such trends do not necessarily point to problems 
 in the distribution of literature within regions , but the patterns 
 are worth discussion in the context of the needs of decision - makers 
 and stakeholders in different parts of the world . increasingly , 
 organizations and governments are relying on systems 
 approaches to understand the consequences of particular policy decisions . these are often interdisciplinary analyses to try and capture the 
 complexities and feedbacks associated with increasing biofuel production , 
 across the stages of the production chain , for multiple environmental 
 and economic impact domains and from local to global scales . common 
 approaches include life cycle analyses ( lcas ) , and general and partial equilibrium models ( e.g. , global 
 trade analysis project [ gtap ] and others ) . although promising , these approaches are often hampered by the 
 large amounts of input data required , as well as fundamental uncertainties 
 in model processes such as market elasticities and local land management 
 practices . we found that some topics such as ghg emissions and land 
 use / land cover are commonly discussed in an interdisciplinary fashion . whether this is in a substantive enough way to inform modeling efforts 
 and/or policy decisions remains unanswered . on the other hand , many 
 topics appeared relatively isolated , clouding the prospects for an 
 integrated analysis on the impacts of biofuel production across multiple 
 nations . first , 
 we describe trends only for recent peer - reviewed literature from journals 
 indexed by isi web of knowledge . clearly , research published before 
 2000 or after 2009 and in the gray literature such as in government 
 and industry reports will also be important to understanding the impacts 
 of biofuels . and while use of additional online databases of peer - review 
 research might have augmented the literature in our analysis , over 
 the last five years , thompson reuters has greatly expanded and evened 
 their indexing of high - quality social science and international journals 
 ( testa , j. 2011 . thus , use of other databases could have altered our results , though 
 likely not substantially given the broad journal coverage of the isi 
 web of knowledge . furthermore , it is unlikely that this literature - based 
 approach captures all the relevant information on research effort ; 
 it could be augmented with other approaches such as interviews and 
 tracking grants awarded by governmental and nongovernmental institutions , 
 among others . we also note that biofuels research is a rapidly expanding 
 field that may be only beginning to show signs of slowing ( figure 1c ) . second , we had no expectation that the categories 
 representing our topics of interest would be equally covered or connected 
 in the literature . this is because some categories may be inherently 
 broader ( for example , water resources versus the narrower feedstock 
 logistics ) , more important , or overlap more than others . however , 
 our approach for a network analysis on a priori selected 
 keywords and the visualization of connections between topics in the 
 literature is a procedure that has been used many times in social , 
 ecological , and computer sciences ( 6,17 ) and is rooted 
 in the mathematical and physical domains of network theory . despite the methodological and epistemological 
 limitations of our approach , we hope this assessment will provide 
 direction to the research community toward understudied topics and 
 regions , and stimulate greater integration of often disparate fields . agriculture is already the most intensive land use globally , comprising 
 over a third of the global land area ( http://faostat.fao.org/site/291/default.aspx ) and 70% of freshwater use , while 
 contributing to massive changes in earth s biogeochemical cycles . biofuels have the capacity to alter global land 
 use patterns , underscoring the importance of augmenting understudied 
 areas and topics in this rapidly developing industry . the optimal 
 mix of scientific effort to support its sustainable development is 
 unknown , and likely depends on several objective and subjective criteria . for instance , industrialized nations have more often emphasized understanding 
 the environmental consequences of biofuel production and use , while 
 several countries in africa and international bodies have prioritized 
 food security or socio - economic impacts . our 
 intent is , by exposing the structure of recent biofuels research efforts , 
 to raise awareness about scientific and economic areas still relatively 
 unexplored and promote the discussion of their applicability to a 
 range of stakeholders . such an approach increases the likelihood of 
 accurately anticipating the direct and indirect impacts of biofuels 
 policies , while reducing the impacts of unintended consequences .
despite rapid growth in biofuel production worldwide , 
 it is uncertain 
 whether decision - makers possess sufficient information to fully evaluate 
 the impacts of the industry and avoid unintended consequences . 
 doing 
 so requires rigorous peer - reviewed data and analyses across the entire 
 range of direct and indirect effects . to assess the coverage of scientific 
 research 
 , we analyzed over 1600 peer - reviewed articles published between 
 2000 and 2009 that addressed 23 biofuels - related topics within four 
 thematic areas : environment and human well - being , economics , technology , 
 and geography . 
 greenhouse gases , fuel production , and feedstock production 
 were well - represented in the literature , while trade , biodiversity , 
 and human health were not . 
 gaps were especially striking across topics 
 in the southern hemisphere , where the greatest potential socio - economic 
 benefits , as well as environmental damages , may co - occur . 
 there was 
 strong asymmetry in the connectedness of research topics ; greenhouse 
 gases articles were twice as often connected to other topics as biodiversity 
 articles . 
 this could undermine the ability of scientific and economic 
 analyses to adequately evaluate impacts and avoid significant unintended 
 consequences . at the least 
 , our review suggests caution in this developing 
 industry and the need to pursue more interdisciplinary research to 
 assess complex trade - offs and feedbacks inherent to an industry with 
 wide - reaching potential impacts .