Transnational Actors and Transnational Governance in Global Environmental Politics

Transnational actors and transnational governance now form core elements of global environmental politics alongside intergovernmental diplomacy and institutions. This article explores how and under what conditions this transnationalism has arisen, as well as its implications for world politics. It considers what effects transnational actors and governance have had on political outcomes, their relation to states and intergovernmental institutions, and normative questions around their legitimacy and accountability. The critical role of transnational actors and institutions in environmental politics has made the field a laboratory for broader questions concerning the evolution of global governance in world politics more generally. As global environmental challenges continue to magnify and affect other spheres of political activity, understanding these dynamics will become increasingly important. what can establish Whose interests are represented by and how, and whose are


INTRODUCTION: TRANSNATIONAL ENVIRONMENTAL POLITICS IN THE ANTHROPOCENE
Over the postwar period, environmental issues have gone from relatively local or discrete concerns related to pollution, species loss, or resource degradation to planetary-scale, systemic challenges of global politics. The economic expansion that followed the end of World War II, and its resulting damage to air, water, and land, gave rise to the modern environmental movement, signified by the first Earth Day in 1970. Contestation over environmental regulation became a feature of local and national politics. As globalization diffused industrial production and resource extraction around the world, followed by local but also globally linked social movements, environmental issues became increasingly transnational.
While international cooperation on the environment dates from the nineteenth century, the 1972 Stockholm Earth Conference (the environmental Bretton Woods) marked the emergence of what has become a large and complex set of environmental regimes. Today, more than 1,300 multilateral environmental agreements govern all aspects of the natural world (Mitchell 2018). Alongside them, a host of transnational actors, networks, and governance institutions have emerged to form a critical part of global environmental governance. For example, in the climate regime alone, in 2018 the United Nations recorded more than 190 transnational initiatives aiming to address climate change, in which more than 12,000 sub-and nonstate actors participated. These networks included subnational jurisdictions that are home to 20% of the global population and companies whose aggregated annual revenue exceeds the gross domestic products of China and the United States combined (UNFCCC 2018).
Despite a half century of efforts to ensure sustainability, at the start of the twenty-first century, environmental impacts have expanded to affect the basic natural systems-the climate, oceans, biodiversity, and other so-called planetary boundaries-on which human activity depends (Steffen et al. 2015). Ecologists now term the present age the Anthropocene to signify that the actions of humanity are the single greatest influence on the Earth (Biermann et al. 2012, Pattberg & Zelli 2016. The result is a powerful new form of interdependence in which the actions each of us take-individually and collectively-affect the world on which everyone else living today, as well as future generations, depends. The implications for world politics are stark. Environmental issues are now seen as integral to conflict, migration, development, and other core concerns of political science ( Javeline 2014, Keohane 2015, Busby 2018. It is therefore perhaps unsurprising that international relations scholars in the United States have identified global climate change as the single most important policy challenge (Green & Hale 2017).
The rising substantive importance of environmental issues in world politics has made the field a fertile laboratory for theoretical development. In particular, environmental politics has been a key domain for the study of transnational actors, meaning subnational or nonstate actors that form links and engage in political contestation across national borders, and transnational governance, meaning "processes in which nonstate actors adopt rules that seek to move behavior toward a shared, public goal in at least two states" (Roger & Dauvergne 2016, p. 416).
While transnational actors and governance can be observed across all realms of world politics (Hale & Held 2011, Roger & Dauvergne 2016, from terrorist networks to private regulatory standards, cross-issue area comparisons show transnationalism to be particularly abundant in the environmental realm (Reinsberg & Westerwinter 2019). This prominence perhaps reflects the fundamentally "intermestic" nature of environmental issues, which manifest at-and link acrosslocal, transborder, and global scales (Young 2002). Transnationalism is also reflected in the history of global environmental politics. In the early twentieth century, transnational networks of European conservationists created the first international environmental bodies, and after World War II, the International Union for the Conservation of Nature was created as an international organization (IO) with both national governments and independent scientific and conservation organizations serving as members (Boardman 1981). Moreover, the technical and scientific nature of many environmental issues has created an important role for experts, nongovernmental organizations (NGOs), and their associated epistemic communities (Haas 1992). At the same time, the nature of environmental issues often allows sub-and nonstate actors such as cities and private firms to have significant impacts on environmental challenges. This combination of factors creates the possibility for a diverse array of actors to link across borders to acquire authority, contest outcomes, and take on governance functions in environmental politics.
This article summarizes key research questions and debates around transnational actors and transnational governance in global environmental politics. It begins by focusing on transnational actors themselves, asking when and how they form links across borders and under what conditions they can affect political outcomes. Scholars have developed theories to describe interactions between transnational actors, the broader movements of which they are part, and their relations with international institutions and states. Second, the article turns to the question of transnational environmental governance. A rich literature conceptualizes the phenomenon and explains when and how it emerges. Increasingly empirically and theoretically rigorous, the literature on transnational governance spans a range of epistemological approaches in the discipline. Third, the article considers perhaps the key debate that the rise of transnational governance has animated in the field: How does transnational environmental governance relate to traditional intergovernmental and national forms of governance? Is it complementary to state-based governance, or an alternative to it?
The fourth section below considers the political implications of the expansion and pluralization of global environmental governance. Does this increasingly dense, heterogeneous, multiactor landscape of governance yield more effective management of environmental challenges, or is it instead a dysfunctional outcome representing regulatory weakness? Scholars have developed theories of regime complexity, polycentricity, and fragmentation to explore this issue. A related question, discussed in a fifth section, revolves around the normative desirability of these arrangements. Under what conditions can transnational governance establish legitimacy? Whose interests are represented by transnational networks, and how, and whose are obscured?
The article concludes by looking at emerging research questions in the field. We are still in the early days of the Anthropocene, but already it is clear that environmental issues will continue to increase in political salience and global scope. It seems likely that the field will therefore continue to evolve. Will the hybridization of global environmental politics continue to increase and spread to other issue areas? How will transnational environmental governance fare at a time of significant disruption to global governance at large? And will the rising political salience of climate stresses and impacts prompt a shift to more state-based, "hard" forms of governance in the area? These and similar questions suggest that the study of transnational actors and governance will continue to be a source of theoretical insights for political science in the years to come.

WHY DO TRANSNATIONAL ENVIRONMENTAL ACTORS EMERGE, AND WHAT IMPACTS DO THEY HAVE ON POLITICAL OUTCOMES?
The emergence of transnational environmental actors is one important element of the broader transnationalization of world politics in the postwar period. The web of economic connections across borders, underpinned by technological changes in communications and transport, created both the interest in and the possibility of politics beyond the state. Newly empowered groups ranging from multinational corporations to NGOs took advantage of these opportunities, www.annualreviews.org • Transnational Environmental Politics bringing more and more issue areas out of the purely domestic realm as transnational flows and interdependence progressively knit together once-distant peoples and places. Indeed, the emergence of environmental issues as a subject of world politics created an additional vector for the expansion of transnationalism, one of the new domains of international relations (others being health, human rights, global finance, and foreign direct investment) in which growing interdependence brought a more diverse array of actors to engage in political activities across borders (Haufler 2001, Hall & Biersteker 2002, Slaughter 2004, Koenig-Archibugi 2005, Green 2014).
As in other issue areas, transnational environmental actors emerged through two related macroprocesses. First, the issues and institutions that affected the environment became increasingly transnational or global in scale (Young 2002, Tarrow 2005. Second, environmental groups increasingly formed cross-border networks (Keck & Sikkink 1998).
The modern environmental movement that arose and diffused around the world in the late 1960s and early 1970s had important transnational elements. The first Earth Day's core slogan, Think Globally, Act Locally, precisely captured the cross-border links between discrete local issues (Boardman 1981). In the later part of the twentieth century, the environment was, alongside human rights and other social issues, a key area in which scholars documented increasing transnational links across social groups, leading some to speak of "global civil society" (Tarrow & Della Porta 2005, Dryzek 2012). This transnationalization of environmental politics took various forms (Tarrow 2005). A protest over a certain environmental grievance in one country, such as nuclear waste, might inspire groups in other countries to also protest that issue. Alternatively, groups might frame local grievances as global challenges, e.g., promoting paper recycling as a way to combat global deforestation.
However, perhaps the most significant manifestation of this growing transnationalism was the emergence of cross-border advocacy networks as key actors in world politics (Keck & Sikkink 1998). Common in value-oriented areas such as human rights or the environment, advocacy networks are composed of NGOs, local social groups, experts, and other nonstate actors that share values. Operating transnationally allows these networks to gain additional power by sharing resources or information or by leveraging political relationships across borders. For example, a local group seeking redress from its home government can appeal to allies in other countries to enlist their own governments or IOs to apply pressure on the recalcitrant government (Keck & Sikkink 1998). This "boomerang effect" shows how transnational linkages can under certain conditions significantly alter the balance of power and political outcomes in domestic politics.
In addition to linking to each other across borders, environmental actors also grew more transnational by targeting their activism at global institutions. As global governance institutions grew more numerous, powerful, and salient after the Cold War, environmental activist groups (alongside labor, human rights, and other groups) increasingly demanded that they alter policies to avoid environmental and social harms (Zürn 2018). Scholars studied how the 1999 protests around the World Trade Organization convention, dubbed the Battle in Seattle, and similar protests against other global organizations, like the World Bank and the International Monetary Fund, became points of convergence for transnational actors and therefore helped forge transnational linkages among them (Della Porta 2015). In addition to targeting intergovernmental organizations themselves, activists also organized transnationally against multinational corporations, which they perceived to be the driving force behind an ecologically destructive form of globalized capitalism (Newell 2008).
In parallel to protesting forms of global governance perceived to harm the environment, transnational actors have also been important in building and strengthening intergovernmental treaties and organizations to protect the environment. Transnational actors have been a growing presence in multilateral fora across all issue areas (Tallberg et al. 2013), but environmental actors have composed one of the most numerous and influential constituencies, and environmental intergovernmental fora have been increasingly open to transnational actor influence (Betsill & Corell 2008;Tallberg et al. 2014Tallberg et al. , 2018. A series of global summits and intergovernmental negotiations around sustainability in the 1990s, especially the 1992 Rio Earth Summit and its offshoots, provided a "coral reef" on which environmental activists could increasingly build transnational linkages. A large literature has emerged to ask why and how transnational actors engage in multilateral negotiations and what effect they have on outcomes. A key power resource of many transnational actors is expertise, which IOs need for successful policy making. IOs offer NGOs or others access to the policy-making process in exchange for their specialized information, resulting in a degree of NGO influence (Tallberg et al. 2018). Transnational actors are also particularly important for shaping public perceptions via the media, drawing on either expertise or their ability to speak for broader social interests or constituencies. This gives them some degree of power to bestow, or withhold, legitimacy from intergovernmental outcomes, which can shape the external pressures to which diplomats are subject (Newell 2000). As in domestic lobbying, transnational actors often mix insider and outsider tactics depending not only on their goals but also on the type of membership base to which they are beholden (Dellmuth & Tallberg 2017). Expertise, framing, and other means of influence are often more important in the early, agenda-setting phase of negotiations, shaping states' and IOs' interests in advance of intergovernmental deliberation (Betsill & Corell 2008). Overall, transnational actors' engagement in international institutions is conditioned by the varying opportunity structures they face (Dellmuth & Bloodgood 2019).
Finally, while much of the literature has stressed the common frames and interests that transnational environmental actors share, it is also important to note the cleavages within the global environmental movement. For example, when outsider antiglobalization activists began to focus increasingly on climate change, they came into conflict with insider groups that had been working within the negotiations under the United Nations Framework Convention on Climate Change (UNFCCC). Because civil society groups in the UNFCCC are organized around formal constituencies, contestation between different groups emerged over who represented the authentic voice of the global environmental movement ). This cleavage came to a head at the 2009 Copenhagen climate summit, when divisions between civil society groups over what they could endorse became one factor undermining the ability of governments to agree on a successful outcome (Hadden 2015).

WHY AND HOW HAS TRANSNATIONAL ENVIRONMENTAL GOVERNANCE EMERGED?
In addition to engaging in political contestation across borders, transnational actors have also engaged directly in building transnational governance. The emergence of transnational environmental governance must of course be understood as part of the emergence of transnational governance generally, driven by the same postwar trends described above. While forms of transnational governance can be found throughout history (Cutler 2003, Hale & Held 2011, the rise of complex interdependence in the postwar period created conditions for its importance to expand and focused scholarly attention on the subject (Nye & Keohane 1971, Mansbach et al. 1976.
Scholars in some spheres of world politics see transnational governance as less important than state behavior and intergovernmental politics (Roger & Dauvergne 2016), but in global environmental politics it has been a major focus for decades (Newell et al. 2012). That said, though transnational governance occurs across the full range of environmental issues, scholarship has clustered around a narrower array of specific topics. Two of the areas of greatest focus have been www.annualreviews.org • Transnational Environmental Politics private sustainability standards, such as dolphin-safe tuna labels and Forest Stewardship Council certification (Cashore et al. 2004, Auld 2014, and transnational climate governance, such as networks of cities or businesses that collectively set emissions reduction targets (Andonova et al. 2009, Bulkeley et al. 2014. While much of this work began with a qualitative, concept-building approach rooted in particular cases, it has become increasingly quantitative, comparative, and theoretical (Prakash & Potoski 2006b, Cao & Prakash 2011, Abbott et al. 2016, Roger & Dauvergne 2016, Cao & Ward 2017, Reinsberg & Westerwinter 2019.
Within this literature, the most common explanation for the emergence of transnational governance generally, and transnational environmental governance in particular, is that it provides new ways for actors to accomplish their objectives beyond state-based regulation or intergovernmental treaties and organizations. This functionalist logic tends to emphasize three key dynamics.
First, scholars note that the transnational nature of environmental issues can make them difficult to regulate purely through state-based forms of governance. Under globalization, attempts at regulation could potentially lead polluting firms to move to territories with weaker environmental rules, generating a "race to the bottom." Vogel (1995) and others suggest that the opposite may also be true; trade could provide a way for countries to "export" higher environmental standards as they import goods from jurisdictions with weaker environmental laws, since producers would need to meet consumer demand for sustainable goods. Researchers have shown that this so-called California effect often operates through transnational governance, such as transgovernmental networks of environmental regulators (Raustiala 2002) and voluntary private regulations (Prakash & Potoski 2006a, Vogel 2008. Such schemes provide a way for producers to credibly demonstrate their sustainability to consumers. Seen in this way, transnational environmental governance represents a functional response to the increasingly transnational nature of environmental challenges, in which purely state-based regulation would not be sufficient. This kind of transnational governance may be particularly useful when intergovernmental approaches to managing transboundary challenges are weak (Hale & Held 2012). For example, a key motivation of many actors engaging in transnational climate governance is the perception that national and intergovernmental responses have proven inadequate to the task (Bulkeley et al. 2014).
Second, with particular reference to corporations, scholars emphasize that transnational governance can be a useful tool for market differentiation and risk management (Prakash & Potoski 2006b). Credible sustainability governance systems allow firms to separate themselves from competitors and ensure that they will not face protests or other challenges to their brand (Cao & Prakash 2011). For this reason, scholars note that voluntary sustainability standards have been particularly effective in consumer-facing industries. In addition, private governance may allow firms to pre-empt hard regulation (Haufler 2001). If policy makers and other constituencies trust or believe an industry is managing its own environmental problems, they may forgo regulation that could be less suited to firm interests (Vogel 2009).
Third, scholars have studied how different kinds of transnational actors can cooperate with each other, and with states and intergovernmental organizations, to perform governance functions across borders. Abbott & Snidal's (2009a) influential concept of a "governance triangle" describes how different state and nonstate actors can combine their attributes to achieve governance outcomes beyond what they could have accomplished individually. The scientific and technical nature of many environmental issues means that nonstate actors with expertise-typically researchers or NGOs-are needed to perform certain governance functions (Green 2014). Scholars have also noted how multinational corporations can use their financial resources, market influence, and transboundary reach to establish themselves as "global governors," and how NGOs possess moral authority that is often needed for transnational governance to be legitimate and credible (Avant et al. 2010). Cities and other subnational governments, moreover, often possess significant authority to address seemingly local issues with transboundary impacts, such as greenhouse gas emissions (Betsill & Bulkeley 2006). Linking to each other across borders allows these different actors to amplify and potentially combine the governance attributes each possesses. For example, multi-stakeholder regulatory standards like those of the Forest Stewardship Council are effective because they result from bargains between firms that control production, experts that contribute scientific knowledge, and NGOs that provide (or withdraw) legitimacy (Pattberg 2011).
While these arguments emphasize functional explanations, they also note that transnational governance arrangements, like intergovernmental organizations, may advance the interests of one set of actors over another. In other words, transnational governance has distributional implications. For example, scholars have documented how rival voluntary governance schemes, some promoted by NGOs and others by corporations, have competed for market share in commodities such as timber or fisheries as environmental interests battle with profit-seeking actors for legitimacy (Marx & Wouters 2015). Other scholars have gone further, arguing that the emergence of transnational environmental governance represents a privatization or marketization of governance that tends to empower business interests and technocratic, expertise-based viewpoints over others (Clapp 1998, Levy & Newell 2004).
Finally, more sociological accounts see the growth of transnational environmental governance as the product of shifting norms and discourses concerning how to achieve sustainability, and increasing interactions between different social actors. It is striking, for example, how voluntary sustainability standards that arose first around issues like forestry and chemicals were eventually replicated across a much wider array of commodities. Noting this pattern, scholars have described the growth of transnational environmental governance as the creation of an organizational field, meaning that a specific transnational institutional form for addressing environmental issues has become widely adopted by firms and NGOs and recognized as legitimate by states and IOs (Dingwerth & Pattberg 2009). An analogous explanation focuses instead on discourse theory, tracking the emergence of transnational governance as part of a broader shift in how to govern climate change that has occurred between the Copenhagen climate conference and the Paris accord (Bäckstrand & Lövbrand 2019). Focusing instead on the individual level, Paterson et al. (2014) show how transnational networks of professionals involved in carbon trading have driven the diffusion of that regulatory approach, including via transnational governance networks. Focusing not on individuals but on the "ecosystem" as a whole, Abbott et al. (2016) suggest an "organizational ecology" approach, in which the growth of private forms of governance is a response both to the slowdown of intergovernmental governance and to the relative ease with which new forms of governance can proliferate to fill the resulting gaps in authority.
Despite the range of lenses through which the literature views the emergence of transnational environmental governance, it is important to stress that functionalist, critical, and sociological explanations are largely complementary. Each answers related but distinct questions about the transnational turn in global environmental governance, and the field has largely avoided unproductive paradigm wars between competing epistemologies.

DOES TRANSNATIONAL ENVIRONMENTAL GOVERNANCE COMPLEMENT OR SUBSTITUTE FOR STATE-BASED GOVERNANCE?
The rapid expansion of transnational governance raises an important question: What is its relation to states and to intergovernmental forms of authority and governance? During the 1990s, scholars debated whether new forms of governance were likely to supplant nation states (Strange 1996, Korbin 1999. While it is clear that the state is not being replaced anytime soon, the question remains how new forms of governance might serve to reallocate authority and power from one set of actors and institutions to another. Do new forms of governance in the environmental realm supplant state power, or are they complementary or even epiphenomenal to it? To help answer this question, scholars have found useful Green's (2014) conceptual distinction between delegated and entrepreneurial forms of private authority. Delegation from states and intergovernmental organizations to nonstate actors is widespread in environmental politics (Green 2010). In this form of private authority, states retain ultimate control, even if familiar principal-agent dynamics attenuate their authority. In entrepreneurial arrangements, in contrast, agency lies with nonstate actors, who create and promulgate forms of governance on their own. Of course, institutions born through entrepreneurial authority include multi-stakeholder initiatives and public-private partnerships in which nonstate actors can be coequal partners with states or intergovernmental organizations (Andonova 2010).
While noting the agency of nonstate actors, scholars have emphasized how even entrepreneurial forms of authority and governance can be shaped by the shadow of the state. Indeed, national and local contexts and policies have been shown to be powerful determinants of the scope, effectiveness, and impact of transnational governance in practice. For example, local land tenure arrangements or national alternative programs can subvert the effectiveness of transnational forestry certification (Espach 2006, Bartley 2010. More generally, more authoritarian, closed domestic institutions that limit the independence and agency of NGOs and other suband nonstate actors, or that discourage transnational linkages, see less engagement in transnational governance (Drezner & Lu 2009, Dolšak & Prakash 2017). This is not surprising, as this liberal mode of governance, with its emphasis on nonstate actors and cross-border links, may run against existing authority relationships in statist, authoritarian political systems such as the People's Republic of China. However, this effect is mediated by market incentives and state preferences. Many export-oriented Chinese companies, or those that aim to sell to luxury domestic markets, participate in voluntary sustainability standards (Schleifer & Sun 2018). Moreover, when authoritarian regimes gain benefits from engagement in transnational governance, as China has done from transnational carbon-trading standards, they make use of it .
But the finding that state and local institutions and policy preferences condition transnational governance does not necessarily mean that there is a zero-sum relationship between the two. Indeed, research on transnational climate governance finds that sub-and nonstate actors are significantly more likely to join transnational networks when their home governments favor proclimate policies, all else equal , Cao & Ward 2017). This general pattern does not always hold. For example, the United States saw an increase in transnational climate governance from cities, businesses, and other actors following the 2016 announcement by President Trump that the United States intended to withdraw from the Paris Agreement (Urpelainen & Van de Graaf 2018). The literature therefore concludes that the complementarity of transnational environmental governance to state-based forms of authority ultimately depends on state preferences.
Indeed, a significant literature has grown up around the insight that states and intergovernmental organizations do not just condition transnational environmental governance but actively strive to create, shape, and steer it (Andonova 2010). Abbott & Snidal (2009b) describe how states and intergovernmental organizations "orchestrate" what they term governance intermediariesbusinesses, NGOs, and other transnational actors-to tackle problems they cannot fully address themselves. Such arrangements are common in transnational environmental governance (Abbott & Hale 2014. States and intergovernmental organizations often possess legitimacy, focality, or more material resources that can help them activate transnational actors that share their overall goals. Orchestration is looser than delegation in that states and intergovernmental organizations are not in charge of their partners, but it assigns more agency to traditional actors in world politics than strictly entrepreneurial forms of authority. As  shows, orchestrated arrangements are often driven by specific "governance entrepreneurs" in IOs who seek to advance their objectives by forming partnerships with outside actors and friendly member states. Orchestration and other forms of linkage between new and old types of governance have become so prevalent in some global environmental regimes that some authors now speak of "hybrid multilateralism" to reflect the substantive importance of transnational actors and governance . First explored in the context of the 2012 Johannesburg World Summit on Sustainable Development, this prominent role for sub-and nonstate actors is now extensively studied in the climate regime (Hale 2016. The 2015 Paris climate conference officially called on transnational actors to adopt the global goals negotiated in the Paris Agreement and created a voluntary process inside the UNFCCC to orchestrate and track their efforts to advance them (Hale 2016). The merging of intergovernmental and transnational governance in global environmental politics represents a fascinating evolution in the nature of global institutions.

COMPLEXITY, POLYCENTRICISM, AND FRAGMENTATION
The proliferation of transnational actors and governance around environmental issues has led scholars to interrogate the dynamics of pluralistic, multi-actor, densely institutionalized political environments (Biermann et al. 2009, Zelli & van Asselt 2013. The literature has advanced several distinct but related conceptual frameworks to describe how the multitude of actors and institutions in a given area relate to each other, as well as the properties of such complex systems as a whole. Perhaps most prominently, Ostrom and others have described climate governance, in particular, as a "polycentric" system (Ostrom 2009, Jordan et al. 2018) within which transnational linkages and transnational governance play an important part (Bulkeley et al. 2018). While global mitigation of greenhouse gases may represent a global collective action problem, Ostrom notes that a multitude of actors address aspects of the larger problem individually and collectively at a range of scales-local, regional, and transnational. Ostrom (2009) characterizes this polycentrism positively, arguing that complex problems require complex solutions, and others have stressed that polycentrism at least allows for progress beyond multilateral gridlock (Hale 2017). Other authors are more circumspect. While polycentrism accurately describes the reality of climate governance today, it does not necessarily mean that such an approach will become an effective response to climate change or other sustainability challenges ( Jordan et al. 2018). Indeed, by explicitly focusing attention on those areas where progress is possible, it may overlook more recalcitrant actors and harder cases, and the more coercive politics that may be needed to shift them.
Scholars have also highlighted other potential trade-offs around polycentrism. The concept of regime complexity (sometimes called institutional complexity) considers interactions between different international institutions (Oberthür & Gehring 2006, Oberthür & Stokke 2011. Of particular interest to scholars are situations in which different sets of rules and governance processes begin to overlap, possibly generating redundancies or conflicts. For example, a foundational paper shows how regimes as diverse as trade, health, and biodiversity all govern plants' genetic resources, raising conflicts over which sets of interests-economic profit, public health, nature-are prioritized (Raustiala & Victor 2004). Because environmental issues tend to blur into these other areas, they are perhaps particularly prone to the kinds of institutional overlaps that characterize regime www.annualreviews.org • Transnational Environmental Politics complexes. The issue of climate change, which reaches across nearly every sphere of economic activity, is an example par excellence (Keohane & Victor 2011). While much of the work on regime complexity and institutional interaction has focused on intergovernmental institutions, the importance of transnational institutions and links around environmental issues adds significant further complexity (Abbott 2012, Green & Auld 2017).
Instead of looking at specific interactions between institutions and regimes, other scholars have looked at the "fragmentation" of issue areas more generally (Biermann et al. 2009, Zelli & van Asselt 2013. Fragmentation in this sense can refer to more than just the proliferation of institutions, rules, and actors; it can also describe thin social linkages, disparate norms, or other factors that undermine the overall cohesion of the system. From this perspective, global environmental politics is highly fragmented in an institutional sense, lacking any single world environment organization and featuring a wide proliferation of transnational governance. Under such conditions, transnational actors are more able to deploy such tactics as forum-shopping (trading between different options) and forum-linking (combining otherwise disparate issues or institutional processes) to suit their interests (Orsini 2013). Arguably, however, transnational linkages may help to bring some order to this pluralistic institutional setting by diffusing common norms and thickening relational ties. For example, many transnational climate governance institutions have adopted the temperature targets of the Paris Agreement (Bulkeley et al. 2018), suggesting that a proliferation of institutions need not lead to fragmentation of goals (Hale 2017). Moreover, network analysis shows that many actors are linked by participation in common, overlapping networks, suggesting the social structure may not be as fragmented as it might seem prima facie (Widerberg & Stripple 2016, Pattberg et al. 2018. Ultimately, then, there is no consensus in the literature on whether the growing pluralization of global environmental politics, of which transnationalism is a major driver, leads to better or worse outcomes than a counterfactual world of more centralized global environmental politics. Given the current polycentric system, scholars have advocated greater linkages between transnational governance and intergovernmental processes as a way to address the potential negative consequences of fragmentation (Chan et al. 2015).

LEGITIMACY, ACCOUNTABILITY, AND REPRESENTATION
Normative questions around legitimacy, accountability, and representation form a core area of the literature on transnational actors and transnational governance in global environmental politics (Bernstein 2005, Bäckstrand 2008). It is useful to separate the literature on the growing engagement of transnational actors in multilateral fora from the literature on transnational governance, as they raise related but distinct normative issues.
As noted above, transnational actors have increasingly demanded, and received, representation in intergovernmental environmental processes and institutions. Over time, this participation has become institutionalized. In the UNFCCC, for example, nonstate delegates representing a diverse array of social groups now outnumber diplomats. Transnational actors are organized into nine official stakeholder constituencies, such as farmers, youth, business, and environmental NGOs (Kuyper & Bäckstrand 2016. Such actors typically claim that their participation is necessary in order to represent both the general public interest and certain viewpoints and constituencies (e.g., indigenous people, women, nature) whose voices would not otherwise receive representation. Scholars have interpreted this growing role as a nascent form of deliberative or stakeholder-based democracy, heralding its potential to democratize global environmental decision making, or at least to help hold IOs and states to account (Bäckstrand 2006, Macdonald 2008, Scholte 2011. Increasing participation and influence from transnational actors are not unproblematic, however. There is no guarantee that the transnational actors who attend environmental summits represent the global public in some consistent fashion roughly analogous to pluralism at the domestic level (Dahl 1999). Nor is it always clear that NGOs are accountable to the groups they claim to represent (Scholte 2004). Moreover, scholars have questioned how increasing participation may also allow narrower economic interests to exert undue influence over global environmental governance, as business groups receive representation alongside civil society (Newell 2000). After all, equal participation does not mean equal influence, given differential resources and relational power across transnational actors. Even within civil society, large global NGOs have the greatest ability to access and operate in international fora, raising questions of whose interests are served.
There can also be significant struggles between nonstate actors themselves. For example, the UNFCCC constituencies are each represented by an organization, called a focal point, that is supposed to speak for all. This arrangement creates its own series of issues around representation and accountability, as each sector of civil society finds different ways to negotiate its common position and hold its official focal point to account (Kuyper & Bäckstrand 2016). For example, the environmental NGO constituency was unable to agree upon a single viewpoint, so two competing coalitions had to share its official speaking time during negotiations. Civil society's ability to hold states accountable may be undermined by their own competition with each other for access and resources (Balboa 2017).
While scholars generally see the greater role of transnational actors in global environmental governance as positive, transnational environmental governance raises further questions. After all, it is natural to interrogate how private actors may legitimately make rules or otherwise exercise any form of governance across borders. Indeed, precisely because legitimacy cannot be assumed, much of the literature on transnational environmental governance explores exactly this question, documenting how would-be transnational rule makers strive to be seen as authoritative and legitimate in the eyes of prospective rule takers (Green 2014). Scholars have noted how such institutions as the Forest Stewardship Council seek procedural legitimacy by institutionalizing decision-making processes that give forestry companies, affected communities, and ecological voices equal seats at the table (Pattberg 2005). Transparency is also seen as a basic normative requirement for transnational governance (Hale 2008). In addition, transnational institutions may explicitly adopt public rules or goals to add a further sheen of legitimacy, such as the UN Global Compact's reliance on treaty law (Ruggie 2001) or voluntary carbon trading standards' reliance on the Kyoto Protocol rules . While scholars have been thorough in documenting where transnational governance institutions fall short of achieving legitimacy, the study of transnational environmental governance has also allowed scholars to explore novel forms of representation and accountability beyond the state (Dingwerth 2007).
As transnational actors have become more important and as transnational governance has proliferated, the increasing pluralization of global environmental politics adds further normative complexity (Widerberg & Pattberg 2017). Accountability concerns are magnified by the fragmentation of global environmental governance (Boran 2019, Chan et al. 2019). The sheer scale and scope of transnational governance make monitoring difficult, despite efforts (like the UNFCCC's NAZCA portal) to centralize information on what sub-and nonstate actors have made what commitments on climate change. Without stronger monitoring frameworks, actors may be able to announce pledges with little fear of reputational consequences should they break them (Kuyper et al. 2018a).
Finally, the growing links between transnational governance and intergovernmental institutions add yet another layer to the normative puzzle. Because states and intergovernmental organizations orchestrating transnational actors choose which intermediaries to engage, and whom to leave out, they shape governance outcomes with little oversight or deliberation . A review of orchestration platforms in the UNFCCC reveals imbalances in representation between actors from the North and South, and questions around the effectiveness of the minimal provisions for transparency (ClimateSouth 2018). Moreover, linking transnational governance to multilateral fora raises questions about tensions between sub-and nonstate actors' multiple roles as both stakeholders lobbying decision makers and actors executing governance functions. Scholars note that the growing role of transnational actors as partners and agents in global governance may diminish their ability to demand accountability and criticize existing arrangements (van der Ven et al. 2016).

CONCLUSION: EMERGING RESEARCH QUESTIONS
The emergence of modern environmental issues as a subject of world politics in the second half of the twentieth century served as one important pathway for the development of transnational actors and governance. With environmental issues now not only crossing borders but operating at the scale of planetary systems, we may expect transnational environmental politics to become increasingly central to world politics as a whole. But this shift does not necessarily imply a linear continuation of the trends scholars have studied to date. Several questions loom on the horizon of the field.
First, will the trend to bring transnational actors and governance into multilateral processes continue, further hybridizing global environmental governance? As noted above, the UN climate regime has increasingly formalized the role of transnational actors not just as stakeholders but as actors. Similar efforts are now being made to adopt a similar framework in the biodiversity regime (Pattberg et al. 2019) and around the Sustainable Development Goals (Chan et al. 2019). Will other issue areas follow this trend, and if so, how might more formalized governance roles for transnational actors affect political outcomes and normative concerns (Kuyper et al. 2018b)? Could this trend revive debates over whether transnationalization could indeed subvert state power?
Second, how will transnational governance of the environment fare as global governance as a whole is increasingly in flux? The liberal international order faces deep challenges related to growing multipolarity, the rise of nationalism and populism, and other long-term trends that generate gridlock (Colgan & Keohane 2017, Hale & Held 2017. Given these disruptions, three scenarios seem possible for transnational environmental governance. On the one hand, transnational environmental governance may increasingly substitute for state-based governance. If countries continue to fail to manage growing environmental concerns, other actors in world politics will have incentives to cooperate with each other to fill the resulting governance deficit. Such efforts may be incomplete or second-best (or not) (Hale & Held 2012), but under this scenario we could see a growing shift in authority to private or hybrid transnational institutions.
On the other hand, increasing geopolitical conflict and nationalism may eliminate the space in which transnational governance has grown in recent decades. If states find themselves in conflict with each over transborder issues, they may be less willing to let third parties exercise influence or authority. At the same time, increasing nationalist and authoritarian tendencies in domestic politics will limit the ability of sub-and nonstate actors to link across borders. We see a general trend toward shrinking space for civil society not only in authoritarian regimes but also in weakly institutionalized democracies, with governments focusing in particular on closing off funding from abroad and other such cross-border linkages (Poppe & Wolff 2017). Worryingly, there has been a rising trend of violence against environmental civil society groups (Watts & Vidal 2017, Butt et al. 2019).
A final scenario is also possible. Transnational environmental governance and linkages may prove resilient, even if the liberal institutional order falters, precisely because of their complexity, fragmentation, and softer institutional forms that exist across different scales. Following Ostrom (2009), cooperation may be easier to sustain within specific communities of actors that share bonds of trust and reciprocity, even as relations between states become more fraught.
Just as global institutions show signs of strain, the global environment is continuing to deteriorate, raising the likelihood of wider socioeconomic disruptions. Environmental politics is already shifting in response, with climate change being debated in the UN Security Council (Dellmuth et al. 2018, Scott & Ku 2018, playing a bigger role in a growing number of national elections, and driving an emerging transnational youth social movement (Farmer et al. 2019, Fisher 2019. The third question regarding the future of transnational environmental actors and governance, therefore, is how they will change as environmental issues shift from "low politics" to "high politics." Might rising salience increase the weight of state-based governance and intergovernmental diplomacy and institutions in global environmental politics? For example, could increasing divergence in climate-related economic policies between countries lead states to deploy carbon tariffs (Keohane et al. 2017)? Could the emergence of geoengineering lead to arms control-style treaties pertaining to research and deployment of climate engineering technologies (Lloyd & Oppenheimer 2014)? Could resource scarcity and humanitarian disasters lead to a greater role for state militaries in seeking control of commodities and addressing the negative environmental impacts (Burke et al. 2015)? As environmental politics grows more existential and crises accumulate, politics may increasingly shift from soft to hard institutions as states reassert control.
Whatever the future of transnationalism in global environmental governance, it has to date given political scientists a rich series of theoretical insights into forms of transborder political activity and governance with important implications for all areas of world politics. Given the rising challenges that global environmental problems are unleashing, the field promises to be a critical subject of inquiry going forward.

DISCLOSURE STATEMENT
The author is not aware of any affiliations, memberships, funding, or financial holdings that might be perceived as affecting the objectivity of this review.