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The concept of barriers is increasingly used to describe the obstacles that hinder the planning and implementation of climate change adaptation. The growing literature on barriers to adaptation reveals not only commonly reported barriers, but also conflicting evidence, and few explanations of why barriers exist and change. There is thus a need for research that focuses on the interdependencies between barriers and considers the dynamic ways in which barriers develop and persist. Such research, which would be actor-centred and comparative, would help to explain barriers to adaptation and provide insights into how to overcome them.

Adaptation to the unavoidable consequences of climate change has become a necessity globally. However, practitioners, policymakers and scientists encounter and report on many barriers that impede adaptation planning and implementation^{1, 2}. Implementation of adaptation is not keeping pace with the ever-increasing need: the 'adaptation deficit'³ is getting wider. It is therefore critically important to identify and analyse barriers to adaptation to identify possible opportunities to overcome them. This Perspective takes stock of current knowledge and outlines the contours of a research agenda to identify, explain and overcome barriers to adaptation.

Generally defined, barriers to adaptation are challenges, obstacles, constraints or hurdles that impede adaptation⁴. For example, investment to increase the robustness of infrastructure to more frequent extreme weather events might be delayed due to a lack of financial resources. Our Perspective focuses on barriers that are conceptualized as being related to human actions or decisions, and, as opposed to limits, surmountable in principle (for example, through concerted effort). We propose a more precise definition of barriers to adaptation below.

There is an increasing body of case study literature on the barriers to adaptation. This literature carefully describes and categorizes barriers in different ways^{5, 6, 7}. Research is, however, still far from conclusive on causal explanations for the occurrence of barriers and on how they can be overcome. This Perspective is based on an intensive reflection during an international workshop on the barriers to adaptation to climate change held in Berlin in 2012. This workshop synthesized participants' expertise and knowledge of the literature with a qualitative approach (see http://www.climate-chameleon.de/htm_engl/workshop_engl for detailed documentation). The workshop results are further substantiated in this article by drawing on a large sample of peer-reviewed papers, some published interim.

The following three sections report on the state of research on barriers to adaptation and identify limitations of current research. To address these research gaps, we propose components for a research agenda in the fourth section. We argue that more systematic causal explanations are crucial for overcoming barriers. Case studies need to be compared and synthesized into higher-order findings that enable generalization and/or explication of how contextual factors modify the general insights. We conclude with crucial proposals to generate more broadly applicable and transferable knowledge on how decision-makers could avoid, reduce or overcome those barriers perceived as problematic.

The IPCC's fifth assessment report characterizes adaptation barriers (synonymous with adaptation constraints) as “factors that make it harder to plan and implement adaptation actions or that restrict options”.⁴ Only a few studies on barriers to adaptation provide a clear-cut definition beyond the IPCC characterization⁷. To complicate matters, researchers use the term differently. Some scholars use the terms 'limits' and 'barriers' interchangeably^{8, 9}, but more often they have different meanings. Whereas barriers are considered surmountable or mutable, limits are seen to be absolute or unsurpassable^{10, 11, 12}. The concept of barriers has also been defined in relation to adaptive capacity. Barriers are understood as either a reason for adaptive capacity not being translated into action^{8, 13, 14}, or as one reason for low adaptive capacity^{15, 16}.

Moser and Ekstrom¹¹ define barriers as obstacles that make adaptation less efficient, less effective or may require changes that lead to missed opportunities or higher costs. They can be overcome, avoided or reduced by individual or collective action with concerted effort, creative management, changed ways of thinking, political will, and reprioritization of resources, land uses and institutions. Barriers can arise from three sources: the actor(s) making adaptation-related decisions, the context (for example, social, economic or biophysical) in which the adaptation takes place or the system that is at risk of being affected by climate change (called 'system of concern'). This conceptualization follows a positive (that is, descriptive or explanatory) approach instead of a normative one in which barriers are judged as inherently problematic.

Eisenack and Stecker¹⁵ also take a positive approach, and argue for a precise specification of (1) the adaptations to which a barrier refers, and (2) the means necessary to implement these adaptations. The conditions that give reason to why these means are not employed for the specified adaptations are called barriers. This conceptualization emphasizes that barriers are relative to the specified adaptive actions that are considered, to the actors that may exercise them and to the specific situation in which they may be taken. In line with other work¹⁷, Eisenack and Stecker¹⁵ and Moser and Ekstrom¹¹ recognize the role of norms and values in understanding barriers. Research on barriers, however, does not need to make such value judgements. There might be barriers that are judged as being problematic by one actor and viewed as beneficial by others. The actor would prefer to remove these barriers, while others would probably work with them strategically.

Taking account of these considerations, we propose the following refined definition here: a 'barrier to adaptation' is (1) an impediment (2) to specified adaptations (3) for specified actors in their given context that (4) arise from a condition or set of conditions. A barrier can be (5) valued differently by different actors, and (6) can, in principle, be reduced or overcome. In this definition, conditions are the attributes of adaptations, actors, and their context.

The growing number of case studies and theoretical work has produced a large collection of commonly reported barriers^{5, 7, 17, 18, 19, 20, 21}. Several attempts have been made to categorize these barriers^{5, 11, 15}. Ekstrom and Moser⁶, for example, identify four categories of barriers most commonly encountered in the local urban context: institutional, attitudinal, financial and political. Biesbroek et al.⁵ identify seven barrier categories: (1) conflicting timescales, (2) substantive, strategic and institutional uncertainty, (3) institutional crowdedness and institutional void, (4) institutional fragmentation, (5) lack of awareness and communication, (6) motives and willingness to act, and (7) lack of resources.

At this abstract level, most of these barrier categories are not specific to the governance of climate adaptation. Other change, policy and management processes encounter similar barriers. Governance of adaptation faces many of the same difficulties involved in dealing with other complex problems. Some scholars, however, argue that there is a set of barriers that are particular for adaptation^{22, 23}. For example, conflicting timescales can be very pronounced, as short-term interventions based on a long-term vision demand an enduring commitment by taxpayers, politicians and the private sector; persistent uncertainties about the nature and scale of risks and the effectiveness of adaptations feature strongly, particularly in the context of large-scale adaptation investments^{22, 24}; and institutional fragmentation is a persistent problem, as many adaptation strategies depend on the interaction of various sectors, for example, in water management, spatial planning and infrastructure design, and from various policy levels, from local to global^{6, 25}.

These general and descriptive barrier categories subsume a wide variety of specific barriers that are highly context and actor specific. As an example, in the Chilean capital Santiago, institutional fragmentation hinders urban adaptation as the regional decision-making authority is dispersed among 52 mayors²⁶. In other cities, the most crucial barrier can stem from institutional fragmentation among administrative departments²⁷. With regard to political barriers, in some cases, elected officials delay adaptation due to high costs²⁸; in others, delays are due to the complexity of the system to be adapted or the perceived trade-offs among different constituencies^{27, 29}.

Why do these commonly reported barriers appear? What makes them often so persistent? Few systematic explanations are offered in the literature. Many studies are primarily descriptive and do not trace the origins of barriers, whereas others identify different, and sometimes diverging, reasons for the same type of barrier. For instance, while a low priority for adaptation in urban policy agendas has been traced to low awareness about climate impacts among citizens and administrative staff³⁰, the same barrier has been attributed to unclear roles and responsibilities in the Australian multi-level governance system³¹. A financial constraint impeding the assessment of adaptation options might result from an actor having failed to secure an appropriate budget (even though the municipality is in a healthy financial situation), or from a widespread financial crisis (in this case the cause is contextual)^{6, 32}. Low problem awareness at the local level is sometimes traced back to a low priority for adaptation at higher institutional levels³⁰; in other instances, it results from unclear responsibilities for adaptation at the local level³¹. Missing leadership, and also dominant leadership by certain actors, can lead to an absence of appropriate decision-making routines^{27, 33, 34, 35, 36}. And, finally, a lack of local leadership is frequently explained by missing top-down support^{27, 37}, but too much involvement from higher levels can also discourage local leadership^{38, 39}. Importantly, these diverging findings do not indicate disagreement in the scientific literature, but stem from differences in the cases studied.

Moreover, some studies indicate that different barriers cannot be understood in isolation. Long-term financial shortages give rise to and magnify other barriers, such as those stemming from lack of information, inadequate interagency coordination and specific personal beliefs^{30, 40}. Other studies conclude that the absence of appropriate decision-making routines can be the underlying reason for inappropriate budgets²⁷. Uncertainty and lack of awareness can impede adaptation and interact bidirectionally: uncertainty can undermine the motivation of actors to become more aware, while lack of awareness can inhibit efforts directed towards reducing uncertainty⁴¹.

To complicate matters, barriers are not static but change over time. For example, budget constraints can become more pressing over time, for example, due to a broad financial crisis or local budgetary priorities given to other policy issues^{6, 32}. Adaptation investment in long-lived infrastructure is strongly shaped by competition regulation and expectations about the future^{35, 42}. At the same time, the current needs, options and costs of adaptation depend on investment decisions made in the past^{38, 43, 44}. Both past decisions and current barriers to adaptation may lead to path dependencies that constrain future action^{6, 27}.

Taken together, current barrier research offers a broad and diverse empirical and conceptual base. Few studies, however, explain the occurrence of barriers. If explanations are offered, they mostly apply to the unique case under investigation, with little grounds yet for generalization. Although single studies illustrate the importance of understanding how barriers are related to each other or how they change over time, there is a need to address these questions explicitly and systematically for a broader set of cases. Existing meta-studies (including the IPCC's fifth assessment report⁴) categorize barriers into variably generic or fine-grained lists. Barrier categories are, however, primarily descriptive and do not yet offer systematic explanations.

So far, there are only a few studies that explicitly investigate how barriers perceived as problematic might be overcome. Yet some studies of instances where adaptation is already occurring provide insights on enabling factors that either prevent barriers from emerging or that help actors to deal with them^{6, 45, 46, 47, 48}.

Of those actions that have already been implemented, only a few have been large or costly^{27, 49}, and many are essentially extensions of previous policies. A related and important enabling condition is the integration of adaptation into other policies (frequently called 'mainstreaming', for example, by including climate change projections in water management, urban planning or health^{50, 51<}