International climate adaptation assistance: Assessing public support in Switzerland

PLOS ONE | https://doi.org/10.1371/journal.pone.0317344 February 12, 2025 1 / 26 OPEN ACCESS Citation: Andonova L, Zucca A, Montfort S, Dolšak N, Prakash A (2025) International climate adaptation assistance: Assessing public support in Switzerland. PLoS ONE 20(2): e0317344. https://doi.org/10.1371/journal. pone.0317344 Editor: Sung Min Han, Hankuk University of Foreign Studies, REPUBLIC OF KOREA Received: July 2, 2024 Accepted: December 27, 2024 Published: February 12, 2025 Copyright: © 2025 Andonova et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data availability statement: Data is posted on Harvard Dataverse: https://dataverse. harvard.edu/dataset.xhtml?persistentId=- doi%3A10.7910%2FDVN%2F3YWEEV&ver- sion=DRAFT Funding: Andonova: Graduate Institute Geneva Seed Grant Simon Montfort: Doc.CH funding by the Swiss National Science Foundation (SNF grant number 207269). RESEARCH ARTICLE International climate adaptation assistance: Assessing public support in Switzerland Liliana Andonova1, Agnese Zucca1, Simon Montfort2, Nives Dolšak3, Aseem Prakash 3* 1 Graduate Institute of International and Development Studies Geneva, Geneva, Switzerland, 2 École Polytechnique Fédérale de Lausanne, Lausanne, Switzerland, 3 University of Washington, Seattle, Washington, United States of America * aseem@uw.edu Abstract Lower-income countries account for a small share of accumulated greenhouse gas

emissions but are highly vulnerable to climate-induced events. In response, industrialized higher-income countries, the major contributors to greenhouse gas stock, have pledged policy packages to support developing countries to adapt to climate change. Foreign aid and international migration often figure prominently in such packages. We employ a survey-embedded conjoint experiment to assess public support in Switzerland for interna- tional climate assistance packages which consist of six attributes: (1) the country receiving the package (Algeria, Kenya, Bangladesh, and the Philippines); (2) the volume of Swiss bilateral climate aid to this country; (3) the number of climate migrants from this country in Switzerland; (4) types of extreme weather event this country faces; (5) Swiss trade with this country; and (6) the country’s record of voting with Switzerland in the United Nations Security Council. We find that while Swiss respondents are indifferent to aid volume, their support for the policy package diminishes as the number of migrants increases. Respon- dents support policy packages for countries that trade with and vote alongside Switzerland in the Security Council. Respondents also have country-specific preferences: they support assistance to the Philippines, disfavor Algeria, and are indifferent to Kenya and Bangla- desh. Ideology, cultural beliefs, and benchmarking with peer countries of Global North or past Swiss aid and immigration records do not change support for the policy package. Introduction

Global climate policy has an important domestic component. As Putnam’s [1] two-level game framework suggests, leaders take positions on global issues while being watchful of domestic reactions. The global-local connection has been debated in the context of climate mitigation policy. Historically the lack of progress on climate policy was largely attributed to global free riding. Increasingly, scholars recognize the important role of domestic distributional conflicts in impeding decarbonization [2–5], and alternatively, the significance of domestic political incentives for stimulating transnational climate initiatives [6,7]. In this paper, we extend the domestic-global connection to the study of climate adaptation politics. A well-established literature examines public support for adaptation at the domestic and local levels [8–11]. We contribute to this literature by examining domestic support for

PLOS ONE | https://doi.org/10.1371/journal.pone.0317344 February 12, 2025 2 / 26 PLOS ONE Support for adaptation assistance in Switzerland global climate adaptation assistance [12–14]. More specifically, we examine to what extent and in which ways domestic support for overseas climate adaptation among donor countries might depend on how citizens view the relationship of their own country with the country that will receive adaptation assistance. Thus, our paper provides a new direction to the study of climate politics by suggesting that while climate mitigation might be impeded by distri- butional implications, climate adaptation might be influenced by domestic perceptions of countries that receive adaptation support. Historically, climate policy research focused predominantly on mitigation [15]. This was probably motivated by the concern that a focus on adaptation might reduce incentives to invest in mitigation; a moral hazard problem [16]. Some noted that adaptation was a “taboo” in the climate discourse [17]. While adaptation has slowly begun to feature more in climate discussions, it is not clear how it might be financed. The United Nations Framework Con- vention on Climate Change (UNFCCC) recognizes that advanced industrialized countries bear substantial responsibility for climate mitigation due to their significant contribution to the accumulated stock of greenhouse gases (GHG). Importantly, “shared but differenti- ated responsibility” should hold for adaptation as well. Because climate change is already in

motion, Global North countries have a responsibility to support adaptation in Global South countries, which bear a disproportionate burden of climate change due to their high levels of exposure to climate risks along with meager resources to address them [18,19]. Thus, scholars note that climate adaptation has an important justice component [20]. The reality is that while Global North countries have made pledges at international forums to support climate adap- tation overseas, the actual appropriations have fallen short [21]. Moreover, it is not clear what counts as adaptation aid, and whether Global North countries are double counting develop- ment aid as adaptation aid [22]. Why are Global North countries showing inadequate commitment to overseas climate aid? In part, this might be due to its low policy salience or low public support among their citi- zens. Moreover, most governments, even in Global North countries, struggle with managing multiple priorities within a limited budget. While opinion polls suggest that a majority of the population in the Global North is worried about climate change and wants governments to take action it is less clear if they are willing to support spending that creates adaptation bene- fits overseas. This paper seeks to advance the understanding of factors driving domestic support for overseas adaptation aid [13,14]. While the foreign (development) aid literature explores public support in donor countries [23] and the implications for different ways of disbursing aid [24],

the subject of public support for climate adaptation aid remains relatively underexplored (exceptions include Gampfer et al. [25] and Uji et al. [26]). More specifically, we examine how international issue linkages and domestic politics might interplay to shape public support for bilateral climate adaptation aid. We build on Uji et al. [26], by asking survey respondents to choose between policy pack- ages that include different levels of two forms of adaptation assistance: adaptation finance and accepting climate migrants. While Uji et al. examined public support in Japan using a conjoint experiment, we modified their instrument to the Swiss context. This allows us to make several contributions. First, the replication of the methodology in another donor country expands the comparative understanding of the factors that shape public support for overseas adapta- tion assistance in the form of adaptation aid and acceptance of climate migrants, as well as the overall external validity of the results. Second, we replicate the methodology in a political context where immigration is a highly salient issue, namely Switzerland and Europe more broadly [27–29], while migration is not a highly salient issue in Japan [26]. Moreover, aid and migration politics seem to be closely linked in Europe. Unlike Japan, European countries have Competing interests: The authors have declared that no competing interests exist.

PLOS ONE | https://doi.org/10.1371/journal.pone.0317344 February 12, 2025 3 / 26 PLOS ONE Support for adaptation assistance in Switzerland sought to manage migrant flows by providing sizeable aid to some countries (especially on Africa’s Mediterranean coast) [30]. This means that the financing aspect of climate adapta- tion aid and immigration is likely to be more salient in the European context, especially with the vocal opposition from right-wing parties to climate mitigation and immigration. The implication is that the political context in which the two forms of adaptation (migration and aid) are discussed differs in Japan and Switzerland, and their comparative understanding is of paramount significance for the future of adaptation aid at large. Finally, from a theoretical perspective, the study of public support for adaptation aid in Switzerland provides an oppor- tunity to elaborate theoretically on the two-level linkages between international collaboration on strategic issues and domestic support for adaptation aid. The rest of the paper proceeds as follows. The theoretical section elaborates on the logic of adaptation policy packages and outlines a set of hypotheses about public support. Section 3 provides a more detailed justification of the case selection and specifies the methodology of data collection and analysis. Section 4 presents the results and discussion before we conclude with implications for policy and future research. Theoretical framework International climate adaptation assistance is embedded in the broader debate on foreign aid

and immigration. Because individuals seldom evaluate issues in isolation, we adopt a policy package approach in theorizing and evaluating public support for climate adaptation assis- tance to specific countries. This implies that public support for bilateral adaptation aid could be influenced by factors such as immigration from this country, Swiss trade with this coun- try (instrumental motivations), how this country votes with Switzerland in the UN Security Council (convergence of policy positions in international forums), the location of the country (geographical bias), and the type of extreme weather event this country faces (empathy with countries with similar problems, or likely media salience of specific events). To identify different dimensions of our policy package we elaborate on the idea that issue linkages could influence public support for bilateral assistance. Building on Uji et al. [26] we ask respondents to consider packages that allow them to take into account climate migration and levels of adaptation financing as two forms of adaptation assistance. The logic is that exposure of Global South countries to climate-related extreme weather events and their eco- nomic, infrastructure, and displacement impacts can be viewed as a “push factor” [31], which encourages individuals and households to emigrate overseas. Adaptation assistance can work via two mechanisms. Financial aid could enhance the resilience of the local community to extreme weather events (think of drought-proofing agriculture, new ways to tackle urban heat

islands, or building coastal mangroves) and therefore lower the incentives to migrate overseas. Further, by enhancing resilience, adaptation aid could support economic growth, which may, in turn, reduce the incentives to emigrate, although we recognize the possibility of a non- linear relationship between economic growth and migration [32]. While most migration takes place within the country or spills over to neighboring countries [33–35], there is a perception (often fueled by populist parties) that a sizable percentage seeks to emigrate to the Global North. Thus, we also examine whether, in donor countries, the public willingness to support bilateral climate finance to specific overseas countries might also depend on their attitudes towards these countries. Our analysis tests the argument that donor and recipient characteristics may also influence public support for overseas adaptation assistance. The political economy of aid literature has long stipulated that the nature of donor and recipient countries and their relations in different international forums affects who gives assistance, in what form, how much, to whom, and why

PLOS ONE | https://doi.org/10.1371/journal.pone.0317344 February 12, 2025 4 / 26 PLOS ONE Support for adaptation assistance in Switzerland [36–38]. Scholars have examined these issues in the context of foreign aid in totality as well as specific components such as humanitarian aid and environmental aid [39,40]. However, the extent to which different types of international linkages interact with specific country contexts to influence adaptation assistance has not been fully elaborated or explored in the literature. Acknowledging that recipient characteristics as well as economic and political linkages of the donor with recipient countries influence aid decisions, we explicitly build such expectations in our conjoint experiments. Given the “aid fatigue” in many donor countries due to perceptions about the misuse of foreign aid in recipient countries [41], and the fear that adaptation assistance might crowd out domestic donor priorities, the first hypothesis captures a baseline expectation, similar to the Japan study, that public support for the policy package will diminish as the volume of bilateral aid increases. Hence: Hypothesis 1: Support for the policy package will diminish when the volume of climate aid is high. In the European context, immigration is a salient yet complex subject in an era of popu- lism. There is a well-developed literature examining public support for immigrants in host countries [42–46], including immigrants with different characteristics such as skill level,

ethnicity, religion, and gender. There is also an emerging literature examining public support for accepting climate migrants [47–49]. Scholars have examined how aid and migration are related [50–53], including how adaptation aid might influence migration [54]. The European Union’s generous aid package to Turkey and, more recently, to Tunisia suggests that industri- alized countries are using foreign aid to hold back the inflow of migrants. Yet, it is not clear whether the public in donor countries sees the link between climate assistance and immi- gration. Arguably, given the populist backlash against immigration, the baseline expectation would be that the public in the Global North is less likely to support a policy package as the number of climate migrants increases. Hence: Hypothesis 2: Support for the policy package will be lower when the number of climate migrants to be accepted from this overseas country is high. Hypotheses 3 and 4 aim to elaborate the logic of how strategic linkage of issues at the international level may influence domestic support for different forms of climate adaptation assistance. Concerning bilateral aid, scholars note that donors have multiple motivations, including humanitarian, economic, and political. Specifically, countries might value their international standing in global forums and reward countries that vote alongside them in these venues [55]. Scholars note these sorts of “vote buying” in the context of the UN General Assembly [56–58], the UN Security Council [59,60], as well as more specialized forums such

as the International Whaling Commission [61]. While such strategic linkage between aid and support at international forums takes place at the intergovernmental level, it is less clear how such linkages at the global level might impact public support for adaptation assistance. Theoretically, we elaborate on approaches that emphasize the relevance of public opinion for development aid policies and multilateral climate finance [24,25,62] to explore if the public responds to strategic issue linkages when expressing support for bilateral adaptation packages. More specifically, the conjoint experiment tests for such linkages in the context of the UN Security Council, given that Switzerland recently became a non-permanent member of the Council, and its role as such is relatively salient in the public’s view [63,64]. Thus, we hypothesize:

PLOS ONE | https://doi.org/10.1371/journal.pone.0317344 February 12, 2025 5 / 26 PLOS ONE Support for adaptation assistance in Switzerland Hypothesis 3: Support for the policy package will be higher for countries that vote with Switzerland in the UN Security Council. Trade relations constitute another international factor that could affect public support in donor countries for climate assistance packages. Some research suggests that trade displaces aid, while others find that donors provide aid to countries with whom they have more trading relationships [24,36,37,65]. Bilateral trade has furthermore been deployed strategically to gain traction on other international priorities such as human rights, labor rights, or the environ- ment, and more recently on climate finance [61,66,67]. In international environmental poli- tics, issue linkage with trade regimes has served to incentivize international agreements and treaty ratification [68] and in supporting domestic commitments for climate mitigation [69]. Uji et al. tested the impact of Japan’s imports and exports with an overseas country on support for adaptation assistance, capturing the potential relevance of domestic distributional implica- tions [26]. Here we investigate whether the public view of linkages between international trade relationships and aid, rather than distributional implications, influence public support for adaptation packages. We hypothesize: Hypothesis 4: Support for the policy package will be higher for countries that trade more with Switzerland.

The elaboration of H3 and H4 furthermore allows us to explore in greater depth, both theoretically and empirically, to what extent the linkage between climate aid and issues such as trade and support in UN forums might have broader repercussions for the level of public support through conditional effect on other relevant attributes examined in this study. Hypothesis 5, in turn, explores the relevance of country characteristics related to migra- tion patterns, such as geography, on public support for adaptation packages. Public support for foreign aid is influenced by the geographical location of the recipient country [56,57,70]. Examining the relevance of distance or proximity to Switzerland on respondents’ support for policy packages may be reflective of the European context at large, given the high and divisive attention placed on immigration waves from relatively close destinations such as North Africa and Syria. Thus, unlike respondents in other contexts such as Japan, where migration is not a salient issue, respondents might fear larger (climate) migration from a country that is prox- imate to Switzerland. Second, such risk perceptions might be stronger if the donor country has previously experienced high immigration levels from specific overseas countries. In recent years, Southern and Western Europe – including Switzerland – have witnessed a surge in immigration from African and Middle Eastern countries. Scholars note the emergence of bias and specific negative frames for specific migrant communities [71]. Research has further-

more revealed a tendency to instrumentalize and politicize immigration from these countries by populist right-wing parties [29]. Thus, it is important to recognize that, sometimes, race, ethnicity, and geography overlap. Support for adaptation assistance to specific countries might also be influenced by such implicit or explicit political, cultural, or racial biases [34,49]. To unpack these related issues, our survey instrument features four potential recipient countries that differ in terms of their distance from Switzerland and cultural characteristics such as religion and language spoken within each region. We chose Algeria, Kenya, Bangla- desh, and the Philippines for several reasons. The countries in our study are located either in Asia or in Africa. They show high levels of vulnerability to climate change and have experi- enced extreme weather events. Their geographical distance to Switzerland varies, and so does the historical and current presence of these migrant communities from these countries. We selected two countries in Asia and Africa that vary in racial and cultural characteristics such as

PLOS ONE | https://doi.org/10.1371/journal.pone.0317344 February 12, 2025 6 / 26 PLOS ONE Support for adaptation assistance in Switzerland language, religion, and colonial history. Referencing actual rather than hypothetical coun- tries is intended to simulate a more realistic context, to which respondents could relate more directly. Based on the preceding discussion, we hypothesize: Hypothesis 5: Support for the policy package will be lower for countries located in closer proximity (Algeria and Kenya) as opposed to a far-off continent (Bangladesh and Philippines) in reference to Switzerland. H5 suggests that we may expect context-specific dynamics to influence support for climate adaptation packages, reinforcing the value of the comparative perspective. The relevance of geographical regions and distance is likely to reflect the underlying politics of migration and vulnerability, as well as cultural and historical legacies associated, for instance, with colonial- ism. The inclusion of countries with different cultural and historical backgrounds within each region further allows us to explore indirectly if cultural biases related to religion and language, for instance, could be detected with respect to public support for adaptation packages. This is to our knowledge one of the first studies to explore the underlying politics that may influence public support in the Global North for climate assistance for overseas countries located both across and within continents. Finally, the last two hypotheses, consider how different types of extreme weather events

might influence public support for adaptation assistance, again with attention to the Swiss context as a donor country. Given that important regions and sectors in Switzerland, includ- ing water, tourism, agriculture, and the Alpine regions more generally, are highly vulnerable to climate change, the Swiss public might be more sympathetic to countries that experience similar extreme weather events (floods and droughts as opposed to cyclones or sea level rise). Swiss residents may relate more directly to the consequences of such events and the resulting need for adaptation. Alternatively, the intensity of other extreme weather (such as cyclones and floods) tends to be captured dramatically in the media, which might enhance public support for adaptation assistance to overseas countries [34,48,49]. Such a portrayal can evoke empathy for countries that have contributed little to climate change but whose human security and economies are threatened by sudden-onset events over which they have little control. There may be a perception, because of media attention, that sudden-offset severe events are likely to bring about more severe damage, human displacement, and suffering and thus require immediate adaptation assistance. We thus distinguish between a “linked fate” hypothesis, referring to shared experience with climatic weather events (Hypothesis 5a), and a “sudden-onset events” hypothesis related to their anticipated high impact and salience (Hypothesis 5b). Hypothesis 6a: Support for the policy package will be higher for countries that experi-

ence floods and droughts as opposed to sea-level rise and cyclones. Hypothesis 6b: Support for the policy package will be higher for countries that experi- ence sudden-onset events, such as floods and cyclones as opposed to slow-onset events such as sea level rise and droughts. Case selection and model In terms of case selection, we focus on Switzerland because it has a consistent record of implementing climate mitigation policy commitments under the UNFCCC [72–74]. The most recent Climate and Innovation Act, known also as the Net-Zero Climate Law, sets interme- diary objectives and means for reaching net-zero emissions. The government proposal was

PLOS ONE | https://doi.org/10.1371/journal.pone.0317344 February 12, 2025 7 / 26 PLOS ONE Support for adaptation assistance in Switzerland supported by popular vote in June 2023 with 59.1% of the votes. Broadly, opinion polls report a high level of awareness of vulnerability to climate change as well [75]. This reflects, in large part, the ecological sensitivity of the country’s Alpine region (including the issue of reced- ing glaciers), its importance for core sectors such as tourism, agriculture, water, and energy, as well as citizens’ identification with and attachment to the Alpine landscape. Thus, Swiss respondents are likely to have a high appreciation for the need to adapt, even as the country seeks to mitigate global climate change. Furthermore, because there is a strong element of direct democracy in Switzerland, where voters are called to express their opinion on specific policies multiple times every year, the use of surveys is highly appropriate in the Swiss context to assess public support for a given policy [76]. The results can inform policymakers about specific frames or features that drive individ- uals’ support for overseas climate adaptation assistance. The Swiss context also captures the broader trends in European and global politics to both the unfolding trajectory of climate change and the rise of right-wing parties. Following the highly salient heatwave of 2003, Europe has experienced a steady increase in average tempera- ture, droughts, and sudden onsets of floods and wildfires. Such events are widely covered in

the media. At the same time, with the rise of right-wing parties, issues such as climate change, development aid, and migration have become more contentious and salient. By probing pub- lic opinion on the question of overseas adaptation assistance in the case of Switzerland, this paper seeks to provide new empirical evidence on the interface of domestic public opinion with climate aid and immigration. Switzerland constitutes a strong candidate for replicating the Japan study by Uji et al. [26]. While immigration is not a salient issue in Japan, it is a highly salient one in Switzerland, due to the politicization and instrumentalization by right-wing parties [27,28,29]. In addition, the rising immigration flows have motivated European countries to offer aid to overseas countries to stem immigration [30] – a strategy officially acknowledged as one of the objectives of Swit- zerland’s development assistance by the Swiss government [77]. The financing of development aid, moreover, has become a contentious issue in Swiss politics – with drastic cuts proposed and supported by right and center-right parties in recent years [78]. These characteristics suggest that both adaptation aid and immigration are more salient and contested in the Swiss context as opposed to Japan. Replicating the study in the Swiss context and comparing results across these cases is important for several reasons. First, in and of itself, the replication of the methodology in another donor country expands the comparative understanding of the factors that shape

public support for policies that increase adaptation aid and acceptance of climate migrants, and the overall external validity of the results. While replicability is a core tenet of rigorous and impactful research, it tends to be practiced much more readily in the natural sciences, compared to social sciences. This is often a consequence of data availability, concerns about comparability of units of analysis, and discounting the practical value of such research, unlike in life sciences where replication is a core and necessary element of practical applications. We contend that to improve policy understanding of the politics that could unlock support for increased adaptation assistance, systematic examination of such factors across different donor countries is necessary. Second, not only do Japan and Switzerland differ in key respects when it comes to the salience and politicization of migration and aid, but also as previously mentioned demo- graphic and political situations observed in Switzerland are part of a broader European trend. As such, while cautious of the generalizability of results beyond the Swiss context, this com- parison may offer a first understanding of preference formation in contexts where immigra- tion and aid are highly salient and politicized topics.

PLOS ONE | https://doi.org/10.1371/journal.pone.0317344 February 12, 2025 8 / 26 PLOS ONE Support for adaptation assistance in Switzerland Methods Empirically, we examine public support for bilateral climate adaptation assistance via a conjoint experiment nested in a survey experiment in Switzerland. As public policy is at least partly responsive to public opinion [79], surveys are an important tool to assess preferences about existing or proposed policies. Specifically, a conjoint exper- iment, contrary to other experimental designs, allows for a concurrent assessment of the causal effect of different policy attributes on public support for climate adaptation policies. Such conjoint experiments reduce social desirability bias because respondents can implicitly trade off attributes against each other. This makes them particularly suited to studying the drivers of policy preferences. Validation efforts of survey-experimental conjoint methods have shown that this experimental approach closely replicates the outcomes in popular votes [80]. If individuals are boundedly rational, their policy preferences might be influenced by how policies are framed. This is why we embedded the conjoint in a survey experiment where respondents are randomly assigned to two treatment frames that highlight aid and migration policies of peer countries as well Switzerland’s past policies on these subjects. Survey experiments are sometimes criticized for their drawbacks in terms of external valid- ity, they only offer a snapshot of a given case at one point in time, and they may not always

mirror real-world situations. The conjoint design of our experiment and its setting mitigate some of the above concerns. On the one hand, conjoint experiments are thought to have a better generalization potential compared to other survey experiments because they better approximate real-world situations where individuals are asked to evaluate policies with mul- tiple attributes [81]. Such evaluation exercise is, moreover, quite common in the semi-direct democratic context of Switzerland, where individuals often evaluate policies and their trade- offs when called to vote upon them [76]. Lastly, the method lends itself to rigorous replication and comparative understanding of the politics of climate aid, in this case concerning a prior analysis of Japan. Sampling strategy, sample, and survey administration We administered an online survey-embedded conjoint experiment to a sample of 876 Swiss residents aged 18 and above. Respondents were drawn from a Qualtrics panel. We established quotas on gender, age, and spoken language to ensure the sample was representative of the Swiss population, which resulted in a representative sample of 876 respondents. Qualtrics XM ran this survey between July 4 and July 19, 2023, in the three Swiss official languages (German, French, and Italian). Internet panels have their drawbacks, compared to population-based probability sam- pling, particularly regarding their representativeness of older age groups of society with lower internet access [82]. However, studies have shown that Qualtrics panels perform well

when compared to population-based random samples and better than those of other pro- viders [83,84]. Furthermore, the possibility of setting quotas – and their use – can mitigate these concerns. The data in our analysis is representative across key dimensions of the Swiss population. After Human Subject Approval was obtained by the authors, the experiment was pre- registered (https://osf.io/wahxb?view_only=73e3c88760ed478e87c90a4fcc86add2). More- over, given that the survey was to be administered in three different languages of Switzerland (German, French, and Italian), authors informally pre-tested the questionnaire with native speakers before the official launch to make sure that the questions were written clearly and understood by respondents across linguist groups.

PLOS ONE | https://doi.org/10.1371/journal.pone.0317344 February 12, 2025 9 / 26 PLOS ONE Support for adaptation assistance in Switzerland Informed consent, anonymity, and confidentiality: Informed consent was sought at the beginning of the survey. Each participant had to read the informed consent statement and could decide whether to accept and agree to participate or exit the survey. In the informed consent statement, participants were informed of the purely scientific purpose of the paper and the lack of commercial and government-related purposes. Anonymity and confidential- ity guarantee related to data collection, storage, and publication were explicitly stated in the informed consent statement. The statement also included an email address at which partic- ipants could direct their concerns, which included the name of one of the authors’ institu- tions. The objective of the study was explained to participants, who have been informed at the consent stage of this process that the authors intended to study public opinion on climate change. They were also informed of the length of the survey. The informed consent statement is available in the survey instrument in Supporting information S1 File. Conjoint experimental design In the design of our experiment, we ask respondents to compare two overseas adaptation assistance packages in different countries. In each package, respondents consider two forms of assistance – aid and immigration along with different types of connections the recipient

country has with Switzerland. The ability of conjoint experiments to capture such multi- dimensionality makes them better able to mirror real-life situations. Conjoint experiment design further lessens the risk of social desirability bias because respondents can implicitly trade off one attribute against another [85]. We draw broadly on the approach implemented by Uji et al. [26] in the context of Japan. First, all respondents initially read a paragraph laying out the causes of climate change, explaining the role played by industrialized countries, including Switzerland, in the accu- mulation of greenhouse gas (GHG) emissions, and how developing countries facing extreme weather events can be supported, either by providing adaptation aid or by accepting climate migrants. The possibility that Switzerland is expected as an industrialized country to provide adaptation assistance was presented in a neutral, non-normative tone, to decrease the risk of social desirability bias. The introductory paragraph was followed by a question that served as an attention check. In the experimental phase of the survey, we randomly assigned participants to one of the three groups: a control group and two treatment groups. The objective was to assess if providing information about peers or what Switzerland has done in the past regarding aid or immigration, might frame the conjoint experiment for respondents in different ways. Data on Switzerland’s overall acceptance of migrants was obtained from the UNHCR Refugee Data

Finder, while the data on bilateral climate aid comes from the OECD Development and Coop- eration Profiles. The first treatment group (peer benchmarking) received information about annual levels of bilateral climate aid and refugee acceptance in OECD countries. The second treatment group (self-benchmarking) received similar information about Switzerland. The third group, serving as the control or reference category, received no benchmarking information. After reading the benchmarking information, respondents were provided with instructions about the task they were about to complete and were informed they were about to be shown different hypothetical policies that the Swiss government could adopt to support developing countries in adapting to climate change. Participants were asked to compare two policy packages, each with six dimensions. Then they were asked to select the one they would recommend the Swiss government to adopt. They repeated the choice task six times. This type of paired-options, forced-choice conjoint design is believed to promote higher engagement of respondents with the task at hand [86].

PLOS ONE | https://doi.org/10.1371/journal.pone.0317344 February 12, 2025 10 / 26 PLOS ONE Support for adaptation assistance in Switzerland The six attributes in the conjoint are: (1) the recipient developing country (Algeria, Kenya, Bangladesh, Philippines); (2) the level of climate aid Switzerland will provide to the recipient country per year (0 CHF, 30 million CHF, 60 million CHF, 90 million CHF, 120 million CHF); (3) the number of immigrants from the recipient country Switzerland will accept per year (0, 250, 500, 750, 1,000, 1,250); (4) the type of extreme weather event that the recipient country experiences (drought, sea level rise, flood, cyclones); (5) the volume of trade between Switzer- land and the recipient country (0, 500 million, 100 million); (6) the percentage of times the recipient country voted with Switzerland at the UNSC (0%, 40%, 80%). To present respondents with credible information, we set the levels of the trade attribute using official data available on the website of the Swiss Federal Department of Foreign Affairs on the trade volume between Switzerland and each one of the recipient countries (https:// www.eda.admin.ch/, accessed 2003). Table 1 displays attributes, their labels in the survey, and the possible levels. The value any attribute took randomly varied within and across choice tasks. The only restriction to randomization was that the policies constituting a pair within each choice task always referred to two different countries, to avoid instances in which a respondent would be

similar to those spoken in Switzerland will integrate more easily in Switzerland”; “Immi- grants from countries that are culturally similar to Switzerland will integrate more easily Table 1. Conjoint experiment attributes and levels. Attributes Labels Levels Recipient country Recipient developing country Bangladesh/Philippines/Algeria/ Kenya Level of climate immigration Number of climate migrants to accept from this country per year 0/250/500/750/1,000/1,250 Level of climate aid Climate aid to give to this country (CHF) per year 0/30 million/60 million/90 mil- lion/120 million Trade volume Value of Swiss trade with this country (CHF) 0/500 million/1,000 million Extreme weather event Extreme weather event Drought/Sea level rise/Floods/ Cyclones UNSC voting alignment Percentage of this country’s votes in line with Switzerland’s position at the UN Security Council 0%/40%/80% https://doi.org/10.1371/journal.pone.0317344.t001

PLOS ONE | https://doi.org/10.1371/journal.pone.0317344 February 12, 2025 11 / 26 PLOS ONE Support for adaptation assistance in Switzerland in Switzerland”; “I am proud of Swiss culture and history”; “I believe that development aid reduces poverty”. The survey included questions aimed at measuring standard socio-demographic variables (gender, age, area of residence, education, occupation, economic sector, income, and polit- ical ideology) which can independently influence the respondents’ support for the overseas adaptation package. The full questionnaire is available in Supporting information S1 File. Replication data is available at the Harvard Dataverse: https://dataverse.harvard.edu/dataset. xhtml?persistentId=doi%3A10.7910%2FDVN%2F3YWEEV&version=DRAFT Results and discussion We use marginal means (MMs; [87]) analysis to estimate the respondents’ favorability for different policy attributes and levels among a given attribute. Conventional measures of Fig 1. Example: choice task with benchmarking on OECD countries’ policy. https://doi.org/10.1371/journal.pone.0317344.g001

PLOS ONE | https://doi.org/10.1371/journal.pone.0317344 February 12, 2025 12 / 26 PLOS ONE Support for adaptation assistance in Switzerland interaction effects such as Average Marginal Component Effects (AMCEs) require the arbi- trary choice of a baseline category, which can affect the directionality, size, and significance of coefficients. In contrast, MMs summarize outcomes by averaging predicted probabilities across all levels of attribute or covariate combinations. By averaging these predicted probabil- ities, MMs provide an intuitive and comprehensive summary of the favorability of the policy package with a given attribute level [87]. Because of these advantages, MMs have become an established method for conjoint survey experiments. Thus, we interpret marginal means from a forced-choice conjoint survey experiment as the favorability of a policy package with a given attribute level to gauge public support. Fig 2 presents MMs, whereby the vertical dashed line represents the 50% (0.5) threshold. Coefficients higher than 0.5 reflect that a policy proposal with the respective attribute level increases the favorability of a climate adaptation policy (as long as the confidence interval around the point estimate stays on the right of the 0.5 level), while coefficients lower than the threshold reflect lower favorability (as along the confidence band around the point estimate stays on the left of the 0.5 level). We discuss the results along the different dimensions of our argument and specific hypotheses. Descriptive statistics are presented in Supporting informa- tion S2 Fig.

Limited unconditional support for climate aid or climate migration We found no statistically significant effects for adaptation packages at any level of climate aid. This is surprising because supporting Global South countries in climate adaptation is now an important issue in global policy conversations. Previously, Global North countries had pledged annual climate aid to the tune of US$100 billion. This was revised upwards to US$300 billion at the 29th Conference of Party meeting in Baku (although this is far below US$1.3 tril- lion demanded by Global South countries). Of course, how much of the aid will be devoted to climate adaptation remains unclear, yet climate aid has gained more prominence on the world agenda. We do not find Swiss respondents expressing a preference for policy packages with high or low levels of adaptation aid (Hypothesis 1 is not supported). While the limited preference of respondents for adaptation aid is consistent with the findings by Gampfer et al. [25] in other country contexts, it is somewhat surprising in the Swiss context, as Switzerland has a reputation for being one of the first Global North coun- tries to provide adaptation aid [88]. According to the latest available data from the OECD Development Assistance Committee (DAC), Switzerland increased climate aid as a percentage of bilateral aid to 23% in 2020–21 compared to 19.5% the previous year, with a greater share devoted to adaptation assistance compared to mitigation [89]. However, despite its stronger

emphasis on adaptation compared to other OECD countries, Switzerland still lags in terms of the share of bilateral aid directed to climate issues compared to the OECD average [89]. More recently, the Swiss Agency for Development and Cooperation and the State Secretariat for Migration have established a new program for climate change and migration “to create better prospects” for youth in North and West Africa and establish safeguards against youth traffick- ing and abuse [90]. Such policy initiatives are illustrative of the concern about proximity and the political salience of the region, as well as linkages between climate adaptation, develop- ment prospects, and migration. Such concerns, however, are not reflected at the level of public opinion according to the results of our conjoint survey experiment. Migration is also a form of adaptation: by accepting climate migrants, Switzerland could support overseas climate adaptation. However, we find that Swiss respondents favor policy packages with zero immigration and disfavor high levels of immigration (1,000, 1,250). This likely reflects the populist backlash against immigration in many European countries, includ- ing Switzerland (Hypothesis 2 is supported).

PLOS ONE | https://doi.org/10.1371/journal.pone.0317344 February 12, 2025 13 / 26 PLOS ONE Support for adaptation assistance in Switzerland Attributes that capture exposure to different types of extreme weather events similarly do not have a significant impact on public support for policy packages (Hypotheses H6a and H6b are not supported). This is somewhat surprising for Switzerland, given the high awareness about vulnerability to weather extremes domestically. The above discussion is summarized in Fig 2. below (and Supporting information S3 Table) Recipient developing country Number of climate migrants to accept from this country per year Climate aid to give to this country (CHF) per year Value of Swiss trade with this country Extreme weather event Percentage of this country's votes in line with Switzerland's position at the UN Security Council 0.425 0.450 0.475 0.500 0.525 0.550 Kenya Philippines Algeria Bangladesh 1,250 1,000 750 500 250 0 120 million 90 million 60 million 30 million 0 million 1,000 million 500 million 0 million Cyclones Floods Sea level rise Drought 80% 40% 0% estimate level Fig 2. Baseline favorability for the overseas climate adaptation assistance policy package. https://doi.org/10.1371/journal.pone.0317344.g002

PLOS ONE | https://doi.org/10.1371/journal.pone.0317344 February 12, 2025 14 / 26 PLOS ONE Support for adaptation assistance in Switzerland Issue linkage with trade and UN voting alignment The results of the survey experiment show that Swiss respondents prefer adaptation policy packages for trading partners and for countries that vote with Switzerland in the UNSC. As shown in Fig 3, 80% alignment of votes with the Swiss position in the UN Security Council increases the favorability of policy packages, while the 0% alignment scenario reduces sup- port, with the mid-range (40%) being statistically non-significant (Hypothesis 3 is supported). Similarly, respondents favor policy packages for countries with medium and high levels of trade with Switzerland, while support is significantly reduced for countries with zero volume of trade (Hypothesis 4 is supported). These findings suggest that respondents tend to view climate adaptation assistance as part of the broader spectrum of international economic and policy engagement between Switzerland and developing countries – that is, they are not com- partmentalizing climate policy. The significant impact of the high share of UN vote alignment and high volumes of trade on public willingness to support adaptation packages to developing countries suggests that the public may view such countries as strategic allies in international affairs more broadly and with respect to climate adaptation more specifically. These findings prompted us to further

explore how such two-level issue-linkage might condition the effects of other conjoint attri- butes. Importantly, similar results were found in the case of Japan for vote alignment in the UN General Assembly, and countries to which Japan has a high volume of exports. The signif- icance of these factors across the different political contexts of two donor countries, in terms of trading partners, geographies, and policies related to climate, migration, and economic development, suggests that two-level issue linkage deserves further exploration with respect to public motivation to support for increasing adaptation assistance. Notably, the results on the conditional effects of trade volume and vote alignment in the UN Security Council provide further evidence of the significance of trade relations and UN Security Council position alignment for respondents’ preference of policy packages. As shown in Figs 3 and 4, the highest bilateral trade volume (1,000 million) is associated with a positive shift in respondents’ favorability of substantial bilateral aid packages, rang- ing from 30mn/year to 90mn/year. Similarly, respondents tend to favor packages with low and medium levels of climate migration (250 to 750 per year). By contrast, when there are no trading relationships, respondents are indifferent about policy packages containing 0 million of climate aid (the confidence interval overlaps the dashed line at 0.5, indicating indifference). Policy packages with positive levels of climate aid reduce respondents’ favor-

ability towards policy packages. These results are broadly robust to a different modeling strategy, specifically, nonparametric ANOVA regression [91], allowing the selection of the most important interaction effects between the different conjoint attribute levels (Support- ing information S17–S19 Tables, S20 Fig, S21–S22 Tables). These results show that policy packages in which countries do not trade with Switzerland decrease the policy package’s favorability in almost all treatment combinations of the attribute levels in the conjoint survey experiment. Fig 4 further shows that the conditional effect of an 80% alignment of votes with the Swiss position at the UN Security Council increases the favorability of policy proposals with the highest range of bilateral climate aid packages (90mn/year and 120mn/year), as well as for packages with low- and medium-level migration (250–750 persons/year). Significantly, the interaction with high levels of UN Security Council alignment and trade volumes makes respondents more likely to favor a broader range of countries (Philippines, Bangladesh, and Kenya). By contrast, the conditional effect of 0% alignment in the UN Security Council is associ- ated with negative or non-significant favorability for packages. These results strengthen our

PLOS ONE | https://doi.org/10.1371/journal.pone.0317344 February 12, 2025 15 / 26 PLOS ONE Support for adaptation assistance in Switzerland confidence in the robustness of our findings concerning Hypotheses 3 and 4 on how linking issues such as trade and foreign policy positions is likely to strengthen the support for policy packages of climate adaptation assistance. Yet, if such issue linkage strategies were to be used for increasing adaptation aid and willingness to accept climate migrants, their consequences 0 million 500 million 1,000 million Recipient developing country Number of climate migrants to accept from this country per year Climate aid to give to this country (CHF) per year Extreme weather event Percentage of this country's votes in line with Switzerland's position at the UN Security Council 0.35 0.40 0.45 0.50 0.55 0.60 0.35 0.40 0.45 0.50 0.55 0.60 0.35 0.40 0.45 0.50 0.55 0.60 Kenya Philippines Algeria Bangladesh 1,250 1,000 750 500 250 0 120 million 90 million 60 million 30 million 0 million Cyclones Floods Sea level rise Drought 80% 40% 0% Marginal Means Fig 3. Trade conditions the effect of the other five conjoint attributes on the favorability of policy packages by respondents. https://doi.org/10.1371/journal.pone.0317344.g003

PLOS ONE | https://doi.org/10.1371/journal.pone.0317344 February 12, 2025 16 / 26 PLOS ONE Support for adaptation assistance in Switzerland for environmental equity might also be negative, as aid might not go to countries that are the most vulnerable and least resilient, but to those with a strong competitive advantage and strong international trade engagement and institutions. 0% 40% 80% Recipient developing country Number of climate migrants to accept from this country per year Climate aid to give to this country (CHF) per year Value of Swiss trade with this country Extreme weather event 0.40 0.45 0.50 0.55 0.60 0.40 0.45 0.50 0.55 0.60 0.40 0.45 0.50 0.55 0.60 Kenya Philippines Algeria Bangladesh 1,250 1,000 750 500 250 0 120 million 90 million 60 million 30 million 0 million 1,000 million 500 million 0 million Cyclones Floods Sea level rise Drought Marginal Means Fig 4. UN Votes condition the effect of the other climate adaptation assistance conjoint attributes on the favorability of policy packages by respondents. https://doi.org/10.1371/journal.pone.0317344.g004