IntroductionMitigating climate change is urgently needed to avoid severe negative impacts of climate change to nature and society1. This not only requires the implementation of mitigation strategies to reduce CO2 emissions (e.g., switching to renewable energy or lifestyle changes), but also actively removing remaining CO2 emissions through negative emission technologies and practices (NETs), which capture and store carbon underground through natural and/or chemical processes2,3,4,5. Implementing NETs is not only a technological and ecological challenge, but also a societal challenge, which is critical to address to ensure that sustainable transitions take place in a responsible and fair way2,6,7,8. In international climate negotiations, the allocation of responsibility for mitigating climate change and the fair distribution of burdens of implementing NETs have been widely discussed2,8. To fairly allocate the burdens to each country, the concept of “common but differentiated responsibilities and respective capabilities” have been widely discussed by philosophers9, economists and political scientists10, and applied in modelling mitigation policy or carbon removals11,12. Similarly, it has been argued that countries that emitted most CO2 (referred to as the responsibility principle) and are most capable of paying the costs (referred to as capacity principle) should remove most CO2 from the atmosphere to secure a fair distribution of efforts13,14,15,16,17. However, these fairness principles mostly remained normative criteria in research and policy making. The question remains to what extent these principles are also critical in shaping citizens’ fairness judgments, especially whether these two principles are equally important for their perceptions of fairness or one is more important than the other. Furthermore, while the indicators of perceptions of a country’s responsibility and capacity have mostly been linked to the country’s historical and current emissions, and national wealth, respectively13,14,15,16,17, some other indicators crucial to the capacity of implementing NETs (e.g., land availability and knowledge) have not yet been studied, so we know little about how these are linked to public perceptions of fairness. Our study aims to bridge the gap between normative principles and citizens’ perceptions of fairness by investigating the relative importance of perceived responsibility and capacity of implementing NETs, and understanding how different indicators of perceived responsibility and capacity of implementing NETs are related to citizens’ fairness judgments. Understanding the extent to which these factors are related to perceived fairness of implementing NETs in different countries is crucial, as perceived fairness is one of the key factors influencing public acceptability of mitigation technologies and policies and thus may significantly impact the acceptability of NETs and legitimacy of decision-making in the implementation of NETs7,18,19,20,21.Following the normative principles, we will examine to what extent perceived responsibility of a country for emitting CO2 that caused climate change, and perceived capacity of a country to implement NETs affect the perceived fairness of implementing NETs in a given country. Previous studies suggest that both perceptions of responsibility and (financial) capacity affect to what extent people find it fair that a country takes climate action: people generally think it is fairer that countries that are more responsible for causing climate change and have more financial resources take actions to mitigate and adapt to climate change15,16,22,23. Based on this, we hypothesize that the more CO2 a country has emitted, and the more capacity a country has for implementing NETs (e.g., resources and knowledge), the fairer citizens think it is that a country implements NETs.It has been argued that both the perceptions of responsibility and capacity need to be considered to allocate the burdens of mitigating climate change14,16. Yet, the question remains whether these perceptions play equal roles in shaping citizens’ fairness judgments. Therefore, extending previous research, we aim to examine whether perceived causal responsibility and perceived capacity of a country can best explain perceived fairness of implementing NETs in a country. Some studies examining which factors affect the perceived responsibility of countries to mitigate climate change showed that people think that countries that have relatively high CO2 emissions are most responsible to act, while countries’ financial resources seem to be of secondary importance20,21. We theorise that perceived causal responsibility may have a stronger influence on perceived fairness of implementing NETs than perceived capacity, since morality is likely to play a key role in fairness judgments—someone who causes the problem should be held accountable, while someone capable of solving the problem is less morally obliged to do so24,25. For example, even if a country is highly capable of implementing NETs but contributes little to CO₂ emissions, people may perceive this country as less morally obligated to implement NETs. On the other hand, if a country is less capable of implementing NETs yet highly responsible for past CO2 emissions, people may still consider the country to be morally obligated to implement NETs.The next question is which indicators of perceived responsibility and capacity are most important to understand public perceptions of fairness of implementing NETs, as this provides a more detailed view of which issues are most important to people. We expand previous research by employing multidimensional conceptualisations of perceived responsibility for causing climate change and perceived capacity to implement NETs. Based on previous research, we reason that three indicators can shape perceptions of a country’s responsibility for causing climate change: past CO2emissions13,14,15,22,26, current CO2emissions13,15,26, and past efforts in reducing CO2emissions15. While higher past and current CO2 emissions imply higher responsibility for causing climate change, more effort in limiting climate change in the past may signal less responsibility for causing climate change. However, previous research has mainly examined past efforts qualitatively15, without comparing its relative importance with past and current emissions in shaping fairness judgments. While perceived past and current emissions are mostly linked to perceived causal responsibility 13,15, less is known whether perceived past efforts in reducing CO2 emissions are related to perceptions of fairness to implement NETs.Extending previous work, we propose that the perceived capacity of a country to implement NETs not only depends on its financial resources14,15,16,22,23, but that other resources, in particular available land and space, and knowledge and technology development, are also crucial to the capacity to implement NETs. Hence, we hypothesise that perceptions of a country’s capacity to implement NETs likely depend on a country’s national income14,15,16,22,23, available land and space12,27,28, and knowledge to implement NETs27,29,30. Wealthier countries are likely perceived to be more capable of implementing NETs. Yet, land availability may also be a crucial indicator of the perceived capacity to implement NETs, as NETs may require massive land for planting trees or underground space for carbon storage2,3. Similarly, knowledge to implement NETs (e.g., research and development capability, receiving funding, patents registration) likely affects the perceived capacity to implement it30, especially for novel but immature NETs.Hence, we aim to extend previous studies by studying the relative importance of different indicators of perceived responsibility and capacity for the first time in an integrated way, giving a comprehensive picture of how the public builds their perceptions of fairness based on different indicators of responsibility and capacity, respectively. In doing so, we include indicators that have been understudied in previous research on perceptions of fairness principles in the context of mitigation or adaptation policy, and currently remain mostly normative standards with a few applications in modelling11,12,28,29.We aim to unfold fairness perceptions across different geographical, socio-economic, cultural, and political contexts by conducting our study among representative samples in six European countries: western (Germany, the Netherlands), southern (Spain), northern (Finland), to central/eastern (Poland, Lithuania) Europe. These countries also vary from relatively high CO2 emissions (Germany) to low CO2 emissions (Lithuania); and their capacity to implement NETs (i.e., GDP per capita)12, from relatively high GDP per capita (the Netherlands, Finland) to low GDP per capita (Poland, Lithuania). By doing so, we can explore whether the patterns of the relationships are similar across countries, even though the levels of causal responsibility and capacity may differ across countries. Furthermore, we will compare the perceived and actual responsibility and capacity of NETs to explore whether public perceptions align with reality.This research focuses on two types of NETs: afforestation and reforestation (AR) and direct air capture with carbon storage (DACCS), representing nature-based and technology-based NETs, respectively. Both AR and DACCS have high potential in removing CO2 emissions, with DACCS estimated to be more efficient, potentially removing 5–40 GtCO2-eq per year globally, while AR is expected to remove around 0.5–10 GtCO2-eq per year3. Although AR is cheaper than DACCS, it is considered a less secure type of NETs, as AR may release CO2 to the atmosphere again due to risks like wildfires and pests. In contrast, DACCS is likely to store CO2 underground for thousands of years3. In addition, people generally perceive AR more positively (e.g., less risky and having less negative impact on nature) and more acceptable than DACCS31,32,33. Comparing the results for both NETs enables us to unveil fairness perceptions and the underlying factors for these fundamentally different technologies and practices.Methodologically, we strengthen existing evidence by integrating experimental and correlational designs in order to ensure internal and external validity, respectively. The experimental design allows us to test how a country’s responsibility for causing climate change and its capacity to implement NETs influence fairness perceptions in a controlled setting, while keeping other factors constant, enabling us to draw conclusions on causality. The correlational design, on the other hand, cross-validates the findings within a complex real-life context, considering participants will be evaluating fairness of implementing NETs in their own country.ResultsEffect of a country’s CO2 emissions and availability of resources on perceived fairness of implementing NETs in that countryOverall, participants considered it fairer to implement AR than DACCS. The detailed descriptive statistics of perceived fairness of implementing both types of NETs for each experimental condition and each country are available in the Supplementary Material, Table S2. Below, we discuss the effects of a country’s CO2 emissions and availability of resources on perceived fairness of implementing each of these NETs.As Fig. 1a shows, we found a significant main effect of a country’s CO2 emissions on perceived fairness of implementing AR, with a small effect size, F(1, 5237) = 130.91, p