The Demand for Voluntary Carbon Sequestration – Experimental Evidence From a Reforestation Project in Germany

With the increasing recognition of the use of reforestation measures as a complement to conventional carbon emissions avoidance technologies it is important to understand the market valuation of local forest carbon sinks for climate change mitigation. We conducted a framed-field experiment among a Germany-wide sample to provide a revealed preference study on the individual willingness to pay (WTP) for carbon sequestration through forests. Our particular focus is on the role of local co-benefits of climate protection activities. In addition, we add geodata to our experimental data to analyze the impact of spatial variation on the individual WTP. We find that the WTP for carbon removal exceeds the WTP for mitigation efforts found in previous studies. While spatial distances does affect the likelihood to contribute to a local carbon sink, it does not affect the average amount given. Additional survey data finds that trust in forest measures is higher compared to mitigation via an emissions trading scheme, which could explain the comparably high WTP.


Introduction
At the Paris Climate Convention, countries around the world agreed to limit global warming to 2°C -preferably 1.5°C -above pre-industrial levels.To fulfill this target, economies have committed to reach net zero greenhouse gas (GHG) emissions by the mid of the century.Net zero by 2050 is an ambitious yet important goal that requires a rethinking of traditional GHG avoidance approaches and calls attention to negative emission technologies (NETs) as a complementary method to lower the atmospheric CO2 concentration levels.NETs are based on carbon sequestration and cover approaches that remove carbon dioxide from the atmosphere, and capture and transfer it back to geologic reservoirs and ecosystems (Herzog and Golomb, 2004).
Following the discussion on the remaining carbon budget and the insufficiency of solely applying conventional GHG avoidance methods, NETs are already firmly anchored in 87% of IPCC scenarios that give a more than 50% chance of meeting the 2°C target (Fuss et al., 2014).A range of natural and technological approaches (though still lacking the capability to serve at larger scales) exists for removing carbon dioxide from the atmosphere.Especially measures based on forest carbon sink have recently received a particular amount of attention from both politics and the public.
The Paris Agreement (PA) assigned a key role for combatting climate change through the protection and maintenance of forests (UNFCC, 2015: para. 5).Several transnational initiatives have recently stressed the need for increasing forest coverage.The European Commission has released the EU Forest Strategy for 2030 that aims to increase the EU forest coverage, plant tree billion trees by 2030, and create payment schemes for forest owners for the provision of ecosystem services.At the One Planet Summit for Biodiversity, French President Macron announced additional funding of $14 billion for the Great Green Wall for the Sahel and Sahara Initiative (GGW) in January 2021.At COP 26 in November 2021, over 130 countriesaccounting for more than 90% of the world's forests -have committed to the Glasgow Leaders' Declaration on Forests and Land Use to reverse forest loss and land degradation.Further popular programs are the Trillion Tree Campaign, launched in 2006 by the United Nations Environment Program (UNEP), the Eden Reforestation Project, the International Tree Foundation or Plant your Future.
The estimated potentials for carbon sequestration from forest measures such as reducing deforestation, forest management, and afforestation differ depending on chosen activity, region, time horizon and methods used (Nabuurs et al., 2007;Neumann et al., 2016;Obersteiner et al., 2018;Bastin et al., 2019).Nonetheless, research suggests that these activities inhibit a large potential to support the necessary ambition to meet climate stabilization targets (National Academies of Sciences, Engineering, and Medicine, 2019;Pires, 2019;Austin et al., 2020).
One reason being its comparable low marginal costs in particular in case of fast-growing tree species (Forster et al., 2021).But even with growing scientific and policy recognition (Fuss et al., 2014;Geden et al., 2019;Hilaire et al., 2019), far less is known about natural NETs as "unconventional" avoidance efforts and, in particular, in the interplay with traditional forms of carbon mitigation.However, a successful integration of forest carbon sinks into climate policies requires a profound understanding of the markets valuation of carbon sinks considering the socioecological context and potential trade-offs.This also includes a better understanding on the value of forests serving as a non-permanent GHG removal option but with a potential to contribute to lower peak emissions in the short-to mid-term.
From an economic point of view, analyzing consumers' willingness to pay (WTP) for reforestation projects for climate protection provides an interesting source of experimental variation to elicit the role of co-benefits for voluntary GHG reduction efforts.The IPCC (2014) classifies co-benefits (or ancillary benefits) as positive effects that a certain climate policy measure aiming at one particular objective (e.g.reducing GHG emissions) has on a further related objective.Co-benefits from reducing GHG emissions include both positive impacts on environmental concerns like air pollution or biodiversity but potentially also effect economic indicators such as innovation and technological spillovers.While the primary public good component (i.e. the benefits from reduced GHG emissions) is globally at scale, most of the cobenefits from reforestation projects (e.g., reduced air pollution and noise, clean water, a livelihood for local people and improvements in health and biodiversity) particularly accrue in a local context.This highlights the role of place attachment on the WTP for a local forest carbon sink.
We conducted a framed-field experiment among a Germany-wide sample.Therewith, we provide to the best of our knowledge the first revealed preference study on the individual WTP for the carbon sequestration services of forest.Our experimental variation is inspired by the idea that an active communication of co-benefits can encourage mitigation activities (Bain et al., 2016).
Especially in the context of afforestation, these local co-benefits are highly visible and perceptible.Highlighting co-benefits can help to mitigate free-riding incentives, which are frequently observed in previous experimental research on the WTP for GHG mitigation efforts through purchasing and withdrawing emissions allowances from existing emissions trading schemes (ETS) (e.g., Löschel et al., 2013;Diederich and Goeschl, 2018;Löschel et al., 2021).
Survey evidence reveals that individuals rank carbon storage as the most important contribution of (tropical) forests, followed by biodiversity and support to local communities (Baranzini et al., 2010).As co-benefits are expected to be positively correlated with spatial distance to the local carbon sink, we further add a distance measure and geo-data (i.e.forest coverage, rusticity, spatial distance) to our experimental data allowing an in-depth analysis on the impact of spatial variation on the individual WTP.As a follow-up, we run a survey among a group of individuals that has been uninvolved in the experiment to elicit predominant perceptions and existing knowledge on the function of forests.Insights from the survey enable us to explain our results.
Our empirical findings indicate substantial differences to the previous empirical literature investigating the WTP for individual GHG avoidance activities.In particular, we find that the public valuation of a local forest carbon sink exceeds that of conventional mitigation of emissions by far.We do not observe a strong empirical link between highlighting the co-benefit component and the individual WTP.Instead, we find evidence for a negative relationship between spatial distance and the willingness to contribute to a local carbon sink.Distance does play a role on the decision whether to contribute voluntarily to the local carbon sink or not.It does not influence the contribution levels.

Framed-Field Experiment
We embedded our framed-field experiment in a survey conducted jointly by the University of Münster and an online comparison platform for electricity tariffs that operates all over Germany from 16 to 25 March 2020.The customers of the comparison platform took thus part in our experiment.The survey was thematically unrelated to our experiment and investigated consumer behavior in the electricity retail market.The survey software Qualtrics was used to implement the online questionnaire and generate personalized survey links.The links were distributed via email using MailChimp.In the invitation email, participants were informed about the general purpose and expected duration of the survey as well as their fixed payment of 20 € for complete participation.They could receive further payments between 6 € and 40 €, depending on their answers within the survey on electricity tariffs.The final sum of payments was distributed as a voucher that could be redeemed at over 500 shops.The survey consisted of four parts.Personal traits of participants such as risk preferences, time preferences, trust, need for recognition, paternalism and keeping control of decisions were investigated 1 .All answers were collected using a strategy method.
After completing the online survey, participants were, for the first time, confronted with the opportunity to donate their fixed payment (20 €) to a carbon sink project located in Mannheim, Germany.For this, we collaborated with the city of Mannheim that hosts the Bundesgartenschau 2 (German National Garden Show) in the year 2023.For the event, sealed areas and brownfields are transformed into a green area creating new recreational spaces, a species conservation area and an additional local carbon sink by permanently planting about 1.000 trees, and at the same time improving the city`s air quality and climate.
For the donation decision participants could determine if and how much they want to give with the help of an adjustable slider.Unknown to the potential contributors, subjects were divided randomly into two treatment groups that vary the information given on the carbon sinks project: the sink (S) and the co-benefit sink (CBS) treatment (see Table 1).In both treatments, participants received relevant information on the need for global climate protection and the role of NETs based on the PA and the IPCC reports.In particular, we explained the role of carbon sinks as a form of NET within this process.In addition, participants received information on the average CO2 absorption capacities of trees based on an example of a beech, which on average absorbs 100kg of CO2 in eight years (Klein, 2009).In order to make this information more readily accessible to participants, we provided the information that 100kg of CO2 approximately equal the emissions value of a 550km car trip.Finally, we gave participants information on the reforestation project in Mannheim and that their donation would be used to plant additional trees.In the CBS-treatment, we additionally included information on the local co-benefits.We highlighted the recreational value, local air quality improvements, an increased 1 The questions on risk preferences, time preferences und trust were closely related to the GPS (Falk et al., 2016;Falk et al., 2018).The question on the need for cognition were closely related to the NFC-K (Beißert et al., 2014). 2 The German National Garden Show is an exhibition on horticulture hat enjoys great popularity.It takes place every two years in varying German cities and lasts 189 days.In 2019, the show took place in Heilbronn and attracted 2.3 million visitors.
balance of temperature, and improved biodiversity.After participants received these information, we asked them on a next screen if and how much they would like to donate for the removal of 100kg CO2 from the atmosphere through the reforestation project.The likelihood to give (i.e. the extensive margin effect) and the amount given (i.e. the intensive margin effects) are our main outcome variables for the WTP analysis.As the survey platform records the geographical position of each participating subject, we are able to link these positions with further geo-coded data. 3For an overview and explanations of the sample characteristics, see Appendix Table A1.1 -A1.3.Appendix 3 includes the translated treatment texts used in the questionnaire.

Observational Data on Geographical Indicators
The attitude towards planting trees may not only affected by the physical distance to the forest carbon sink but also by the characteristics of the spatial surrounding of the participants' location such as e.g.forest coverage, agriculture or rurality.Participants living in areas with a high degree of forest coverage and enjoying the value of trees may see the value added of additional trees even in more distant areas.Contrary, these individuals may not see the need to spend money to afforest additional areas.Czajkowski et al. (2017) found that geographical particularities may influence the respondents' WTP in a way that the respondents' WTP was higher the closer they lived by a forest and the scarcer forests were in the area they lived in.To control for these potential effects, we match our experimental data with geo-data from the INKAR-Database (BBSR Bonn, 2020) from the German Federal Office for Building and Regional Planning.The database covers over 700 indicators ranging from labor market, education, social services, demographics, income, housing, public finances, transport and the environment.Most of these indicators are continuously collected since 1995 and are clustered along the different German constitutionally distinct and legally independent political levels.The lower the level the less area they cover allowing a more precise analysis of the indicators impact.The lowest level available for the indicators used in our analysis are districts.For an overview and explanations of the variables, see Appendix Table A1.4-A1.6.

Survey Data
Our expected experimental results may also depend upon the beliefs of participants concerning the different effects of a reforestation project.Assessing prior or post beliefs can be instructive for interpreting the results (Haaland et al., 2020).Especially in the CBS treatment, where we stress the local co-benefits, the response to the information depends on the participant's prior beliefs and knowledge and to which extent this information updates existing beliefs.When participants are already fully aware of the role of co-benefits of forests, stressing them may not have the intended effect.Even in the S treatment participants may account for the benefits of the local program.While the field-experiment itself did not permit to measure the participant's beliefs, we instead conducted an extensive survey among students of the University of Münster to assess the knowledge about forests and general attitudes towards environmental donations such as voluntary giving in form of purchasing certificates or tree donations.We used the Online Recruitment System for Economic Experiments of the University of Münster to recruit participants; 567 students participated.The survey covered questions on knowledge on ETSs and reforestation measures.We elicited whether people prefer to buy credits from emissions-saving projects, plant trees within a reforestation project, or neither.Further questions on the participant's faith on certificates or reforestation programs were included as well as knowledge on forest co-benefits.
The summary table of survey variables are in Appendix Table A1.7.The complete and translated survey is in Appendix 4.

Statistical Power Analysis
We base the optimal sample size calculations for our experiment on results from the experimental study by Löschel et al. (2021) as it is closest to our design.The authors use a local sample in China and report an extensive margin effect of -31% when turning from the local (Beijing, 66% of the subjects contribute) to the global setting (Shenzen, 44% of the subjects contribute).To be able to detect a similar effect size, a power analysis with an underlying two-sample proportions (Pearson's χ2) test (with α=0.05, p1=0.66,p2=0.44)indicates that at least 150 experimental observations are needed to achieve a statistical power of 0.7.We were able to recruit 160 subjects for our experiment.Therefore, we expect our experimental setup to be "well-powered" for being principally able to detect similar treatment effects compared to those reported by Löschel et al. (2021).

Hypotheses
Individual environmental conservation efforts are commonly described as donations to a pure public good, or, if linked to the consumption of a private good, as contributions to an impure public (green) good (Kotchen, 2006).As individual costs for conservation efforts usually are expected to outweigh by far the individual benefits from the increase in environmental quality, strong free-riding incentives are expected to prevent high contributions to global public goods like GHG mitigation.A series of revealed preferences studies that experimentally investigate the WTP for GHG emissions via purchasing and withdrawing emissions allowances from existing ETSs indeed report a positive but low average WTP for climate protection (Löschel et al., 2013;Diederich and Goeschl, 2018;Löschel et al., 2021).On the other hand, over the last years, offset issuances and retirements at the voluntary carbon market (VCM) have increased considerably.
Over the past 20 years, VCMs have funneled more than $5 billion into emission reduction and removal activities, ranging from renewable energy to forest conservation (Forest Trends' Ecosystem Marketplace, 2020).In particular, forest projects among the most popular offsets and made up about 42% of all credits issued in last five years (World Bank, 2020) Direct use values can be experienced directly and cover recreation, education and tourism but also timber, fuelwood and edible plants.Indirect use values provide environmental services such as biodiversity, carbon storage, improved air quality, soil protection and hydrologic functions (Bateman and Lovett, 2000;Núñez et al., 2006;van der Horst, 2006).
Co-benefits are expected to play an important role for voluntary GHG reduction efforts.There is empirical evidence that communicating co-benefits can encourage mitigation activities (MacKerron et al., 2009;Ninan and Inoue, 2013;Bain et al., 2016).The experimental literature on consumers' WTP for emission certificates however reports mixed results.Löschel et al. (2021) find a higher share of contributors to climate change mitigation and a higher median WTP when 4 Non non-use values include the bequest value, altruist value and existence value.
local co-benefits are taken into account.Diederich and Goeschl (2018) and Baranzini et al. (2018) find no effect of highlighting local co-benefits.
Co-benefits of afforestation are highly visible and perceptible such that we expect them to mitigate free-riding incentives.Based on the literature and the forest context, we expect contributions to be higher when additional local benefits from CO2 removal through reforestation are stressed compared to a setting when they are not stressed (i.e.WTPCBS > WTPS).However, the provision of these local public goods (e.g., improved local air quality, higher biodiversity) may provide further sources for additional (more local) free-riding behavior.It therefore remains an empirical question if and how subjects react to stressing the local co-benefits.We formulate our second hypotheses H2 as follows: Hypothesis H2.H0: WTPCBS = WTPS HA: WTPCBS > WTPS Other than local benefits, also local favoritism may influence contribution behavior.
Understanding the effect distance may have on voluntary contributions is an important factor for framing donation appeals.In the context of carbon offsetting, few studies on the WTP for emissions reductions have investigated the spatial dimension and present a rather heterogeneous picture.Diederich and Goeschl (2018) offer participants to buy either local EU-based or developing country offsets and find no locational preference.Anderson and Bernauer (2016) find that domestic offsetting is always preferred over international, only an efficiency argument increases the support for international abatements.Buntaine and Prather (2018) find in two behavioral experiments that American subjects have strong preferences for local activities.Using a choice experiment, Bakhtiari et al. (2018) show that individuals' marginal WTP for comparable biodiversity conservation measures vary with distance.
In the context of the reforestation project geographical influence may be of particular importance too, as already emphasized by forest valuation studies (Bollen et al., 2009;West et al., 2013;Hamilton et al., 2017;Deng et al., 2018).Participants living within a close distance to a forest project are more likely to benefit its local co-benefits.Torres et al. (2015) find a stronger support for local mitigation when local co-benefits are emphasized.Abildtrup et al. (2013) conclude that the spatial dimension is crucial for the recreational value of forest.With an increase in distance the likelihood to experience and profit from these co-benefits decreases, e.g. the use values would be decreasing in distance mainly due to travel costs.The relationship between non-use values and distance might be driven by a social-distance mechanism.By emphasizing the local co-benefits these beneficial but locally bounded components becomes more apparent to participants, such that we expect an interaction of distance and co-benefits.
If local favoritism holds in our setting and given the variety in places of residence of our subjects, we expect a difference between those who are located close to the reforestation project and those who are located further away.If this holds we would reject the Null-hypothesis (H0: ρ(WTP,c)=0) in our hypothesis H3, which captures the correlation ρ between the spatial closeness c to the reforestation project and the willingness to pay as follows: Hypothesis H3.H0: ρ(WTP, c) = 0 HA: ρ(WTP,c) > 0

Results
160 subjects participated in the experiment.The mean age is 44 and 30% of the participants are female.The S treatment has 73, the CBS treatment 87 observations.Balance tests were performed on age and gender and confirm a balanced sample.Appendix 1 provides a description of all experimental variables (Table A1 -A1.3), geographical indicators (Table A1.4 -A1.6), and the post-survey variables (Table A1.7).In total, participants in our experiment donated 1.797 EUR.
With this money, four Pterocarya fraxinifolia of five to six meters height were planted in May 2021 on the area of the Mannheim National Garden Show.

Univariate Analysis of the Treatment Effects
We compare individual contributions to the local carbon sink across treatments.Figure 1 gives an overview of the share of contributors, and the mean contributions both conditional on giving and of the total sample.Starting with the extensive margin effects (i.e., the share of contributors), results clearly indicate that the share of subjects that contribute to CO2 removal is larger than zero in both treatments (t-test, p=0.000).65.0% of all subjects in our sample contribute a positive amount to the public good.In S, 70.0% of all subjects give a positive amount, this share decreases to 60.9% in CBS.This decrease is however not statistically significant at any conventional level (exact Fisher´s test, p=0.249).We can clearly reject the Null-hypothesis (H0: WTPS= 0) of our first hypothesis (H1).
Continuing with the intensive margin effects on the subjects' implicit WTP for CO2 removal, we denote the amount of money that a subject contributes to the reforestation project as the minimum WTP (WTPmin).Conditional on giving (see Figure 1.b) the mean WTPmin (10.28 EUR in S vs. 9.21 EUR in CBS, t-test, p= 0.4200) do not differ significantly between S and CBS in our sample.
Including all observations (see Figure 1.c), the mean WTPmin amounts to 6.33 EUR/100kg removal.In the S-treatment, the mean WTPmin amounts is 7.18 EUR/100kg removal.In the CBS it is 5.61 EUR/100kg removal.The difference between the mean WTPmin in S and CBS is not significantly different (t-test, p=0.1663).In relative terms, the average contributions amount to 35.9% (S) and 28.1% (CBS) of the initial enumeration.These first insights already indicate that a change in the treatments from a pure CO2 perspective to a scenario where local co-benefits from CO2 removal are explicitly stressed do not lead to a higher WTPmin but -if anything -rather suggest some backfiring tendencies.When we compare our mean WTPmin with studies than inhibit a similar setting in the context of emission mitigation, the WTPmin on carbon removal appears to be substantially higher than the revealed WTP for mitigation activities in Germany.Löschel et al. (2013) found a mean WTP of 1.2 EUR/100kg CO2 and Diederich and Goeschl (2014) a mean WTP of 0.6 EUR/100kg CO2 in settings with greenhouse gas mitigation through purchasing and withdrawing emissions allowances.These observations point into the direction that the public valuation of forest carbon sinks might exceed that of emission mitigation significantly.We discuss potential reasons for this observation using our post-experimental survey data in Section 5.

Relationship between WTPmin and Distance
Based on the existing literature, we hypothesized to find a correlation of distance and our outcomes.The spatial differentiation of the participants' locations allows us to investigate whether the distance towards the forest carbon sink located in Mannheim matters at either the extensive or the intensive margin.Our main distance measure tis car travel distance in minutes and indicates the travel time between the participants' location and the location of the local sink.
The mean distance of a participant's location to the sink is 3 hours and 44 minutes.In addition, we generated a dummy indicating whether someone lives within a 60-minutes-radius of the carbon sink, and categorized the car travel distance in minutes into four categories: living in an under 60-minutes-radius, living in a 61-120-minutesradius, 121-180-minutes-radius, and living above a 180-minutes-radius (see Appendix 2 for a detailed overview).
Starting again at the intensive margin, we find a significant and negative effect: Compared to participants living within the 60-minutes-radius of the sink, with increasing distance the share of contributors' decreases (see Table 2, model 4).This finding is robust when including variables that control for geographical characteristics (see Table 2, model 5).Other than distance, the regression analysis also reveals that at least one geographical characteristic has an impact on giving -that is rurality.The more rural a district is the less likely are participants to give.Forest coverage, natural spaces and recreational areas do in our sample not affect the share of contributors.Checking the given amounts conditional on contributing (see Table 3), we find no effects of distance nor of the variables controlling for geographical characteristics.However, we find robust evidence that giving increases with increasing age.We conclude that we can only partially reject our third hypothesis (H3).Averaged over both treatments, we find a correlation between giving and distance at the extensive margin.However, we do not find that distance has an effect on the given amount.

Correlation between Treatment and Distance
One explanation for the non-effect of the CBS treatment may be given through an interaction of the treatment with distance.We have seen that with increasing distance the share of contributors' decreases.Stressing the local co-benefits in CBS may interact through different channels with distance as conjectured in hypothesis 4. Figure 2 sums the mean given amount conditional on contributing and share of contributors gradually with increasing distance from the local sink differentiated by treatments.Indeed, we see that the graphs do develop differently.
For S, both the graphs follows a rather linear and decreasing trend.For CBS, the share of contributors follows a clear U-shape.The mean given amount conditional on being a contributor follows a slightly increasing, rather linear trend.This suggests that there might be an interaction between treatments and distance.However, especially in closer distance we encounter only little observations not recommending a further regression analysis.Comparing knowledge on the ETS and forests, most participants feel rather uninformed about emissions trading schemes and the EU-ETS.Contrary, the idea to use forests as carbon sink to regulate the climate is comparatively well known (see Figure 3c).Asking participants about their knowledge on the co-benefits that were stressed in CBS, we find that at least student survey participants seem to be well informed about these co-benefits (see Figure 3b).This may well be attributed to the numerous initiatives in Germany that stress the role of trees to complement CO2 mitigation.
The results from our post-survey do too coincide with our experimental finding that the public valuation of forest carbon sinks appears to be higher than the WTP for individual mitigation activities in Germany implemented through purchasing and withdrawing emissions allowances.
Asking participants for their preferred environmental donation, 48% of participants preferred donating to a forest project.Only 22% preferred mitigation via the purchase and withdrawal of emission allowances (see Figure 3d).Investigating potential reasons for such preferences, we asked participants on how much they trusted in the durability of CO2 reduction through forest projects and the purchase of emission allowances.We find a surprisingly low trust in the durability of the emission reduction through emission trading (see Figure 3a).Only 45% have 'rather great' to 'great' trust in the durability of emission mitigation through emission allowances, while 80% inhibit 'rather great' to 'great' trust in forests.We additionally replicated the donation question of our experiment hypothetically.We asked participants to imagine they would participate in a survey lasting about 20 minutes for which they would get 20 EUR.Then, participants had to state how much of their remuneration they would be willing to give for the sequestration of 100kg of CO2 within a reforestation project.This hypothetical setting produced almost identical values with a mean WTPhyp of 6.7 EUR (vs.WTPmin 6.3 EUR) and a median WTPhyp of 5 EUR (vs.WTPmin 4.6 EUR).We are therefore confident that insights from the survey are informative for a better understanding of the ex-ante beliefs in our experiment.
Potentially the high visibility and promotion of tree planting initiatives may have increased the overall acceptance of this measure compared to emission allowances, which may be perceived as rather abstract and diffuse.This is a surprising result as emissions reduction via e.g. the EU-ETS are considered as key climate policy element and reforestation/NETs only as complementary tool.
From the political/scientific perspective this focus on avoided emissions is among others driven by uncertainties on the durability of the carbon stored, i.e. the permanence of the carbon sequestered.Permanence of forest carbon sequestration can be hampered by natural disturbances, but also intentional, e.g. by earlier harvesting (Gren and Aklilu, 2016).Although carbon sequestration in a non-permanent reservoir has also benefits, its value should be lower than that of avoided emissions (Herzog et al., 2003).Thus, we observe a large discrepancy between participants' preferences and the political /scientific opinion.Most striking, our study reveals substantial differences to the prevalent literature investigating the WTP for avoiding greenhouse gas emissions through purchasing emissions allowances.For Germany, Löschel et al. (2013) report a mean WTP of 12 EUR/t CO2, Diederich and Goeschl (2014) a mean WTP of 6 EUR/t CO2 and both a (close to) zero median WTP.Assuming a linearity in the marginal WTP, we report a median WTPmin of 46 EUR/t CO2 and a mean WTPmin of 63.30 EUR/t CO2.This suggests that the public valuation of carbon sequestration via forest carbon sinks does exceed the one of emissions avoided.This impression was reinforced by the results from the post survey among students, which revealed that subjects feel better informed and have higher trust in forest measures to mitigate climate change.Knowledge on and trust in emission trading appears to be weak.This is surprising as especially emissions trading has not only been established in Europe more than a decade ago but there is also empirical evidence from 29 European countries that there is a positive relationship between trust and GHG emissions reductions (Carattini et al., 2015).Natural carbon removal, on the other hand, has only recently entered the climate dialogue.Governments react with reserve and approaches to implement NET technologies are cautious (Fridahl and Lehtveer, 2018;Scott and Geden, 2018;Geden et al., 2019).The reasons for the discrepancy between theoretically evaluated potentials and the so far missing practical implementation is that in the past forest projects had been considered as relatively risky investments.For example, forest measures are prone to the 'permanence problem' as carbon sequestration is reversible either through deforestation or natural disturbances such as droughts and fires.Other uncertainties include the amount and suitability of land to grow trees as well as land use conflicts.These complex ecological structures and an underlying developmentversus-conservation conflict make it very difficult to integrate forests measures in official activities to mitigate climate change.But forest measures can also, when well-directed, provide numerous economic, environmental, and socio-cultural benefits (Canadell and Raupach, 2008).
However, our analysis could not identify that highlighting these co-benefits lead to a further increase at the extensive or intensive margin.A likely explanation are non-observed ex-ante priors of the experiment-participants, which seems reasonable, as especially tree planting projects have become a popular measure.A similar effect is found by Baranzini et al. (2018).Their participants accounted largely for the local benefits such as local biodiversity, leading to a limited effectiveness of their local co-benefits treatment.What however matters in our study and has been reported in other studies is the effect of distance on giving.With increasing distance the likelihood to make a contribution decreases.This is an important insight for designing contribution appeals for such programs.
From a geopolitical perspective, future research requires to understand much better private giving behavior along the spatial dimension as tree-planting initiatives will occur all over the world with a focus on degraded areas to avoid land-use conflicts.For the case of Europe, the regulation on Land Use, Forestry and Agriculture has set an overall target for EU member states for carbon removals by natural sinks equivalent to 310 million tons of CO2 by 2030.This includes a plan to plant three billion trees across Europe by 2030.Additionally, the no-debit obligation requires that emissions from land use, land use-changes and forest do not exceed removals from the same sector.
Understanding the WTP for carbon forest sinks also provides an incentive to foster the development of an institutional framework that explicitly targets the question to what extent forest offsets can be perceptively integrated into the EU-ETS, thereby both acknowledging the challenging question of permanence but also the valuable local co-benefits.Currently, the discussions are predominantly shifted to the EU's Effort Sharing Regulation dealing with GHG emissions from sectors such as transport, buildings, agriculture and waste that are not covered by the EU-ETS.The Effort Sharing Regulation already allows EU member states to buy and sell net accounted removals from land use and forestry from and to other Member States.This provides incentives to increase CO2 removals beyond own commitments (European Commission, 2021).
All these efforts require enabling conditions like reliable monitoring and verification systems and have to deal with the issue of permanence of the carbon stored via natural sinks.Only then, more comprehensive carbon markets can emerge.These markets might include a trading system for carbon removals and its potential inclusion in existing emissions trading systems, like the EU ETS.
We therefore hope that our study initiates additional research on environmental donations, in particular in the context of voluntary (land based) removal activities that addresses not only cobenefits, but also the associated risks.We consider insights from such demand-side reactions to be key for successfully designing these comprehensive carbon markets.The participant should for the question imagine that she gets 20 EUR for answering a questionnaire, which takes about 20 minutes.After answering the questionnaire, you she has the opportunity to donate the remuneration.The variable indicates how much the participant would hypothetically donate for the sequestration of 100kg of CO2 within a reforestation project.

Preferred environmental donation
Indicates whether the participant would prefer an environmental donation to set aside CO2 certificates (EU ETS), or to support a reforestation project in Germany, or none.The Questionnaire The questionnaire is unrelated to the experiment.The questionnaire items can be supplied upon request.

The Treatment Text
[S and CBS: You now have the opportunity to use your remuneration to make a contribution to a climate protection project.You are completely free to decide whether and, if so, how much you wish to contribute.
The following information is intended to provide you with essential background information on the selected climate protection project.
The Paris Climate Convention aims to limit global warming to 2 -preferably 1.5 -degrees Celsius above pre-industrial levels.According to the Intergovernmental Panel on Climate Change, this requires that "net emissions" of greenhouse gases such as CO2 are rapidly reduced to zero.More precisely, zero net emissions means that the amount of greenhouse gases emitted must be at least equal to the amount of greenhouse gases removed from the atmosphere.
Carbon sinks offer an opportunity to remove CO2 from the atmosphere and thus protect the climate globally.A well-known example of a carbon sink are forests: With reforestation the carbon sequestration capacities can be enhanced.In preparation for the Bundesgartenschau (Federal Horticultural Show) in 2023, the City of Mannheim intends to unseal urban areas over the next few months and to then create an additional local carbon sink by planting trees of predominantly native species.According to the current state of planning, the City of Mannheim guarantees permanent maintenance by the municipal park department.
You now have the opportunity to support this project of the city of Mannheim.With your contribution to the reforestation project additional trees can be planted.These trees actively remove CO2 from the atmosphere and bind it over their lifetime.How quickly or how much CO2 a tree binds depends on many factors, such as the tree species, its age, soil quality and water supply.For example, experts at the Forest Centre of the University of Münster calculate that a beech needs to grow for about 80 years to absorb one ton of CO2.On average, this means a beech absorbs 100kg of CO2 in eight years.] [Only CBS: Your contribution will not only help to protect the global climate, but also creates additional habitats for animals and plants and supports local biodiversity.Besides, there are a range of other additional positive side-effects for society.Afforested areas serve as recreational and leisure areas.They improve local air quality by filtering harmful fine particles from the air, and improve the urban climate and the supply of fresh air.Especially in the summer months, reforestation can locally increase the balance of temperature and humidity extremes.] [S and CBS: Please use the slider below to indicate the contribution you would like to make to the reforestation of the tree population in Mannheim.
I would like to support the removal of 100kg CO2 from the atmosphere as part of the reforestation project with: [Slider] Of course you can also decide to contribute nothing.The remaining amount of the participation fee will be sent to you in the form of a voucher as described above.
After evaluating the data of all participants, we will inform you about the results.No individual contributions will be mentioned.] Appendix 4: Survey Questions (Translated from the German original) Page 1 In the past, participants of our studies have repeatedly asked us for the opportunity to donate part of their remuneration to projects protecting the climate and the environment.Please imagine you are a participating in such an incentivized study and answer the following questions against this background.
As you may know, there are different ways to make a contribution for the environment and climate protection.Consider the following two donation options: -Retirement of CO2 allowances under the European Emissions Trading Scheme.Once a CO2 allowance is purchased it can no longer be used for entitlement to emit CO2 on the market -A reforestation project in Germany In the context of such an environmental donation, would you generally prefer to have the opportunity to set aside CO2 certificates as part of the European Emissions Trading Scheme, or would you prefer to support a reforestation project in Germany?-How many tons of CO2 do you think an 80-year-old beech tree can sequester?[Slider] -What is the corresponding emission value of a distance traveled by a car (in km) of this value?
[Slider] -Assume that participation in the study would be remunerated with 20 EUR, for which you would have to answer a questionnaire.Answering the questionnaire takes about 20 minutes.After answering the questionnaire, you have the opportunity to donate your remuneration in parts or fully.How much would you be willing to donate for the sequestration of 100kg of CO2 within the reforestation project?
Page 4 How quickly or how much CO2 a tree can sequester depends on numerous factors, such as the type and age of the tree, the soil quality and its water supply.Accordingly, data on the CO2 sequestration potential of forests varies depending on the calculation base.Experts at the Forest Center of the University of Münster estimated that a beech must grow for about 80 years to absorb one ton of CO2.On average, this means a beech can absorb100kg of CO2 in eight years.This corresponds roughly to the emission value of a distance traveled by a car of about 550 km.
After having received this information, would you want to adjust your donation from the previous page?
□ Yes, I would like to increase the donation amount (to_____€) □ Yes, I would like to decrease the donation amount (to____€) . With the increasing recognition of the appliance of forest measures to complement low-carbon technologies as an additional path for future mitigation activities (and include them into emission trading systems), it is important to understand the markets valuation of carbon sinks in climate change mitigation.It however remains an open question whether the empirical insights on low levels for individual GHG mitigation carry over to a situation where subjects can actively contribute to CO2 removal through contributions to a local forest carbon sink.Based on the theory on the voluntary provision of public goods and the available empirical evidence, our first statistical hypothesis on the WTP to contribute to CO2 removal in the S treatment (of a forest consists of both use and non-use 4 values.Local co-benefits can be especially found in the use values, which are differentiated in direct and indirect use values.

Figure 1 :
Figure 1: Extensive and intensive margin effects

Figure
Figure 3: Survey results

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Rather purchase CO2 certificates □ Rather support reforestation project □ I would equally endorse both projects as a donation option □ I would not support either project as a donation optionPage 2Please think again about the reforestation project.With which of the following functions of the forest are you familiarTrees can absorb and bind CO2 over the course of their lives as they grow.Please imagine again that have the opportunity to support a reforestation project.

Table 2 : Regression analysis share of contributors
Distance is the car travel distance to the carbon sink measured in minutes.For the categorical variable 'Distance category' the base category is '<60 min'.See Appendix 1 for an overview and description of the independent variable and dependent variables.

Table 3 : Regression analysis on giving conditional on being a contributor
*** p < 0.001.Distance is the car travel distance to the carbon sink measured in minutes.For the categorical variable 'Distance category' the base category is '<60 min'.See Appendix 1 for an overview and description of the independent variable and dependent variables.

Table A1 . 4 :
Explanation of used independent and dependent geographical variables

Table A1 . 7 :
Explanation of survey variables

Table A2 . 1 :
Distribution of participants and share of contributors within distance categories □ No, I would not change the donation amount In the following, we are in your basic assessment regarding various climate protection measures.How much do you agree with the following statements?Finally, we would now like to know how good you feel informed about...