5. Result

The below presented Table 2 summarises the assessment of various valuation studies used for cost benefit analyses in the identified nature-based solutions in this report. They were all chosen for their applicability to specific Nordic contexts, including Oslo, Copenhagen, Åland, and Getinge. We took extra care in deciding on the suitability score where three experts assessed it independently. Suitability scores were evaluated based on their relevance to the given locations, cost-based valuation, and the quality of indicators used. Overall, the method of assigning suitability scores to identified studies on monetised values enhances the importance of context-specific applicability and methodological robustness when transferring valuation frameworks to Nordic environmental and urban settings.

Key findings include high suitability scores for studies addressing urban heat islands (Huang et al. 2013) and combined sewage overflow reduction (Karras & Read 2016), both scoring six due to their alignment with local priorities and robust methodologies. Studies of the purification of nitrogen and phosphorus (Söderqvist & Wallström 2017) also scored six, showcasing strong relevance to Åland and Getinge's environmental goals. Similarly, Christie & Rayment (2012), which evaluates educational and recreational ecosystem services, also achieved a score of 6, reflecting its methodological robustness and relevance to the Nordic context. In contrast, studies with limited geographical or thematic overlap, such as increased Biodiversity in wetlands (Meyerhoff et al. 2012), and microplastic filtering (Beaumont et al. 2019), received lower scores of four, indicating increased uncertainty for their value transfer to the Nordic contexts.

Table 2 Correction factors for value transfer between chosen research papers and the six NBS.

Table 3 illustrates the process of adjusting valuation data from chosen studies to reflect the use of purchasing power parity (PPP)-corrected exchange rates and inflation adjustment to convert the estimates to 2023-euros, ensuring comparability and currency standardisation across studies. The columns labelled "Low" and "High" represent the original valuation ranges reported in the studies.

To account for differences in currency value and economic conditions, these original values were converted using PPP-corrected exchange rates, as shown in the PPP € Low and PPP € High columns. Subsequently, these PPP-adjusted values were updated to 2023 prices, reflecting inflation, in the €₂₀₂₃ Low and €₂₀₂₃ High columns. This adjustment ensures that all values are comparable in current economic terms, facilitating meaningful cross-study analysis.

Table 3 Temporal and Spatial adjustments from research papers to the Nordics 2023

The adjusted values from the table, standardised for PPP and inflation, are applied to the identified nature-based solutions (NBS) to estimate the benefits derived from externalities.

This application allows for a consistent and robust calculation of the externalities’ monetary value, ensuring comparability across different NBS projects and providing a clear basis for assessing their cost-effectiveness and contribution to sustainable development goals.

In the rest of the result chapter, we discuss our findings for each of the identified nature-based solutions. The NPV calculations are made on 2, 3, and 4 percent discount rate, and with a time horizon of 70 years. This horizon might seem a bit long for some of the solutions, and a bit short for others, but for comparison we mean its beneficial to use the same time horizon for all our analysis. In the following tables we present the high and low values for all estimations, we also present how the use of 2%, 3%, or 4% discount rate affects the calculated net present value.

For the wetland Nabben outside of Mariehamn at Åland, Table 4 shows the monetised values for identified positive externalities. The net present values for purifications at a 3 percent discount rate are between € 0.98–1,2 M for Nitrogen, € 1,2–1.3 M for phosphorous, and purifying plastics contribute with € 75–750 k. The wetlands cultural support is estimated to be in the span of €36–60 k. Also, the two eco system services Habitat Provision and Carbon sequestering contributes with respectively €190–720 k and €5–170 k to the total monetised value for positive externalities of between €2.5–4.2 M. For the lower discount rate of two percent, the corresponding interval is €3.1–5.4 M, and for a four percent rate the sum is between €2.0 and 3.4 M. The summarised value of all identified positive externalities could be added to the value of protection against sea level rising and storm surges to then be compared with the cost of constructing and maintaining the wetland in a complete cost-benefit analysis.

Table 4 NPV for Nabben wetland

Net present values for the positive externalities for the constructed flood protection in Getinge in Sweden is shown in Table 5. Here the purification of Nitrogen and phosphorous has the highest impact which sums up to between €19 M and €22 M at a 3 percent discount rate. This benefit is remarkably high, and if the purification benefit should be included in the original cost-benefit analysis, the expected amount of purified nitrogen and phosphorous would need to be verified. Further we see that the wetlands habitat provision contributes with between €1.0–3.8 M, while the cultural values are being estimated to be in between €190 k and €280 k for the seventy-year timespan. The wetlands carbon sequestration contributes with between €28 k up to €930 k to the total sum of €20–28M. The corresponding intervals for 2% and 4% discount rate are €26–35 M, and €16–22 M. These numbers should be compared to the calculated surplus of about M€1 over the 83-year time span at a discount-rate of 1.4 percent, based on production, maintenance, and protection against floodings.

Table 5 NPV for flood protection in Getinge

Table 6 that contains the net present values for benefits from positive externalities at the retention basin in Nesbyen, indicates that at a 3 percent discount rate the cultural value over the coming 70 years is estimated to be between 5.5 M€ and 6.5 M€, while the habitat provision contributes to the span k€64–240. This sums up, at a 3.5 discount rate, from M€5.6 to M€6.7. For the two optional discount rates the corresponding spans are M€7.1–8.6 for 2% and M€4.5–5.5 for 4%. Since the recreational value for this NBS is high we would recommend further willingness to pay studies among the people living nearby the basin, before this number is included in a cost benefit analysis.

Table 6 NPV for retention basin in Nesbyen

The results for the analysis on Hovin Stream in Oslo are displayed in Table 7. Like the retention basin in Nesbyen, we also here observe a high cultural value for the people living nearby the stream daylighting (close to M€100 at a 3 percent discount rate). This value might seem extremely high but considering 50 000 inhabitants benefit from the increased cultural value for a time span of 70 year, the annual benefit per capita is not particularly high. The daylighting is also estimated, again at a 3 percent discount-rate, to support water quality in the range of k€100–260 and habitat provision with approximately M€1.3. The Stream Daylighting at Hovin thereby is estimated to contribute with 98 M€ to the society. For the two optional discount rates the sums are estimated to M€125 at 2%, and M€80 at 4%.

Table 7 NPV for daylighting in Hovin Stream, Oslo

The estimated net present values from Karens Minde are, as seen in Table 8, also driven by the recreational values for the 8,800 inhabitants living nearby the 3.7-hectare park. At a 3 percent discount rate this value contributes with M€28 over the 70-year time span, while Habitat provision contributes with k€520, and increased water quality by between €1,000 and 3,000. The total net present value from positive externalities at Karnes Minde thereby sums up to M€29 at a 3% discount rate. With M€37 at 2%, and M€24 at 4%.

Table 8 NPV for climate adaption at Karens Minde, Copenhagen

Table 9, that represents the net present value calculations, at a 3% discount rate, for Skt. Kjelds neighbourhood in Copenhagen, indicates that the Greening supports the habitat provision with k€140 and that the recreational value for the 19,000 inhabitants living nearby sums up to M€10 over the next seventy years. Again, the recreational value is driving the net present value, and the contribution of water quality of k€2–5 has only a minor effect on the total NPV at a 3% discount rate of M€10–11. For the lower discount rate at 2%, the NPV sums up to M€13–14, while span for the higher rate at 4% comes to the interval M€8.3–8.9.

Table 9 NPV for climate adaption at Skt. Kjelds, Copenhagen