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Voluntary carbon offset markets from projects in the tropics are expected to contribute to meeting net-zero climate change objectives in the Global North. In voluntary markets, certified third-party agencies sell carbon credits to buyers who aim to reduce their carbon footprint beyond levels legally required either by national domestic legislation or international commitments1.
Recently, there has been a surge in voluntary carbon credits, with the market value rising from US$473 million (all $ values in US dollars henceforth) in 2020 to edging close to $2 billion at the end of 20222. The value of these offsets is expected to increase at least 100-fold by 2050, as industries and governments aim to meet the 1.5 °C Paris target3. In response to this surging demand, a new offset industry has emerged in which numerous entities develop carbon offset projects and seek their certification from private organizations through verification processes, after which myriad consultancy companies rate their quality and sell the offsets to buyers.
Within the overall voluntary carbon credit market scene, REDD+ offsets have become the leading category, constituting 40% of the market. More importantly, in 2021 the market showed a 166% annual increase in the volume of traded carbon credits coming specifically from REDD+ projects that avoid unplanned deforestation and a 972% increase in programmes that avoid planned (legal) deforestation8, signalling the dynamism of forestry-based credits.
Despite this context of ''REDD+ project-euphoria'', the scientific literature assessing the impacts of voluntary REDD+ offsets remains notably scant9. Carbon credit verification agencies do include monitoring and evaluation as part of their processes to renew voluntary carbon credits. However, the objectivity, transparency and robustness of these assessments have been called into question, undermining the credibility and viability of the voluntary offset market10. This controversy11 has increased the calls within the scientific and policy communities for more independent and rigorous assessments of REDD+ projects9,12,13,14,15.
Supplementary Information A and Supplementary Table 1 summarize published work that uses BACI approaches to assess the environmental effectiveness and livelihood impacts of REDD+ initiatives. The majority of studies focus on non-certified sub-national REDD+ initiatives, or projects that were in the early stages of development (pilot schemes)18,20,21,22,23,24,25,26. The overall picture that emerges from this work on mostly non-certified REDD+ initiatives is that they have had a rather muted impact on deforestation.
Remarkably, just four studies in our review, focus exclusively on actually certified REDD+ projects10,27,28,29. These studies focus on assessing deforestation impacts only and do not assess the livelihood impacts or behavioural mechanisms that could explain any changes in deforestation. They tend to rely on remote sensing data rather than field survey data. Where survey data are included, they tend to be ''ex-post'' rather than collected ''before and after'' in recipient/non-recipient control (that is, comparable) groups. A few studies find mildly positive or at least no negative impacts on welfare and livelihood indicators21,23,30.
In sum, the available body of evidence still leaves many under-researched issues concerning the environmental and economic impacts of voluntary carbon offset schemes. In this paper, we contribute to the need to build the evidence base by reporting on a study that evaluates the impacts of an actual certified voluntary REDD+ project. In particular, we contribute a BACI assessment of the impacts of such a project 5 yr after its commencement and consider the causal pathways through which REDD+ operates. Notably, we also add a cost-to-carbon analysis.
Each pixel shows whether any deforestation took place from 2001 until 2018. The dashed line shows the 4 km buffer zone in which the REDD+ programme took place. Source: ref. 42.
a, Total forest loss from 2001 to 2018 in 8 PAs of Sierra Leone and the average for Sierra Leone. b, Total forest loss from 2001 to 2018 in the 4-km buffer zones of these PAs. The PAs shown are the GRNP, Outamba, Loma Mountains, Western Area Peninsula, Kangari Hills, Tingi Hills, Kambui Hills and Tiwai Island (see Supplementary Fig. 1 for separate graphs for each PA). The break in the lines in 2013 denotes the launch of a new satellite (Landsat 8) resulting in more precise measures of forest loss. Source: ref. 42/UMD/Google/USGS/NASA.
Total forest loss from 2001 to 2018 in REDD+ vs non-REDD+ villages. The village polygons were estimated using population-weighted Voronoi estimations. Data are presented as mean village-level values and shaded areas denote 95% confidence intervals. The vertical black line indicates the start of REDD+. The break in the lines in 2013 denotes the launch of a new satellite (Landsat 8) resulting in more precise measures of forest loss. Source: ref. 42/UMD/Google/USGS/NASA.
To formally test the impact of REDD+ in the GRNP buffer zone, we use a difference-in-difference regression analysis to assess the change in trends over time (Table 1). We find that the REDD+ programme reduced (but not reversed) deforestation in the REDD+ communities by ~1 percentage point (or 30%) compared with non-REDD+ communities. Hence, while the programme reduced the amount of deforestation by ~929 ha yr−1 in the buffer zone, it did not remove pressure on forests completely. Our results are robust to the use of different datasets, using matching combined with difference-in-difference estimates, and alternative definitions of the treatment and control samples (described in detail in ''Robustness analysis'' and in Supplementary Information F).
To benchmark these changes in deforestation, we perform a cost-to-carbon analysis. The REDD+ project led to ~340,000 tCO2 in avoided emissions per year, with an estimated cost per averted tCO2 of $1.12. We further place this calculation into perspective in ''Discussion'' (full details of the calculations can be found in Supplementary Information G).
To measure how economic wellbeing and conservation attitudes in the REDD+ buffer zone communities were impacted by the project, we use detailed primary data from household surveys of N = 841 households collected before (2014) and 5 yr after the programme started (2019). We find an overall increase of 0.222 s.d. in the economic wellbeing index over the 5 yr of the programme, which comprises a substantial and significant improvement (see Column 2 of Table 1). However, this increase cannot be attributed to REDD+ as there is no difference between REDD+ and non-REDD+ communities (the coefficient for the difference is small at 0.022 s.d.).
We also find no evidence that conservation attitudes changed due to the programme (see Table 1, Column 3). Between the survey waves, the index for pro-conservation attitudes lowered substantially in both types of villages, by ~0.226 s.d. Although the attitudinal index is an outcome variable on its own right, it can also be viewed as a mechanism driving the impact of deforestation. We find no evidence of such a causal mechanism at work, so we explore other mechanisms in the next section.
For results on each survey indicator, as specified in our pre-analysis plan, refer to Supplementary Information E presenting standardized outcomes in Supplementary Tables 10 and 11 and unstandardized outcomes in Tables 12 and 13. We also include a series of secondary outcomes (consisting of alternative wealth measures) in Supplementary Tables 14 and 15. Across all of these tables, the interaction term is never large or significant. Our results are also robust to using alternative quasi-experimental methods (described in detail in ''Robustness analysis'' and in Supplementary Information F).
It is important to emphasize that while the Gola REDD+ project has not improved local economic wellbeing or conservation attitudes, we can equally conclude that the project has not resulted in any economic harm or undermining of pro-conservation sentiments. Such ''no harm'' is a vital feature for the viability of REDD+ projects that cannot be overestimated19. More importantly, our finding has enhanced significance in the associated literature, as it stems from a more robust methodological approach.
We next explore other mechanisms that could explain the observed reduction in deforestation rates. Earlier studies have pointed to changes in the local labour allocation as a key factor affecting traditional small-scale agriculture in Sierra Leone5,36,37. Using the same household survey data collected before and after the intervention from both treated and control communities, we explore several such possible mechanisms in Table 2.
We first assess an index of labour availability for the three main types of farms (upland, swampland and plantation). In REDD+ communities, there is a sharp reduction (0.545 s.d.) in access to farm labour (Column 1). In real terms, this reduction translates to a change in labour access of 0.534 on a scale from 0 to 3, where 3 indicates high labour access. This suggests that high labour demand for the activities supported through the REDD+ intervention reduces the amount of labour available for land clearing activities.
Because the REDD+ programme aimed to provide more ecologically sustainable (or forest-friendly) alternative income sources, we examine trends in such sources. We explore income from the sale of non-timber forest products (NTFP). NTFPs are collected in forested areas (including within the GRNP). The activity is encouraged by the GRC, as it is non-invasive and creates incentives for protecting the national park. We find a substantial increase in NTFP incomes of 0.343 s.d. in REDD+ communities in the later period (Column 3). This translates to a 56.7% increase in NTFP income attributable to the REDD+ programme. In contrast, non-REDD+ communities exhibited no statistically significant change in their NTFP income and had a baseline value of ~108.400 Leones ($25).
Finally, we explore whether farmers switched to other crops, such as cocoa. In this context, cocoa is deemed more forest-friendly by the project developers, as cocoa is produced within forests. We find an increase in cocoa harvest size (0.196 s.d., Column 4) in REDD+ communities in the later period, although this is measured with substantial noise and the change is not statistically significant.
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