Ecosystem protection payments pay off Trees take up a lot of CO 2 , so one approach to reducing the rate of increase in atmospheric CO 2 levels is to reduce the cutting down of trees. Jayachandran et al. evaluated a program in which forest owners in Uganda were paid to not cut down their trees. Encouragingly, payments did reduce deforestation, and owners did not compensate by cutting down trees in neighboring forests. Furthermore, even in a scenario where cutting resumed after payments ceased, the beneficial delay in CO 2 release from cut trees, as quantified by the social cost of carbon, outweighed the monetary cost. Science, this issue p. 267

Abstract We evaluated a program of payments for ecosystem services in Uganda that offered forest-owning households annual payments of 70,000 Ugandan shillings per hectare if they conserved their forest. The program was implemented as a randomized controlled trial in 121 villages, 60 of which received the program for 2 years. The primary outcome was the change in land area covered by trees, measured by classifying high-resolution satellite imagery. We found that tree cover declined by 4.2% during the study period in treatment villages, compared to 9.1% in control villages. We found no evidence that enrollees shifted their deforestation to nearby land. We valued the delayed carbon dioxide emissions and found that this program benefit is 2.4 times as large as the program costs.

Land-use change—mostly deforestation—was responsible for 9% of global anthropogenic carbon emissions between 2006 and 2015, making it the second largest source of carbon emissions after fossil fuel combustion (1). Trees absorb carbon dioxide (CO 2 ) through photosynthesis and store the carbon in their biomass. When a tree is cut down, it stops absorbing CO 2 , and, as it decomposes or is burned, its store of carbon is released into the atmosphere.

Curbing deforestation in low-income countries, where most deforestation occurs today, is viewed as one of the most cost-effective ways to reduce global CO 2 emissions (2, 3). The reasoning is that the cost to intensify agriculture and adopt nonwood fuels and construction material in poor countries to meet growing consumption needs without clearing forests is much lower than the cost of many of the technological changes in high-income countries that reduce emissions by the same amount. Thus, having rich countries finance forest conservation projects in poor countries is a promising way to address climate change.

Indeed, the United Nations' REDD+ (Reducing Emissions from Deforestation and Forest Degradation) mechanism was created to financially reward developing countries for preservation of forests. The Paris Agreement negotiated in 2015 bolstered the role of REDD+ in international climate policy (4, 5). To put REDD+ into action, one needs to identify effective on-the-ground interventions that reduce deforestation. This Research Article evaluates a promising and popular type of intervention, namely, financial incentives for forest owners to keep their forest intact. This type of policy is called Payments for Ecosystem Services (PES): Payments are made conditional on voluntary proenvironment behaviors (6–8). PES is the environmental version of conditional cash transfers, a policy instrument used widely in developing countries to incentivize families to invest in child health and education.

Despite the widespread use and growing popularity of PES, its effectiveness and cost-effectiveness are open questions. One key concern is what the PES community calls "additionality" and economists refer to as "inframarginality": Absent the payments, some participants would have engaged in the incentivized behavior anyway. If such individuals represent many of the enrollees, then the program will generate very little gain in forest cover per dollar spent. A second concern is that individuals will simply shift their tree-cutting from land covered by the PES contract to other nearby land ("leakage").

This study was a randomized evaluation of a PES intervention that was implemented in order to measure the causal impacts on forest cover. The PES program offered private owners of forest in western Uganda payments if they refrained from clearing trees. The program was designed and implemented by a local conservation nonprofit, Chimpanzee Sanctuary and Wildlife Conservation Trust (CSWCT) (9). The study was carried out in 121 villages with private forest owners (PFOs); we randomly selected 60 of the villages to be in the treatment group. In treatment villages, the PES program was marketed to PFOs, and they were eligible to enroll. To participate, PFOs had to enroll all of their primary forest (10). During the 2-year program from 2011 to 2013, enrollees who complied with the contract received 70,000 Ugandan shillings (UGX), or $28 in 2012 U.S. dollars, per hectare of forest per year, paid out in cash at the end of each year. CSWCT employed forest monitors who conducted spot checks of enrollees' land to check for recent tree-clearing. The program also offered additional payments in exchange