A new approach is proposed to account for the increasing benefits from scarce ecosystems in policy analysis. As societies become wealthier and ecosystem services become scarcer, the value of these services must rise in benefit-cost analyses. Current policy guidance assumes that ecosystem service values remain constant, leading to an underestimation of their true value. The proposed rule, based on economic theory and evidence, suggests that benefits from ecosystem services increase as real incomes grow and when ecosystem services are declining. This adjustment would correct a downward bias in current estimates and help governments better reflect the importance of ecosystems in policy decisions. Ecosystems provide both market and non-market goods and services, with non-market values often assessed using "shadow" prices. The paper highlights the need for a more accurate method to estimate future willingness-to-pay (WTP) for ecosystem services, considering both real income effects and real scarcity effects. A simple rule based on relative price changes (RPC) is proposed, which adjusts future WTPs according to the growth rates of market goods and ecosystem services. This approach would ensure that the increasing importance of ecosystems is properly reflected in policy analysis. The RPC rule is applied to adjust future WTPs for ecosystem services, taking into account the growing real income and the declining availability of ecosystem services. This adjustment would significantly increase the present value of ecosystem services over time, making long-term projects with positive effects on ecosystems more attractive. The paper also discusses the integration of this rule into policy guidance, emphasizing the need for periodic revisions as new evidence becomes available. The proposal aims to ensure that ecosystem services are treated consistently with other goods, whose values change over time.A new approach is proposed to account for the increasing benefits from scarce ecosystems in policy analysis. As societies become wealthier and ecosystem services become scarcer, the value of these services must rise in benefit-cost analyses. Current policy guidance assumes that ecosystem service values remain constant, leading to an underestimation of their true value. The proposed rule, based on economic theory and evidence, suggests that benefits from ecosystem services increase as real incomes grow and when ecosystem services are declining. This adjustment would correct a downward bias in current estimates and help governments better reflect the importance of ecosystems in policy decisions. Ecosystems provide both market and non-market goods and services, with non-market values often assessed using "shadow" prices. The paper highlights the need for a more accurate method to estimate future willingness-to-pay (WTP) for ecosystem services, considering both real income effects and real scarcity effects. A simple rule based on relative price changes (RPC) is proposed, which adjusts future WTPs according to the growth rates of market goods and ecosystem services. This approach would ensure that the increasing importance of ecosystems is properly reflected in policy analysis. The RPC rule is applied to adjust future WTPs for ecosystem services, taking into account the growing real income and the declining availability of ecosystem services. This adjustment would significantly increase the present value of ecosystem services over time, making long-term projects with positive effects on ecosystems more attractive. The paper also discusses the integration of this rule into policy guidance, emphasizing the need for periodic revisions as new evidence becomes available. The proposal aims to ensure that ecosystem services are treated consistently with other goods, whose values change over time.