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Efforts to avert dangerous climate change by conserving and restoring natural habitats are hampered by concerns over the credibility of methods used to quantify their long-term impacts. Here we develop a flexible framework for estimating the net social benefit of impermanent nature-based interventions that integrates three substantial advances: (1) conceptualizing the permanence of a project’s impact as its additionality over time; (2) risk-averse estimation of the social cost of future reversals of carbon gains; and (3) post-credit monitoring to correct errors in deliberately pessimistic release forecasts. Our framework generates incentives for safeguarding already credited carbon while enabling would-be investors to make like-for-like comparisons of diverse carbon projects. Preliminary analyses suggest nature-derived credits may be competitively priced even after adjusting for impermanence.

It is unclear how much carbon should be stored in temporary and risky offsets to compensate one ton of CO2emissions. Here we cast the social value of an offset (SVO), measured in terms of economic damages avoided, as a well-defined fraction of the social cost of carbon reflecting offset duration, and risks of non-additionality and failure. The SVO reflects the value of temporary storage, and overcomes shortcomings in the climate science and economics of previous contributions1–4. The SVO is policy relevant. An efficient net-zero policy will consist of offsets if their SVO/cost ratio exceeds the benefit/cost ratio of alternatives. The SVO yields an indicator of the equivalence of offsets to permanent carbon storage measured by the ratio of the SVO to the social cost of carbon. We provide a matrix of equivalence factors for different risks, permanence and climate scenarios. Estimation yields a rule of thumb: one offset sequestering one ton for 50 years is equivalent to between 0.33 and 0.5 tons permanently locked away. Equivalence offers a means of replacing perpetual offset contracts by simpler, easy to monitor short-term contracts, has applications to carbon life cycle analysis5and the valuation of carbon debts6, and can be the basis of comparing offsets of different qualities in the voluntary and compliance markets.

We show that economic models of climate change produce climate dynamics inconsistent with current climate science models: (i) the delay between CO2emissions and warming is much too long and (ii) positive carbon cycle feedbacks are mostly absent. These inconsistencies lead to biased economic policy advice. Controlling for how the economy is represented, different climate models result in significantly different optimal CO2emissions. A long delay between emissions and warming leads to optimal carbon prices that are too low and attaches too much importance to the discount rate. Similarly we find that omitting positive carbon cycle feedbacks leads to optimal carbon prices that are too low. We conclude that it is important for policy purposes to bring economic models in line with the state of the art in climate science, and we make practical suggestions for how to do so.