Climate Impacts in Scenarios: Time to Close the Loop?

Abstract Integrated modeling of Earth and human systems, accounting for feedbacks, is key to fully understand climate change consequences and ensuing adaptation needs. In some aspects of climate research, however, closing this loop has proved particularly challenging. A primary example is the generation and use of earth system model (ESM) simulations. Integrated assessment models (IAMs) are used to project socio‐economic activities, emissions and land‐use change. ESM projections, driven by these scenarios, are then used by impact models. According to this modeling chain, however, those impacts do not affect the emissions and land use driving ESMs. Whether this feedback is large enough to warrant explicitly accounting for, needs addressing. Two consequential possibilities, discussed in the literature, are that emissions and land‐use scenarios representing the high and low ends of the plausible range might be too extreme: the high‐end scenarios missing damaging impacts, which could reduce economic activity, therefore emissions; the low‐end scenario ignoring climate feedbacks that make nature‐based carbon removal less effective and hamper‐ the assumed mitigation. In this piece, we describe the challenges that implementing such feedbacks would face, while arguing that recent developments in impact research, data, human and Earth system modeling and emulation make the time ripe for a structured model intercomparison project (MIP). MIPs have benefitted climate modeling for decades. An IAM MIP focused on integrating impacts in emission and land‐use change scenarios could enable testing these feedbacks' implications and assessing whether closing the loop would significantly change our outlook on future climate changes and their consequences.