Assessing Climate-Human Feedback Using a Coupled Earth-Human System Model

Current future projections often adopt a sequential approach: developing socioeconomic scenarios through Integrated Assessment Models (IAMs), running climate models based on these scenarios, and subsequently applying the outputs to impact assessment models. However, this framework overlooks feedbacks between the climate and human systems, particularly how climate impacts influence human activities, which in turn affect Carbon dioxide (CO ₂ ) and other emissions and further alter the climate. To address this limitation, a loosely coupled Earth-human system model was developed to quantitatively evaluate such feedbacks. The model incorporates key interaction processes, including changes in labor productivity, agricultural productivity, and energy demand for heating and cooling in response to climate change. These processes were parameterized using established formulations from previous studies. CO ₂ emissions are endogenously determined by a computable general equilibrium (CGE) model that accounts for feedback effects. For other radiative forcings, the SSP3-7.0 scenario with low near-term climate forcers (NTCF) was applied. Under a business-as-usual (BAU) setting, the CO ₂ emissions projected by the CGE model closely matched the SSP3-7.0 trajectory. The results indicate that all three feedback processes contributed to atmospheric CO ₂ concentration by 2100, with changes of approximately 20-30 ppm. Among the processes considered, only the labor productivity feedback reduced CO ₂ concentrations, whereas the other two processes resulted in increases. Overall, the total feedback effect was estimated to be about one-quarter the magnitude of the natural Earth system feedback and, in contrast to the previous study, exhibited the same directional effect. Further investigation is warranted to assess the cumulative effects of these feedback mechanisms, the uncertainties associated with each process, and the contribution of non-CO ₂ greenhouse gases.