Periodic Reporting for period 3 - Couplet (Transient climate change in the coupled atmosphere--ocean system) Reporting period: 2021-10-01 to 2023-03-31 Summary of the context and overall objectives of the project Worldwide climate change of growing magnitude is expected during coming decades because of global emissions of carbon dioxide and other gases. The severity of impacts of many aspects of projected change is expected to be greater for larger increase in global mean surface temperature. Although computer climate models have hugely improved over the past several decades, knowledge has grown and confidence increased, it is still very uncertain how much global warming will result for a given amount of extra greenhouse-gas heating. This uncertain quantity is called the climate sensitivity parameter. The uncertainty is a serious problem for formulating national and international plans for adaptation to and mitigation of climate change. Research in recent years has revealed that, as well as being uncertain, the climate feedback parameter is not constant. Its inconstancy compounds the difficulty of evaluating and using it.The hypothesis of this project is that the variations of the climate sensitivity parameter reflect inadequacies in scientific understanding of the global energy balance, in particular its neglect of geographical patterns of temperature change. The objective of the project is to develop a new framework for describing the variations of the coupled atmosphere--ocean climate system, taking into account the influences on and the effects of the geographical patterns, and to apply this framework to the analysis of historical and simulated climate change, in order to set refined constraints on the processes, pattern and magnitude of future climate change. Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far In the period covered by this report (the first 30 months of the project) we have carried out and begun work on several aspects of the problem.* We have demonstrated that the climate sensitivity parameter varied over the last 100 years by a factor of two on multidecadal timescales both in reality and in climate model simulations, but the models are quite unrealistic in the timing of the variation. In the models, sensitivity reached its maximum in recent decades, whereas in same period it was at its minimum in the real world, due to an unusual pattern of temperature trends in the Pacific Ocean. * We have found that this pattern is likely to have been produced in reality by a delayed response to explosive volcanic eruptions. This pattern of response is largely absent in the models.* We have examined how the climate sensitivity parameter increases as time passes under constant elevated CO2 because of the way in which the pattern of surface temperature change evolves, and shown that it is greater for higher CO2 concentration.* We have proposed a refined model of the role of climate feedback in the global energy balance, in which the climate sensitivity parameter depends on the change in the vertical profile of atmospheric temperature. This dependence accounts for the effect of the geographical pattern of SST change.* Through comparative analysis of results from a set of model experiments with increasing CO2, we have found conceptual deficiencies in the paradigm which has been commonly used in the last decade for understanding the role of ocean heat uptake in the global energy balance, and we suggest an improved interpretation. Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far) The results outlined have been obtained through analysis, using some novel methods, of climate model experiments, some which we conducted ourselves with new designs, and others that had already been carried out by groups around the world, as part of the Coupled Model Intercomparison Project. There are some emerging connections among these results, and we expect to relate these various lines of investigation through developing the new framework in the second half of the project.