ADJUST1.5: Defining ADJUSTable (emission) pathways to 1.5 degrees C warming, and assessing their feasibility, physical consequences and impacts.

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There is considerable uncertainty in the emissions pathway required to achieve the 1.5 degrees C warming target from the COP21 climate agreement. For example, the latest United Nations Intergovernmental Panel on Climate Change report indicated that 1.5 degrees C is compatible with cumulative carbon emissions of somewhere between 500 and 1500 Gigatonnes of carbon. To restrict warming to 1.5 degrees C, should we aim for net cumulative emissions of 500 GtC or of 1500 GtC? We do not yet have the required knowledge of the climate and carbon systems to decide. In fact, if a warming target of 1.5 degrees C is to be achieved, there has to be scope to adjust our future emissions plans over the 21st century and beyond in response to our evolving knowledge of the climate and carbon systems. This project will define and evaluate Adjustable Pathways to 1.5 degrees C, in which the emissions pathway responds to climate observations over the 21st century to maximise the likelihood of delivering the final warming target. The Adjustable Pathways will be tested using millions of climate simulations from an innovative fast climate model whose ensemble members mimic the warming response range of the state-of-the-art CMIP5 models. We will assess how best to formulate the Adjustable Pathway to 1.5 degrees C warming, considering: (1) The initial rate of emissions reductions; (2) The time-interval between observational re-assessment of the emissions pathway; (3) The maximum rate of change of emissions reductions due to observational re-assessment; and (4) The existence of observational triggers that cause immediate re-assessment of the emissions pathway regardless of the time-interval.We will then assess how the feasibility of Adjustable Pathways to 1.5 degrees C depends on the eventual value of the Transient Climate Response to Emission (TCRE) at 2100, and the future potential evolution of carbon capture and storage technology. We will assess the relative physical consequences of 1.5, 2.0 degrees C and 2.4 degrees C warming for sea level rise and the frequency of extreme sea level events. We will also assess impacts and costs of sea level rise 1.5 degrees C, 2.0 degrees C and 2.4 degrees C warming over the 21st century and beyond to year 2500, focusing on vulnerable, low-lying regions.The unique aspect of this project that enables Adjustable Pathways to be evaluated is the ability to perform millions of climate simulations that reach the specific policy-driven warming targets. Conventional climate simulations use identical emissions pathways, but simulate a wide range of warming responses due to the uncertainty in the climate response. Here, we utilise a fast climate model that has recently developed by the research team, and mimics the range of climate responses shown by conventional models. This fast climate model can be configured to generate climate simulations that all lead to the same warming target, but cover a wide range of emissions pathways due to the uncertainty in the climate response.