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.

Potential Impact:
We will maximise the impact of our research, linked to the two key impact elements of the funding call, through:

(1) Input to the IPCC special report on the 1.5 degrees C target: Our work will be completed in time for the anticipated IPCC special report submission deadline of late 2017.

Our analysis will provide a novel class of future scenario, Adjustable Pathways, for global emissions that will be needed to achieve the global political agreed warming targets over the 21st century. These Adjustable Pathways will set rules on how global emissions pathways can be adjusted in response to climate observations to meet agreed warming targets. We will thus provide analysis on how emissions pathways will need to be adjusted depending on the future evolutions of temperature and anthropogenic carbon uptake.

We will provide analysis of the compatible emissions ranges required to reach 1.5 degrees C and 2.0 degrees C warming targets, stating how these emission ranges to 2100 and beyond correlate with observations of climate over the early 21st century. For example, in a trial ensemble following an Adjustable Pathway to 1.5 degrees C warming, cumulative emissions at 2100 correlates inversely with the additional warming per unit carbon emitted between year 2000 and year 2030 (See Fig. 2, science case).

We will provide analysis on the relative physical consequences of achieving 1.5 degrees C warming versus achieving 2.0 degrees C warming for:
(1) Sea-level rise and the return period of coastal extreme sea-level events; and
(2) Ocean pH changes.

The above analysis will be backed up by a very large ensemble of climate simulations that are observationally constrained to the present day, and agree with the projection ranges for warming and sea-level rise from the CMIP5 climate ensemble up to 2100 (see figure 1, track record).

Our innovative climate model (described in Goodwin, 2016) is already configured to perform the experiments, and is computationally fast to ensure that the analysis is conducted on time for the IPCC special report deadline of late 2017.

Studying impacts and costs of sea-level rise will indicate the potential of avoided damages due to 1.5 degrees C, 2.0 degrees C and 2.4 degrees C warming targets, which we be useful for policy makers and strategic planners (e.g. World Bank, European Commission, national governments). Given the commitment to sea-level rise which will continue much longer than temperature rise it will inform adaptation planning needs as well. We will increase our impact by liaising with German and UK colleagues to improve the DIVA model and submit high-quality publications and attend conferences. Where appropriate, we will also link with and communicate findings to existing projects at UoS (e.g. DECCMA which analyses climate change mitigation and adaptation in delta regions), to strength and improve our outputs, so that they can be used in an international field.

(2) Evidence to the UK Committee on Climate Change: We will use our Adjustable Pathway analysis to generate evidence for the UK Committee on Climate Change concerning the emissions ranges required to achieve the 1.5 degrees C target, and emissions ranges required if the target extends to 2.0 degrees C. Our advice will constitute:
(1) Total global cumulative emission ranges compatible with 1.5, 2.0 and 2.4 degrees C targets;
(2) Ranges for the global emission pathways compatible with 1.5, 2.0 and 2.4 degrees C, showing how quickly the emissions reductions must occur; and
(3) Analysis on how adjustable emissions pathways must be, to account for the uncertainty in the maximum cumulative carbon emission compatible with the agreed warming targets.

We will also undertake additional activities to ensure that the research from our project leads to wider impact through outreach, linking with the CO2 Modeller climate app project, which allows the public to run a climate model on their smartphones, lead by PI (Goodwin).