CURB CO2: Carbon Uptake Revisited - Biases Corrected using Ocean Observations

When we emit carbon dioxide (CO2) to the atmosphere through industrial activity, only around half of that CO2 remains in the atmosphere, with the remainder being taken up approximately equally through photosynthesis by plants on land and being absorbed by the oceans. These anthropogenic CO2 'sinks' are essentially saving us from a large part of the global warming that we would otherwise be experiencing. New evidence suggests that our estimates of how this fraction of CO2 that stays in the atmosphere is changing, and will continue to change, may be too high, meaning that there may be more hope that we can prevent atmospheric CO2 concentrations rising too high than was previously thought.

Whilst we can estimate how much CO2 we are presently emitting, and can measure the concentration of CO2 in the atmosphere, and therefore work out how strong these sinks are (i.e. how much CO2 they are taking out of the atmosphere), we must calculate how this number will change in the future if we are to determine how much CO2 we can emit as a society without exceeding dangerous CO2 concentrations in the atmosphere. This project aims to give us a better understanding of what this future change in the fraction of CO2 staying in the atmosphere is, by correcting a bias we have identified in the models we use to make these projections.

We make projections of how the land and ocean CO2 sinks may change in the future using increasingly comprehensive Earth System Models (which are climate modes which also contain a representation of additional processes such as the carbon cycle). While these models are the best possible tools we have to simulate future climate change, they are still far from perfect. We have shown that in the North Atlantic, which is the most intense ocean CO2 sink, these models underestimate how quickly the CO2 absorption is increasing, and have identified what the models are doing wrong. This project will extend this work from the North Atlantic to the full ocean, and by correcting for the biases that cause the models to under-predict this change, produce new and improved future estimates of ocean CO2 absorption.

One we have our improved estimates of future changes in the strength of the ocean carbon sink, we will account for how the land CO2 sink responds to this, and produce a set of new scenarios describing how much CO2 can be emitted through human activity through time if we are not to exceed the atmospheric CO2 concentrations linked to global warming of 1.5 to 2 degrees C above preindustrial temperatures.

The overarching aim of this project is to provide UK and international governments with the best possible impartial information from which they can plan how best to work towards the global warming targets (the 'Paris Agreement') set at the Paris Climate Conference in December 2015.

The necessarily tight timescale of delivery of this call's outputs means that successful impact can only reliably be achieved through established communication pathways. To this end, CURB CO2 outputs will be routed through the Met Office and AVOID 2 programme to the Committee on Climate Change (CCC), department for Business, Energy and Industrial Strategy (BEIS), as well as other stakeholders that can be accessed through existing Met Office and AVOID2 networks. The results will also be discussed within the Met Office to investigate how they might contribute to the UKCP18 project, which will provide new UK climate scenarios. Subcontractor Jason Lowe, as Head of Climate Services for Government at the Met Office and AVOID2 Chief Scientist, is optimally placed to facilitate this communication.

A central focus of our impact plan will be to produce, publish and communicate key figures placing the new results alongside existing literature synthesis. This is the approach preferred by the CCC, and primary pathway for influencing the IPCC Special Report on the impacts of global warming of 1.5 degrees C. These figures will be delivered to the CCC and BEIS through Met Office briefing documents, AVOID2 reports and personal briefings, but also rapidly published within pre-review 'discussions' papers in the open access journal Earth System Science Data discussions (or similar).

Following the initial publication and reporting of the first-level outputs, we will produce more detailed science-focused publications aimed at high impact factor journals. These papers will explore the missing mechanisms in the CMIP5 model simulations, the impacts of the anticipated additional CO2 uptake on the oceans, and the implications of the modified Airborne Fraction on the effectiveness of BECCS.

Rather than holding a project-specific end-of project meeting, the dissemination of key results arising from this project will be aligned with the final meeting of the EU FP7 Project HELIX (High-End cLimate Impacts and eXtremes - http://helixclimate.eu/home), which will be held in Exeter in Autumn 2017. Key stakeholders will be present at this meeting, and a side-event will be held to make them aware of the project outcomes.

The final component of the impact plan will be to proactively approach the authors of the IPCC Special Report on the impacts of global warming of 1.5 degrees C, making them aware of the results and keeping them up to date with the progress of publications. Whist it is not yet known who the authors on this report will be, it is anticipated that through the project team, AVOID2 and/or HELIX projects we will have, or will be able to obtain access to these individuals.