Our object is to understand how large, and how variable, are sources and sinks of greenhouse gases to the atmosphere from the North Atlantic. We aim to be able to describe how these have changed in the recent past and how they will change in the future under different climate scenarios. Most effort will be concentrated on carbon dioxide, and we will deliver a comprehensive budgeting of natural and anthropogenic components of the carbon cycle in the North Atlantic and understanding of why the air-sea fluxes of CO2 vary regionally, seasonally and multi-annually. Observations of CH4 and N2O and estimates of their regional fluxes will additionally be made. We, in collaboration with our partner institutions in Europe and the US, will undertake surface measurements of CO2 air-sea fluxes made from networks of voluntary observing ships and at fixed sites. These will be synthesised with observations from hydrographic sections of the interior carbon content. We will thus obtain accurate estimates of the uptake, present storage, and net transport of anthropogenic carbon, and variability in the natural uptake and release of atmospheric CO2 by the N. Atlantic. In parallel with direct estimates made from these observations, forward and inverse models (of both atmospheric and oceanic kinds) of these fluxes will be developed. The main hypotheses are (1) that past uptake and variability of CO2 in the region can be quantified by examination of the deep carbon inventory in the Atlantic, (2) that the present observed variability in CO2 uptake is due to a combination of biological and physical processes that are driven by climatic variations, the main factors being captured by ocean carbon simulations embedded in climate models, and (3) these variations (past, present and future) are due to a combination of variability internal to the climate system and external anthropogenic forcing - in proportions we will determine. Objectives are (1) a template for operational forecasting of the fluxes of GHGs into and out of the N. Atlantic, to be implemented as part of ICOS and in combination with ECMWF (2) an understanding of that sink that can be used to improve projections of how the ocean CO2 sink will change in the future, and (3) a quantitative understanding of how and why Atlantic Ocean uptake of anthropogenic CO2 has changed as a result of climate change over the last 100 years.

In order to mitigate the effects of climate change, governments, private companies and individual citizens are taking action to reduce emissions of greenhouse gases (GHGs). Our project will provide new information that can be used to better evaluate the change in emissions that result from these actions. We will help the UK government track the effectiveness of emissions reductions policies that have been implemented to meet the targets laid out in the Climate Change Act (2008), which mandates that GHG emissions are reduced by 80% below 1990 levels by 2050. The UK has played a major part in recent scientific and technological advances in emissions reporting and evaluation. Its GHG emission inventory, which is compiled based on data relating to human activities and rates of emission from each activity, is world-leading. Furthermore, the UK is one of only two countries that regularly submits a second estimate of emissions, those derived from atmospheric measurements, as part of its annual United Nations Framework Convention on Climate Change (UNFCCC) submission. This second "top-down" estimate can be used to assess where uncertainties lie in the inventory and where further development is needed. However, limitations exist in our scientific knowledge and in our technical capabilities that prevent the UK, or any other country, from further improving its emissions reports through the incorporation of atmospheric data. Through the NERC Greenhouse Gas & Emissions Feedback programme, which ended in 2017, we demonstrated the ability to quantify the UK's net national GHG fluxes using atmospheric observations. However, we have not yet been able to separately estimate fossil fuel and biospheric carbon dioxide sources and sinks, or determine the major sectors driving changes in the UK's methane emissions. This proposal will develop new science to address these needs, and pave the way towards the next generation of GHG evaluation methodologies. Our work will span four key areas: 1) Improving models of emissions from individual source and sink sectors to determine when and where GHG emissions to the atmosphere occur from both natural and anthropogenic systems. 2) Utilising new surface and satellite atmospheric GHG observations, such as isotopic measurements of methane and carbon dioxide, and measurements of co-emitted or exchanged gases (oxygen, carbon monoxide, nitrogen dioxide and ethane) to provide information on emissions from different sectors. 3) Utilising enhanced model-data fusion methods for making use of these new observations and for better quantifying uncertainties. 4) Integrating data streams to determine the highest level of confidence in the UK's emissions estimate. To improve the transparency of national reports, scientists and policy makers have been strongly advocating for the combination of such methods in the reporting process. The UNFCCC, at its 2017 Conference of Parties, acknowledged the important role that emissions quantified through atmospheric observations could have in supporting inventory evaluation (SBSTA/2017/L.21). Through our close links to the inventory communities in the UK and around the world, the IPCC and to UK policy makers, we can ensure that our work will be used to update and improve the UK's GHG submission to the UNFCCC and will showcase methods of best-practice.