Geological Net Zero as policy to address the non-inequivalence of carbon emissions and removals in meeting national zero-emission targets in the United Kingdom

Carbon dioxide removal (CDR) will be required to support rapid emission reductions and reach net zero emissions. Recent studies have highlighted different global warming impacts of CDR options depending on the durability of their carbon storage. Geological net zero, which demands that residual fossil CO2 emissions are matched by permanent geological storage of CO2, has been identified as one potential policy approach to address these durability differences, as it recognizes the warming risk of delayed CO2 release from less permanent storage. Considering the UK as a national case study, we investigate the effect a geological net zero policy may have on national climate change mitigation strategies.  
Using the national-scale energy system model UK TIMES, we explore different ways of implementing a geological net zero policy: a strict implementation applied on an annual accounting basis from 2030 forward, a progressive implementation that introduces a more gradual “share” of fossil emissions covered under the policy, and a cumulative implementation to 2050 which allows emissions earlier in the time horizon to be compensated for later.  
Our initial results suggest extreme difficulty in achieving GNZ, highlighting that the UK is unlikely to be able to able to reach geological net zero before 2040, as more than one decade is required to decarbonize the emitting sectors and significantly scale up removal methods with permanent storage. It is also clear that the speed of change required to achieve even this outcome is significant, requiring rapid and deep phase out of fossil fuel use much earlier than traditional scenarios suggest. We find, however, that progressive and cumulative GNZ implementations can get much closer to solving, and offer more ambitious pathways that significantly reduce the UK's cumulative emissions to 2050 compared to the current UK pathways and emissions targets. We quantify residual emissions and determine the sectors with the highest challenges for full decarbonization and find that the availability of key resource biomass as well as the pace of scaling up carbon capture and storage infrastructure have crucial impact on the feasibility of any geological net zero policy.
To our knowledge, this study is the first to assess potential geological net zero policies at national level, providing insights into the opportunities and challenges of faster decarbonization and dependence on geological carbon storage in all sectors of the UK economy. Findings of this study are also relevant for other nations considering more ambitious climate change mitigation policy.