The science of net zero, developed over 15 years ago,* does not include these natural carbon sinks in the definition of net human-induced CO₂ emissions.

Natural sinks play a vital role to moderate the impact of current emissions and draw down atmospheric CO₂ concentrations after the date of net zero, stabilizing global temperatures. Yet governments and corporations are increasingly turning to them to offset emissions, rather than reducing fossil fuel use or developing more permanent CO₂ disposal options. Emissions accounting rules encourage this by creating an apparent equivalence between fossil fuel emissions and drawdown of CO₂ by some natural carbon sinks, meaning a country could appear to have ‘achieved net zero’ whilst still contributing to ongoing warming.

The researchers call on governments and corporations to clarify how much they are counting on natural carbon sinks to meet their climate goals, as well as recognising the need for Geological Net Zero.

Geological Net Zero means balancing flows of carbon into and out of the solid Earth, with one tonne of CO₂ committed to geological storage for every tonne still generated by any continued fossil fuel use. Given the cost and challenges of permanent geological CO₂ storage, achieving Geological Net Zero will require a substantial reduction in fossil fuel use.

We need to stop dumping carbon dioxide into the atmosphere and scale up our ability to get rid of it permanently. We can’t take credit for the absorption of carbon which would happen naturally – the carbon we remove must be in addition to that.

Professor Myles Allen, Department of Physics.

The study was led by Professor Myles Allen, of the University of Oxford’s Department of Physics, known as ‘the physicist behind net zero.’ In 2005, Professor Allen introduced the notion of a finite carbon budget, implying net zero emissions of carbon dioxide are necessary to halt global warming.

He summarised: ‘We are already counting on forests and oceans to mop up our past emissions, most of which came from burning stuff we dug out of the ground. We can’t expect them to compensate for future emissions as well. By mid-century, any carbon that still comes out of the ground will have to go back down, to permanent storage. That’s Geological Net Zero.’

Professor Allen, who founded the Oxford Net Zero initiative, added: ‘This is not a new target: it is a recognition of what we actually meant by net zero emissions back in the late 2000s. As the climate challenge starts to bite, there is naturally pressure to soften it. Universities like Oxford are uniquely placed to make sure that doesn’t happen.’

In the end, this is a global waste disposal problem. Companies selling fossil fuels should be required to dispose responsibly of the carbon dioxide those fuels generate. If they did, this could solve global warming completely within a generation.

Professor Myles Allen, Department of Physics.

The research team stress the importance of protecting and maintaining natural carbon sinks while accepting that doing so cannot compensate for ongoing fossil fuel use. Total historical CO₂ emissions determine how much a country or company has contributed to the global need for ongoing natural carbon sinks. A country like the UK, with large historical emissions and limited natural sinks, has implicitly committed other countries to maintain natural sinks for decades after UK emissions reach net zero. This is not currently addressed in climate talks.

Professor Allen added: ‘We now need to document the scale of this issue by separating out the role of passive uptake in national targets, Nationally Determined Contributions, and carbon markets. Alongside aiming for Geological Net Zero, we need to identify other mechanisms than carbon offsetting to channel resources to the protection of passive carbon sinks.’

The paper ‘Geological Net Zero and the need for disaggregated accounting for carbon sinks’ has been published in Nature. 

*The 2009 ‘Net Zero Papers’ were Solomon et al, Meinshausen et al, Allen et al, Matthews et al, Zickfeld et al, and Gregory et al.