Research by an international group of authors who developed the science behind net zero, demonstrates that relying on 'natural carbon sinks' like forests and oceans to offset ongoing CO2 emissions from fossil fuel use will not actually stop global warming.
The article, published in Nature (18 November), was led by Professor Myles Allen from the University of Oxford Department of Physics with co-authors including UKCEH climate modeller, Dr Chris Huntingford.
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, stabilising 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’ while still contributing to ongoing warming.
Achieving 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 CO2 storage, achieving Geological Net Zero will likely instead require a substantial reduction in fossil fuel use.
As such, the authors stress the importance of protecting and maintaining natural carbon sinks while accepting that doing so cannot then be used to compensate for ongoing fossil fuel use if society wishes to achieve net zero.
Professor Myles Allen, of Oxford’s Department of Physics, who led the study, summarises: "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."
Climate modeller, Dr Chris Huntingford, a co-author from the UK Centre for Ecology & Hydrology said: "Humans have pushed the climate some way from equilibrium, and so even with major emissions cuts, the physics of the system suggests more warming. However, such further warming will be broadly counterbalanced if the land and oceans are allowed to pull down carbon dioxide from the atmosphere, which acts to lower temperatures. Hence, if reduced future emissions levels designed to achieve net zero do not include allowing offsets against this land and ocean effect, it is likely that global warming will stop."
The paper ‘Geological Net Zero and the need for disaggregated accounting for carbon sinks’ was published in Nature on 18 November 2024.
Further information can be found in the University of Oxford news story.