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Understanding the net in net zero

Dr Steve Smith, Executive Director of the Oxford Net Zero Initiative and of CO2RE, explains the science behind net zero and the role of carbon removals in achieving it.

To stabilise global temperature, we need to get to a point where we are not adding any more CO2 to the atmosphere. Despite the amazing complexity of Planet Earth, the data shows a simple, clear relationship between the total amount of CO2 we have emitted since the start of the Industrial Revolution and the global average temperature rise. There are natural processes which add some variability, but underlying that noise there is a linear trend – the temperature we get is proportional to how much CO2 we put into the atmosphere.

To stop the temperature from rising, we need to stop adding to our cumulative total of CO2 emissions into the atmosphere. We can do that by stopping all emissions. Having a decent chance of stabilising the rise at 1.5°C requires us to stop by 2050 – within a generation. If we can’t do that soon enough, we can balance out residual emissions by doing an equivalent amount of carbon removal from the atmosphere. That is what we call net zero.

How much carbon removal we need depends on how successful we are at squashing emissions. Globally we are still emitting 40 billion tonnes of CO2 into the atmosphere every year. We certainly don’t have the capacity to do enough carbon removals to balance out anything like that much. Human-driven carbon removal currently amounts to about 2 billion tonnes a year, over 99% of that through trees being planted and grown.

IPCC scenarios that hold future temperature around 1.5°C all require deep and fast emission reductions, starting now. Even so, on average they suggest we will still need 7 billion tonnes of carbon removal per year by 2050, to reach net zero. In short, to achieve the goals that countries signed up to in the Paris Agreement, we must be super-ambitious both in cutting emissions and in scaling carbon removal – it is not an either/or choice. If we are too late in getting to net zero and the resulting climate impacts are too much to live with, then carbon removal will become an even more important option.

There is a wide range of ways to do carbon removal. We can capture atmospheric CO2 through biomass growth or through chemicals and minerals. We can store it underground in geological formations or put it in the ocean, in products or in woodlands. Each approach has opportunities and constraints that vary by place and context. For this reason, it makes sense to pursue a mix of approaches rather than relying on any one.

Meeting our climate goals requires exponential growth. Our research shows that new carbon removal technologies – such as enhanced rock weathering, storage of carbon from biomass use and direct air capture with storage – must grow by a factor of about 30 over this decade, and about 1,300 by 2050, to be on track to 1.5°C.

That kind of trajectory currently causes both great excitement and great concern. Some see it as an opportunity for profit and livelihoods, while others see it as a distraction from the crucial business of cutting emissions. I hope that carbon removal can become a helpful part of mainstream climate action. We should deal with CO2 as we do with many other waste problems: limit the waste production as much as possible and also clean up the mess.

Read more: The State of Carbon Dioxide Removal 1st Edition