Briefing Note 2 – What Do We Mean by Net Zero?


What Do We Mean by Net Zero?


The Paris Agreement reflects an ambition of the global community to halt anthropogenic climate change. However, although a significant step forward, this ratified document achieves little in isolation. The success or failure of the Paris Agreement rests on the willingness of signatories to implement the required changes to decarbonise our society and reach greenhouse gas balance and net zero emissions.


Ultimately it is national governments that will interpret and enact the Paris Agreement’s temperature goal. This interpretation primarily rests on understanding the meaning of articles 2.1a (“Holding the increase in the global average temperature to well-below 2°C above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5°C above pre-industrial levels”) and 4.1 (“…to achieve a balance between anthropogenic emissions by sources and removal by sinks of greenhouse gases in the second half of this century”).


Understanding these two articles is key to designing targets that are consistent with the Paris Agreement. In particular the statement about ‘balancing sources and sinks’ emphasises the requirement of a net zero target.


Which Gases Should We Include and What Metric of Aggregation Should Be Used?


One option is to include as many of the major contributing greenhouse gases to global warming as possible. 

The three largest single contributing gases are CO2, methane and nitrous oxide; but these don’t all contribute to warming equally. CO2 and nitrous oxide have long atmospheric lifetimes and act as cumulative pollutants. Methane contributes to more warming but has a short atmospheric lifetime of one decade. This means the consequences of reducing emissions for each gas is different.


For CO2, which acts as a stock pollutant, reducing emissions doesn’t stop warming until net zero emissions are achieved. Methane is different because it acts as a flow pollutant. By reducing the rate of annual emissions we can approximately stabilise warming or achieve reduced warming even with continuing (but declining) emissions, but only in the short term. The GWP metric isn’t perfect for exploring the temperature impact of different GHGs. GWP* could act as a solution to this, offering a formula which better accounts for the different lifetimes of pollutants whilst still remaining simple and flexible enough for implementation in policy.


GWP* at the national level offer high historical SLCP emitters the possibility to continue their high SLCP emissions so long as they are stabilised in the second half of the 21stcentury. This is a question of fairness: is it right that historical emitters have the implicit right to continue emitting their SLCPs?

Although the Paris rulebook has chosen GWP100 as the metric for the comparison of greenhouse gases, it doesn’t rule out the possibility of comparing warming impact with GWP* alongside inventory logging with GWP100.


The consequences for the exact interpretation of the greenhouse gas balance target and the articles of the Paris Agreement are clear, and it’s important to highlight the pitfalls of any metric choice for the comparison of greenhouse gases in policy.

Which Emissions Should Be Included?


In order to achieve greenhouse gas balance at a global level it’s important to be aware of where promised mitigation could fall through. It’s argued there is a quantity of emissions mitigation that remains at risk, which in present day scenarios provides ‘mitigation deterrence’ (the interactions between climate responses that prevent, deter or delay mitigation). Essentially, climate policy is not simply additional; it interacts economically, politically and culturally. 


Although required to achieve a net zero world, greenhouse gas removal technologies remain uncertain and could trigger unexpected responses in climate policy and mitigation efforts. If these promised CO2removal (CDR) deployments aren’t later fulfilled we have a large mitigation gap to rapidly close. This doesn’t mean we shouldn’t deploy CDR, but it does mean we must understand the impacts of the choices we make.


What Date Should Be Set for Net Zero?


IPCC has assessed emissions pathways for different warming levels that reach net zero CO2 but they don’t necessarily reach this goal with respect to all greenhouse gases. To an extent this doesn’t matter, as the philosophy of the Committee on Climate Change (CCC) is not-wholly-centered on a net zero target but instead about contributing the highest possible ambition to national emissions reductions.

At a global level many of the available scenarios suggest a net-zero in the centre of the 21stcentury. Differences remain between model output net zero timings and the timing of net zero based on projections of national inventories. Models consistently report larger emissions than inventories, so if you look at inventories we see net zero occurring sooner because they have fewer emissions to abate.


Once we get on the road to net zero we can revise exactly what society might deem to be ‘net zero’, but for now we should push forward with the highest possible ambition to influence the international community.


Ultimately, NDCs are for countries themselves to define. The net zero date should be considered a marker on which to push for the highest possible ambition, not the final act in national climate policy. 


It is the job of each nation and sector to provide a ‘highest possible ambition’ strategy to mitigation. If that can be achieved in a timely manner we will be well on our way to net zero.


What Does Achieving Net Zero Conclude and Recommend?


  • Balancing sources and sinks as stated in the Paris Agreement requires a net zero target;


  • It is clear that the choice of metric has significant consequences for the definition of a net zero target and there is space for flexibility in the defining articles of the Paris Agreement;


  • It is important to highlight the drawbacks of any metric choice that informs policy design.