What Emissions Count?
Net zero at a global scale requires that all emissions of greenhouse gases should be balanced by their removal from the atmosphere. Natural processes contribute to that removal, more efficiently and rapidly for some gases than for others. That complicates the definition of net zero and associated accounting since it requires an understanding of the longevity of different gases, their effects on climate, and the interactions between human activities and natural processes.
Which greenhouse gases count?
The three most important gases driving climate change are CO₂, methane and nitrous oxide. These differ markedly in their ability to absorb energy and how long they remain in the atmosphere. CO₂ and nitrous oxide have atmospheric lifetimes of hundreds of years, and therefore act as cumulative pollutants. Methane makes a greater contribution to short-term warming, but has a short atmospheric lifetime of one decade. This means the consequences of reducing emissions for each gas are different.
For CO₂ and nitrous oxide, reducing emissions will not stop warming until net zero emissions are achieved. Even then other climate change impacts will continue for a while, because of delayed responses in the climate system. Methane is different because of its much shorter atmospheric lifetime. By reducing the annual rate of methane emissions, we can either stabilise the warming it causes or actually reduce its warming effects. Thus for methane, the removals needed to balance additions (i.e. Net Zero) are achieved naturally, rather than depending on additional human actions.
The stabilisation of global temperature therefore requires:
1. No further net emissions of CO₂ and nitrous oxide (or other long-lived greenhouse gases) by human activities; any release to be balanced by removals.
2. Stabilising emission rates of methane (and other short-lived greenhouse gases). If reductions can be achieved, they make it possible to reach global net zero earlier than would otherwise be possible.
This graph contrasts the response to a long-lived pollutant such as CO₂ (red) with a short-lived climate pollutant such as methane - CH₄ (blue). Three cases are shown: emissions rising steadily, emissions constant, and emissions falling to zero, in all cases over several decades. Lower panels show the warming caused by these emissions.
When emissions are rising, CO₂ and methane both cause warming. Temperature continues to rise under constant emissions of CO₂, as CO₂ continues to accumulate. In contrast, constant methane emissions lead to constant methane concentrations in the atmosphere, which hold temperature at an elevated, but nearly stable, level. Temperature continues to rise in response to falling CO₂ emissions, as long as they remain above zero, but temperature falls in response to rapidly falling methane emissions.
When emissions reach zero, the temperature response to CO₂ remains constant for many decades at whatever level it has reached due to cumulative CO₂ emissions over the entire industrial period, while the temperature response to methane declines to near zero within about a decade, because of its short lifetime.
Who should count which emissons and how?
The overall philosophy driving net zero pledges should be that of achieving the highest ambition possible, as rapidly as possible. This means comprehensive accounting for all gases and sources of emissions and ensuring the effectiveness of removals.
There are different ways to account for emissions (and removals) of a country, city or business. Problems remain in this accounting relating to:
The need to account for the ‘embedded carbon’ (greenhouse gases released in manufacture or agriculture) for traded goods and supply chains, currently considered the responsibility of the originating entity.
The need to account for historical emissions that continue to cause warming.
Natural processes and their management. For example, whether accounting should include emissions arising from the loss of natural carbon stores (through forest fires, soil degradation or thawing permafrost). Conversely, the extent to which carbon removed by natural processes, on land or at sea, can be included in the balance sheet.
For nations and cities, geographical boundaries of emissions are also relevant for accounting. For example, whether cities should include the transport emissions relating to commuting, and other consumption-based emissions (e.g. food-related) that might be reduced by changes in individual behaviours.
Business pledges to net zero emissions can either be limited to their direct burning of fossil fuels or include indirect emissions. Indirect emissions can be only emissions generated by the purchase of electricity; or also include all other emissions that arise from the business activity and their products.
Each actor, state, city or business may have different interpretations of the accounting problems above, but the framing for net zero should be that of the highest possible ambition.
While there is some risk of double-counting, that is beneficial if applied to emissions, since Net Zero will then be reached more rapidly. Double-counting of reductions or removals would, however, have the opposite effect.