At the COP26 gathering last week much of the discussion related to “Net-Zero” goals. This concept derives from important physical science results highlighted in the Special Report on 1.5ºC and more thoroughly in the last IPCC report that future warming is tied to future emissions, and that warming will effectively cease only once anthropogenic CO2 emissions are balanced by anthropogenic CO2 removals. But some activists have (rightly) pointed out that large-scale CO2 removals are as yet untested, and so reliance on them to any significant extent to balance out emissions is akin not really committing to net zero at all. Their point is that “net-zero” is not zero and hence will serve as a smokescreen for insufficient climate action. To help sort this out some background might be helpful.
Net-zero CO2 has real geophysical significance
With empirical data and more and better modeling, it has become clear that, to first approximation, the eventual anthropogenic warming from carbon dioxide is tied to the cumulative emissions. This figure is from the AR6 SPM:
The basis of this relationship is the rough balance between the net uptake of carbon into deep pools (mainly the deep ocean) and the rate at which the oceans warm in response to an energy imbalance. We’ve discussed ‘commitment’ issues before, and to zeroth order global temperature is basically stable once CO2 emissions stop. Thus future warming is totally dependent on future emissions. These relationships implies that once cumulative emissions stop (i.e. net-zero is reached), the eventual warming is set.
This is a very important result, and one that underlies the recent pledges to achieve net-zero by 2030/2040/2050 etc. coming as part of the upgrade to Nationally Determined Contributions (NDCs) for the COP26 meeting.
Net-zero greenhouse gas emission does not have any geophysical significance
Within the Paris Agreement there is a section that says:
In order to achieve the long-term temperature goal set out in Article 2, Parties aim to reach global peaking of greenhouse gas emissions as soon as possible, recognizing that peaking will take longer for developing country Parties, and to undertake rapid reductions thereafter in accordance with best available science, so as to achieve a balance between anthropogenic emissions by sources and removals by sinks of greenhouse gases in the second half of this century, on the basis of equity, and in the context of sustainable development and efforts to eradicate poverty.Article 4, section 1
This could be interpreted in different ways. Some folks have made a link between this statement and the emission reporting requirements which uses GWP-100 to convert different gases to a CO2-equivalent to suggest that we should be aiming for net-zero emissions of CO2-e. However this does not have any particular geophysical significance since it could be associated with an increasing, decreasing or stable temperature depending entirely on what is happening to CH4, N2O and CFCs.
For example, if anthropogenic CH4 emissions can be halved (say), that’s roughly 170 TgCh4/yr, which implies that 5100 TgCO2/yr ~ 5 GtCO2/yr could remain if net-zero CO2-e was the goal. While this is much smaller than current emissions of ~ 36 GtCO2/yr, it’s sufficient to maintain a warming trend of ~0.02ºC/decade. Alternatively, if we achieved net-zero CO2 and moved into net-negative CO2 emissions, net-zero CO2-e might occur at some point later while temperatures are declining. (At some point it will be worth looking in detail at what happens when we get 80 or 90% CO2 emission cuts because at that point the detectability of global temperature trends and the importance of small and hitherto neglected sources and sinks will come into play).
IPCC AR6 and the Glasgow text are clearer
During the approval session for the IPCC AR6 Summary for Policy Makers, this was one of the persistent issues. Wherever the authors had made a reference to net-zero CO2 emissions, the Saudis (and sometimes China) tried to amend it to say net-zero greenhouse gases instead. The authors and many other countries took pains to explain that these were not commensurate statements, and the SPM text ended up making it clear that net-zero as a concept only applied to CO2.
Subsequently, the Glasgow text clarified this for the COP process as well:
[The Conference of the Parties] also recognizes that limiting global warming to 1.5 °C requires rapid, deep and sustained reductions in global greenhouse gas emissions, including reducing global carbon dioxide emissions by 45 per cent by 2030 relative to the 2010 level and to net zero around mid-century, as well as deep reductions in other greenhouse gases;Glasgow Pact final text, section 17
This implies that the goal of net-zero for CO2 is now enshrined as a key goal (at least for the 1.5ºC aspiration). However, it is the timetable and cumulative emissions up until ‘net zero’ that will determine the eventual temperatures (with some additional influence from cuts to other GHGs and short-lived forcings). So even though ‘net-zero’ is only coupled to the 1.5ºC aspiration in this text, it’s actually much a broader concept.
Is net-zero as a scientific concept different from net-zero as a slogan?
Some parts of the climate community has been hostile to the use of “net-zero” because, in their eyes, the “net” part allows for bad faith actors to maintain current emissions while promising unrealistic amounts of negative emissions in the future to compensate. This is clearly an issue. Any net-zero pledge needs to be examined for credibility and the proposers need to be held accountable for the claims. I think it’s inevitable that some (maybe even all) such pledges will be optimistic about the magnitude of plausible negative emissions. But is net-zero the same as climate denial?
My answer is no. Net-zero is a well-founded physical goal that is rooted in the science that climate deniers mostly try to ignore. Is the rhetorical flourish here useful? I don’t particularly think so – it mostly serves to confuse greenwashing (a real problem) with the science of the carbon cycle which people already have a hard enough time with.
This isn’t to say that pledges and targets should not be scrutinized – of course they should, and some will be found to be less credible than others. But even though net-zero is not zero it is still geophysically meaningful and a useful concept for the general public to understand because it underlies the important conclusion that future CO2-driven warming is driven by future CO2 emissions.