RealClimate: Net Zero

There was a lot of discussion about Net-Zero goals at the COP26 gathering. The concept is based on the results of physical science that show that future warming is tied to future emissions and that warming will only stop once CO2 is removed. Some activists have pointed out that reliance on large-scale CO2 removal is akin to not committing to net zero at all, as they pointed out that it is as yet unknown. Their point is that net-zero is not zero and therefore will serve as a smokescreen for insufficient climate action. It might be helpful to have some background.

Net-zero CO2 is important in thegeosciences.

It has become clear that the warming of the planet is tied to the cumulative emissions of carbon dioxide. The figure is from the AR6 SPM.

The relationship between carbon emissions and temperature.

There is a rough balance between the net absorption of carbon into deep pools and the rate at which the oceans warm in response to an energy imbalance. Global temperature is stable once CO2 emissions stop. Future warming is dependent on emissions. Once cumulative emissions stop, the warming is set.

This is an important result and one that underlies the recent pledges to achieve net-zero by the year 2050.

Net-zero greenhouse gas emission has no significance.

There is a section within the Paris Agreement.

Parties aim to reach global peaking of greenhouse gas emissions as soon as possible in order to achieve the long-term temperature goal set out in article 2, and to undertake rapid reductions thereafter in accordance with best available science.
Section 1 of article 4.

This could be seen in many different ways. Some people have made a link between this statement and the emission reporting requirements which use GWP-100 to covert different gases to a CO2 equivalent, suggesting that we should be aiming for net-zero emissions of CO2e. This doesn't have any significance since it could be associated with an increasing, decreasing or stable temperature depending on what is happening to CH4, N2O and CFCs.

If net-zero CO2e was the goal, 170 TgCh4/yr would mean 5100 TgCO2/yr 5 GtCO2/yr would remain. It is sufficient to maintain a warming trend of 0.02oC/decade, even though this is much smaller than current emissions. If we achieved net-zero CO2 and moved into net-negative CO2 emissions, net-zero CO2e might occur at some point later. 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.

The Glasgow text is more clear.

This was one of the persistent issues during the approval session. The Saudis tried to change the reference to net-zero CO2 emissions to net-zero greenhouse gases. The authors and many other countries took pains to explain that net-zero was only a concept for CO2 and not for other things.

The Glasgow text made clear this for the COP process.

Reducing global carbon dioxide emissions by 45 per cent by the year 2030 is required to limit global warming to 1.5 C.
section 17 is the final text of the Glasgow Pact.

The goal of net-zero for CO2 is now a key goal. The temperature will be determined by the timetable and cumulative emissions up until net zero. Even though net-zero is only coupled to the 1.5oC aspiration, it is a broader concept.

Is net-zero different from net-zero as a slogan?

The use of net-zero has been opposed by some in the climate community because it allows bad faith actors to maintain current emissions while promising unrealistic amounts of negative emissions in the future. This is 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 is inevitable that some of the pledges will be optimistic about the magnitude of the negative emissions. Is net-zero the same as climate denial?

Net-zero is a physical goal that is based on the science that climate deniers ignore. Is the rhetorical flourish useful? I think it serves to confuse greenwashing with the science of the carbon cycle, which people already have a hard time with.

This isn't to say that pledges and targets shouldn't be scrutinized, but some will be less credible than others. Even though net-zero is not zero, it is still useful for the general public to understand because it underlies the important conclusion that future CO2 emissions will cause future warming.