“Net zero” represents the ambition to hold Greenhouse Gas (GHG) emissions to a level that new emissions have no impact on global average temperature. A necessary part of this process is to eliminate soot emissions from coal and eliminate emissions from incompletely burnt diesel fuel. About 1/3 of the increase in global average temperature can be attributed just to the summer loss of Arctic ice. One can easily see the mutual importance of this aspect of “net zero”.
What is included in GHGs?
- Carbon Dioxide (CO2) is the major contributor to the atmospheric radiative forcing from GHGs (66.0% in 2021). The extra CO2 in the atmosphere since industrialisation has its own half life. It is about 50 to 60 years. This means that “net zero” can include emissions that replace the CO2 that is lost through natural processes – about 1% of the “extra CO2” in last year’s atmospheric level.
- Methane (CH4) is another significant contributor to the atmospheric radiative forcing from GHGs (18.5% in 2021). It has a half life of about 11.5 years. This means that “net zero” can include emissions that replace the Methane that is lost – about 9% of the last year’s atmospheric level. (The methane is converted to CO2, but since quantum of Methane is much smaller than CO2, the additional CO2 is less than a rounding error, so it can be ignored.)
- Nitrous Oxide (N2O) also contributes to atmospheric radiative forcing (4.7% in 2021). It has a half life of about 120 years, which is longer than just about anyone currently lives.
- Fluorinated gases (F-gases) contribute to atmospheric radiative forcing (9.0% in 2021). The multiple gases in this class have different half lives. They can be expected to grow because of the wider uptake of air-conditioners for both heating and cooling.
- Other gases contribute 1.8% to radiative forcing. Atmospheric concentrations of these gases are relatively stable and even have declined since 2019.
Cutting the emissions for each of these gases will have its own challenges. It is not just a matter of cutting CO2 emissions. Indeed, one can quickly see that just cutting CO2 emissions will not achieve the ultimate aim of “net zero”, which is a stable temperature. It cannot be achieved without cuts to Methane, Nitrous Oxide and F-gases.
Target stable temperature
Paris agreement at COP21 set a maximum target stable temperature of +2.0°C over pre-industrial temperatures with the ambition of +1.5°C. Using the IPCC formulae, incorporating pre-industrial atmospheric levels of GHGs, we are already at +1.3°C, including +0.3°C attributable to the loss of arctic sea in each summer.
There is low-hanging fruit here. It is simply to stop soot and unburnt diesel entering into the atmosphere and landing on the ice, as is happening in Greenland.
If we are to hold global average temperature to below 2.0°C and prevent 1 to 2 metre rise in sea levels, we had better add this into our targets for 2030 and beyond.
Even if we can effectively implement this change, +1.5°C may be out of reach. Indeed, if we can prevent the ice sheets of Greenland and Antarctica from melting, something more than +1.5°C may be appropriate. Provided it is less than +2.0°C, this will still be within the Paris agreement. It would also be manageable.
Net Zero – CO2
Net anthropogenic CO2 added to atmosphere from pre-industrial times to December 2021 was (415 – 280 = 135 ppm * CO2 per ppm [2.12 * 3.67] =) 1050 gigatonnes. 1% of this would 10.5 gigatonnes out of about 36 gigatonnes of emissions.
On this basis, a 70% cut in emissions would be equal to “net zero”, taking only CO2 into account.
A cut of this magnitude will probably require technological changes in cement and steel production, since these currently occupy emit up to 5 gigatonnes a year.
A new approach to generating electricity will be required, so that coal and gas have only a minor part to play. The current emphasis on intermittent renewables is only tolerable if periodic black outs are acceptable, which is unlikely. With current technology, more emphasis on storage will be required.
Yet storage, by its nature, is finite. Unless nuclear is found to be acceptable, some managed use of coal and/or gas as a backup for electricity generation will be required.
Vehicle transport (including aeroplanes) has to be considered. To assume that the whole world could and would convert to 100% EVs is a pipe dream. Just consider, EVs are mostly about twice the cost of internal combustion vehicles. A sensible approach would be for tropical and sub-tropic countries to chose bio-fuels as an option to keep ICE vehicles working. Biofuels are an option for aeroplanes, with Qantas and Airbus signing an agreement for joint development of an aviation biofuel facility.
Gas for heating also is a challenge. More thinking is required in this area.
Net Zero – Methane
With methane, it is easy to bring down “emissions” even to below net zero. Here are the known action items:
- Stop methane leaks from gas pipelines and from gas and oil extraction. Emissions from gas pipelines is a simply a matter of proper management. Emissions from existing gas and oil extraction is also a matter of proper management.
- Stop methane leaks from current and abandoned coal mines. This is a matter for the governments of the nations to do a better job of managing this resource within their own territories.
- Reduce methane emissions from ruminant animals. Australia has come up with a seaweed-based food than can effectively do this for all animals who are served within controlled feeding arrangements.
- Recover and use methane from rubbish dumps and from sewerage. Once again, this is a matter for the governments of the nations to do a good job of managing this issue.
Net Zero – Nitrous Oxide
There is no obvious way to bring net nitrous oxide emissions down to net zero.
Like CO2, plants need nitrogen to grow, and nitrogen / ammonia based fertiliser are widely used. In an earlier age, crop rotation was used, with nitrogen-fixing plants being grown in alternate years. This is now being considered as an option to reduce the need for nitrogen-based fertilisers.
Another approach is to increase the carbon content of soils, where this is below the desired level. It is possible that carbon could be captured and used as a bi-product of production processes that generate CO2 emissions. RMIT has done research in this area. This should be encouraged. Perhaps further research will show that this could reduce the need for nitrogen-based fertilisers.
Net Zero – F-gases
There is no obvious way to bring net F-gas emissions down to net zero.
There are a number of different forms of F-gases, but none of these offer a real solution.
CFCs
Even though new use of CFCs has been banned because of their ozone depletion characteristics, they are still used in legacy applications. They have a long half-life (up to about 100 years). They still contribute about 6.7% of radiative forcing, mostly because of their slow decay.
HCFCs
HCFC-12 is allowed until 2030. It has a half-life of about 12 years and currently contributes 1.3% of radiative forcing.
HFCs
The “current hope” for the side is HFC-134a. This is rapidly growing and has a half-life of 14 years. HFCs currently contribute 1.0% of radiative forcing, and atmospheric levels of these gases can be expected to grow quite rapidly.
Air-Conditioning
Perhaps a new way of keeping houses and offices warm without using F-gases could be used. Some of the current offerings are quite expensive, like redesigning houses and offices. More thinking and research is required in this area.