Climate discussions within IMO, UNFCCC, and others often include different references to zero emissions. These include ‘net-zero’, ‘near-zero’, ‘absolute zero’, and ‘zero’. Not surprisingly, the understanding of each of these terms may not be common to all stakeholders, and there are circumstances where some terms are used interchangeably which understandably adds confusion to what is already a complicated discussion.In the interest of facilitating greater clarity and understanding, we examine each of the terms concerning zero emissions and attempt to provide a practical, plain-English, distinction that is based on accepted definitions and current state-of-the-art modelling of GHG emissions. Where established definitions are lacking, we offer an informal explanation and practical description. A very brief explanation follows for each of the terms relating to zero emissions.

The term ‘net-zero’ is used to describe a neutral or zero effect on the climate system by balancing GHG emissions with removals that may or may not be within the marine fuel lifecycle. For reference purposes the IPCC has defined net-zero GHG emissions as follows: "Net-zero emissions are achieved when anthropogenic emissions of greenhouse gases to the atmosphere are balanced by anthropogenic removals over a specified period. Where multiple greenhouse gas emissions are involved, the quantification of net-zero emissions depends on the climate metric chosen to compare emissions of different gases (such as global warming potential, global temperature change potential, and others, as well as the chosen time horizon)."

The term ‘near-zero’ refers to the GHG emissions associated with the use of fuels produced using best available technology and 100% renewable energy that achieve overall GHG gas reductions equal to or greater than 80%, but less than 100% when compared to Low-Sulphur Fuel Oil (LSFO) produced through conventional refining methods. The range of performance (e.g., >80% reduction) is not presented as a precisely defined threshold or definition, but is based on an objective survey of lifecycle models developed independently by experts. Examples illustrating ‘near-zero’ emissions include combustion of renewable marine fuels in dual-fueled engines using a hydrocarbon pilot fuel.

The term ‘absolute zero’ is used to describe an energy source that produces no direct or indirect GHG emissions across the full Well-to-Wake lifecycle. To be more specific, ‘absolute zero’ describes where there are no emissions of carbon dioxide (CO2) or other greenhouse gases (GHG) across all scopes, i.e., where there are no direct emissions from fuel consumption or indirect emissions from energy purchased or any GHG emissions from production to end use.

The term ‘zero’ is frequently used in articles, conversations, and climate policy documents. Despite the widespread use of the term, this term understandably invites questions as to whether the user really means ‘net-zero’, ‘absolute zero’, or ‘near-zero’. Some speakers clarify that for them ‘zero’ means absolute zero. It is also clear that ‘zero’ is used by different authors and speakers to refer to one or more of the above variants concerning ‘zero’ emissions.

Source：ISWG-GHG 13/3/9

Image: IMO