Carbon accounting for system-wide change

What do we want to know?

A key question we want to answer when we undertake any climate change mitigation action is ‘What is the total change in emissions/removals that we achieve?’. We want to know the size of the reduction in emissions (or net increase in removals¹). And we definitely want to make sure that we don’t reduce emissions in one place but unintentionally increase them by a greater amount elsewhere.

Ensuring an intervention is a net removal is particularly important for a pollutant like carbon dioxide (CO₂) that readily mixes in the atmosphere, because emitting a tonne of CO₂ in one part of the world has the same impact on temperature change as emitting it from any other part of the world. This global impact also means that removing CO₂ in one place but increasing emissions by an equivalent or greater amount elsewhere is counter-productive.

So, we really need to know the total system-wide change in emissions and removals caused by our actions. But what methods should we use?

Be careful of using the wrong method

There are some widely used greenhouse gas (GHG) accounting methods that quantify emissions and removals within a defined inventory boundary. For example, attributional life cycle assessment (LCA) quantifies the GHG emissions and removals from all the processes physically used in the life cycle of a product. And there are also corporate-level GHG inventories that include all the GHG emissions and removals from a company’s value chain.

However, these methods are not appropriate for quantifying the total system-wide change caused by mitigation actions as they only include emissions and removals that occur within their inventory boundaries. If an intervention causes changes outside that boundary, e.g., outside the processes physically used in the life cycle of a product, or outside the value chain of a company, then those changes are missed entirely.

To give an example, we might decide to grow crops to make biofuels in order to reduce emissions from fossil fuels. But, increasing demand for crops is likely to increase crop prices, and farmers around the world tend to respond to price signals by increasing production, including through converting natural land to agriculture. The resulting indirect land use change emissions wouldn’t be included within an attributional LCA (if the crops physically used in the biofuel are grown on existing agricultural land), but they are nevertheless a consequence of our actions (see Searchinger et al., 2008).

Much like pushing on one domino can lead to unforeseen impacts downstream, doing something that appears to reduce GHG emissions in one place may indirectly lead to an increase somewhere else (Figure 1). In order to mitigate this risk, one must use a carbon accounting method that accounts for system-wide impacts.

Unfortunately, and all too often, practitioners and even policy-makers still use these attributional methods even though these methods don’t tell us want we want to know. A letter in the journal Carbon Management flags this issue and the inappropriate use of attributional LCA for quantifying removal carbon offset credits.

This isn’t to say that attributional LCA and corporate GHG inventories aren’t useful for other purposes, they just aren’t the right method for assessing system-wide change. So, what method should we be using?

Using the right method

There is a group of GHG accounting methods that are broadly referred to as ‘consequential’ or ‘intervention’ methods (and sometimes also ‘relative’ methods) that specifically aim to calculate the total system-wide change in emissions caused by an intervention or decision. Different instances of this consequential approach focus on different levels of action, e.g., at the product, project, or policy level, but broadly they are all aiming at the same goal. Examples of standardised guidance include ISO 14064-2 for project-level assessments, and the GHG Protocol Policy and Action Standard for government-level policy interventions.

An important feature of many of these consequential methods is that they also provide information on when GHG emissions occur, so that it is possible to understand whether an action creates an initial ‘carbon debt’ that is paid back over time once the action is reducing emissions. For example, there will be upfront emissions from manufacturing and installing a wind turbine, which are later ‘paid back’ once the turbine is operational and is displacing fossil power generation.

Another important feature is that these methods attempt to include all emission sources and sinks that change as a result of the intervention that is studied, in order to calculate total system-wide change. Inevitably the ripple of cause-and-effect pathways from an action can become complex and uncertain, but the aim is to include all the effects that are likely to be significant, and to be transparent about uncertainty.

Applying this method to enhanced weathering

Enhanced weathering, just like any other mitigation action, can be fully assessed using consequential methods. As well as including all the emissions associated with project activities, and the direct removals/sequestration, it’s important to consider possible indirect effects such as indirect land use change due to changes in crop yields caused by the application of crushed rock. One point to highlight is that indirect effects can be positive as well as negative, for instance milled limestone can enhance crop yields, which could reduce land use change pressures elsewhere. Ultimately, once all significant changes are included, we get an answer to our key question ‘What is the total change in emissions/removals that we achieve?’.

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