James Mason / Net zero

Words by James Mason

Illustration by Steve McCabe


‘Net zero’ and ‘carbon neutral’ have quickly become the go-to phrases for ambitious declarations on climate change action. But what do net zero, zero carbon or carbon neutral mean in practice? James Mason, PhD researcher at Tyndall Manchester, explores the terms and how we can tackle the problem.

As local and national climate action targets are announced, it’s important that the assumptions involved are properly understood if we’re to deliver on the aspirations set out in the 2015 Paris Agreement.

This agreement pledges to keep the global rise in average temperature below 2°C and pursue efforts to limit it to 1.5°C. This has spurred interest in terms such as net zero and carbon neutrality – but defining these is not straightforward. Carbon neutral, climate neutral, net zero, zero emissions and decarbonisation have all been used interchangeably within both academic and policy discussions.

There are three key issues to understand in this discussion: which greenhouse gas emissions (GHGs) are included, whether and how offsetting is included, and which sectors it applies to.

On the first issue, it’s important to take into account the atmospheric lifetime of different gases. For example, methane has a greater warming effect than CO2, but it has a short lifetime and doesn’t accumulate in the atmosphere as CO2 does. When the accounting period is spread over several decades, the climate impacts of different gases is dependent on when emissions occur as well as their magnitude. As a result, a single cumulative emission budget or carbon budget should be restricted to CO2 alone.

On the issue of offsetting, unlike (absolute) zero emissions, net zero and carbon neutral imply that some carbon/GHG emissions remain but allow for offsetting through carbon dioxide removal or negative emissions. Consequently, achieving genuine net zero emissions depends on a tight definition of the boundary and requires proper monitoring.

Lastly, we should consider the sector. Opportunities for GHG reduction in sectors such as agriculture and land-use change are limited. Some energy uses are easier to abate than others. Electricity, heat, road and rail transport are expected to reach zero emissions before aviation, shipping and industrial processes, which have fewer options for mitigation and reducing demand. To achieve global net zero CO2 emissions, negative emissions approaches may be crucial in offsetting from these sectors.

At Tyndall Manchester we have developed a transparent model to help UK cities set zero CO2-only emissions targets from energy related sectors. The SCATTER project (Setting City and Area Targets and Trajectories for Emissions Reductions) has been used to help Manchester formally adopt an ambition to become a zero-carbon city by 2038, covering heating, electricity, industry and surface transport with separate targets for aviation and shipping at national level.

A zero-emissions target for CO2 rather than GHGs should be the focus for the Paris Agreement. The remaining GHGs shouldn’t be ignored; rather, they should be addressed alongside the main agreement on CO2 reductions by specifying separate limits for non-CO2. The task of bringing global CO2 emissions down to genuinely zero levels is immense and will require an ambitious and radical approach that goes beyond picking the low-hanging fruit.

James Mason, PhD Researcher, Department of Mechanical, Aerospace and Civil Engineering. 

This article was adapted from the Policy@Manchester blogSetting climate targets: when is net zero really net zero?

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