Net zero: what does it mean and what happens afterwards

Reading Time: 3 minutes

In response to global warming, many countries have put in place ambitions for net zero emissions by a certain date. This blog considers what is meant by net zero emissions and what might happen once net zero is achieved.

Net zero: targets and technologies  

Net zero emissions means that for any emissions that are produced an equivalent volume are captured and stored, to the point where the net amount is zero. While climate change mitigation is often thought of in terms of de-carbon-isation (the cessation of carbon dioxide emissions) it is important to note that there are a variety of greenhouse gases (GHGs). Carbon dioxide (CO2) is the most important GHG, but others contribute significantly to warming, for example, around 30% of warming is thought to have been caused by methane (CH4).

The principle of net zero is at times applied just to carbon dioxide, and at other times to a wider set of GHGs. For the 1.5oC Paris Agreement target to be achieved, it is thought that net zero CO2 emissions must be achieved globally by between 2044 – 52, and all GHGs must be net zero between 2063 – 68. For 2oC of warming, net zero GHG should be achieved globally by the end of the century.

Scotland’s net zero target applies to all GHGs by 2045 and, worldwide, this is one of the most ambitious statutory targets. Not all national net zero targets apply to all GHGs however, for example, the New Zealand 2050 target does not include biogenic methane (primarily from livestock), for which there is a separate target. The use of net zero targets means it is not thought realistic to reduce all GHGs emission to zero (or that it is more plausible to rely on technologies that capture emissions than to cease all production of emissions).

In Scotland and the UK, there will be some residual emissions left over as part of the attainment of net zero (that will need to be offset i.e. counteracted by emission capture (see below)). It is not known precisely what activities will cause these residual emissions, but sectors like agriculture, aviation and waste are thought likely candidates. At the UK level it has been estimated that residual emissions will be about 15% of current emissions.

As mentioned, net zero accounting allows for the use of technologies that capture and store emissions, these are sometime termed negative emission technologies (NETs) or greenhouse gas removals (GGR). Possible technologies include direct air carbon capture and storage (CCS) – which extracts carbon dioxide directly from the atmosphere and permanently stores it –  or bioenergy with CCS – carbon dioxide is captured from the combustion of biomass resulting in negative emissions as carbon is captured both by the CCS and the biomass growth. Tree planting and peatland restoration are prominent ‘nature-based solutions’ to capturing and storing emissions.

Sectoral emissions in Scotland are currently mapped out until 2032 (via the Climate Change Plan update in 2020) and in 2032 it is envisaged that negative emission technologies will offset about a quarter of remaining emissions. While some believe that Scotland is ‘exceptionally well suited’ for NETs like direct air capture CCS and Bioenergy-CCS, others think that NETs have ‘over-promised and under-delivered’ and there is too great a reliance on them in Scottish net zero plans. The Acorn project and a Scottish ‘cluster’ in Aberdeenshire hopes to pioneer CCS and while it has received support from both UK and Scottish Governments’ there is still uncertainty about its long-term funding and viability at scale.

What happens after net zero?

What happens to the global climate when net zero is achieved is also worth thinking about. It is not the case that global temperatures will return to pre-industrial levels (at least for thousands of years). Once there is net zero GHGs, the planet will stay at a warmer level unless there is significant use of negative emissions technologies . As there are different GHGs with different atmospheric lifetimes, and global warming potential, the impact is complex but prolonged higher temperatures are expected. According to the CarbonBrief website:

  • ‘if all human emissions that affect climate change fall to zero – including GHGs and aerosols – then the IPCC results suggest there would be a short-term 20-year bump in warming followed by a longer-term decline’

While there is not thought to be any greater warming once net zero is reached there is the potential for tipping points to be breached and impacts may continue to progress despite direct emissions coming to an end. Some analyses suggest that nearly a metre of additional sea-level rise is locked-in, with melting glaciers / ice sheets (key drivers in sea level rise) lagging behind surface temperature increases.

There is much to consider when thinking about net zero – what it refers to and what happens if it is achieved – and there are other SPICe blogs on the net zero concept. A follow-on to this blog will consider what the impacts of climate change are likely to be in Scotland and what is being done to adapt and prepare for these

Niall Kerr, Senior Researcher, Climate Change and Net Zero