Editing IPCC:AR6/WGIII/Chapter-6 (section)

=== 6.6.1 What is a Net-zero Energy System? ===

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Limiting warming to well below 2°C requires that CO 2 emissions from the energy sector be reduced to near zero or even below zero ( [[#6.7|Section 6.7]] ; Chapter 3). Policies, technologies, behaviours, investments, and other factors will determine the speed at which countries transition to net-zero energy systems – those that emit very little or no emissions. An understanding of these future energy systems can help to chart a course toward them over the coming decades.

This section synthesises current understanding of net-zero energy systems. Discussions surrounding efforts to limit warming are frequently communicated in terms of the point in time at which net anthropogenic CO 2 emissions reach zero, accompanied by substantial reductions in non-CO 2 emissions ( [[#IPCC--2018|IPCC 2018]] , Chapter 3). Net-zero GHG goals are also common, and they require net-negative CO 2 emissions to compensate for residual non-CO 2 emissions. Economy-wide CO 2 and GHG goals appear in many government and corporate decarbonisation strategies, and they are used in a variety of ways. Most existing carbon-neutrality commitments from countries and sub-national jurisdictions aim for economies with very low emissions rather than zero emissions. Offsets, carbon dioxide removal (CDR) methods, and/or land sink assumptions are used to achieve net-zero goals (Kelly [[#Levin--2020|Levin et al. 2020]] ).

Precisely describing a net-zero energy system is complicated by the fact that different scenarios attribute different future CO 2 emissions to the energy system, even under scenarios where economy-wide CO 2 emissions reach net zero. It is also complicated by the dependence of energy system configurations on unknown future conditions such as population and economic growth, and technological change. The energy system is not the only source or sink of CO 2 emissions. Terrestrial systems may store or emit carbon, and CDR options like BECCS or DACCS can be used to store CO 2 , relieving pressure on the energy system (Chapter 3). The location of such CDR options is ambiguous, as it might be deployed within or outside of the energy sector (Figure 6.21), and many CDR options, such as DACCS, would be important energy consumers ( [[#Bistline--2021a|Bistline and Blanford 2021a]] ) ( [[#6.6.2|Section 6.6.2]] ). If CDR methods are deployed outside of the energy system (e.g., net negative agriculture, forestry, and land-use CO 2 emissions), it is possible for the energy system to still emit CO 2 but have economy-wide emissions of zero or below. When global energy and industrial CO 2 emissions reach net zero, the space remaining for fossil energy emissions is determined by deployment of CDR options (Figure 6.21).

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'''Figure 6.21 | Residual emissions and carbon dioxide removal (CDR) when global energy and industrial CO''' 2 '''emissions reach net-zero.''' Residual emissions and CDR in net-zero scenarios from the AR6 Scenarios Database show global differences across warming levels (light blue = scenarios that limit warming to 1.5°C (&gt;50%) with no or limited overshoot and scenarios that return warming to 1.5°C (&gt;50%) after a high overshoot; yellow = scenarios that limit warming to 2°C (&gt;67%) and scenarios that limit warming to 2°C (&gt;50%); dark blue = scenarios that limit warming to 2.5°C (&gt;50%), scenarios that limit warming to 3°C (&gt;50%), scenarios that limit warming to 4°C (&gt;50%), and scenarios that exceed warming of 4°C (≥50%)). In each case, the boxes show the 25th to 75th percentile ranges, and whiskers show the 5th and 95th percentiles. Lines and circles within the boxes denote the median and mean values, respectively.

This section focuses on energy systems that produce very little or no CO 2 emissions, referred to in this chapter as ‘net-zero energy systems’. While energy systems may not reach net zero concurrently with economy-wide CO 2 or GHG emissions, they are a useful benchmark for planning a path to net zero. Note that the focus here is on energy systems with net-zero CO 2 emissions from fossil fuel combustion and industrial processes, but the lessons will be broadly applicable to net-zero GHG energy systems as well. Net-zero GHG energy systems would incorporate the major efforts made to reduce non-CO 2 emissions (e.g., CH 4 from oil, gas and coal as discussed in [[#6.4|Section 6.4]] ) and would also need to incorporate more CDR to compensate for remaining non-CO 2 GHG emissions. Energy sector emissions in many countries may not reach net zero at the same time as global energy system emissions (Figure 6.25 and Cross-Chapter Box 3 in Chapter 3).

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