Abstract
To the editor – The Glasgow Agreement statement on the " phase-down " of coal means that limiting warming to well below 2C will require more ambitious emissions cuts through other means. To achieve mid-century carbon neutrality, aggressive policies are needed to reduce point-source emissions primarily through measures such as increased use of renewables. Since many developing nations will be unable to make this transition by 2050, negative emissions technologies (NETs) will also have to be scaled up rapidly to offset residual emissions from fossil fuels through carbon dioxide removal (CDR) 1,2. Many start-ups have followed the trend of the emerging "drawdown economy" to commercialize NETs through the sale of credits from CDR, but there remains a gap in providing top-down decision support for their strategic deployment considering local and regional constraints. The ramp-up of CDR to the gigaton scale has serious implications for energy systems due to the electricity requirement of many NET alternatives 3. Timing is also critical; recent modelling results focusing on bioenergy with carbon capture and storage and direct air capture show that early deployment can reduce cumulative costs of achieving net-zero emissions in the European Union 4. Exploring diverse NETs portfolios is critical due to their potential to avoid risks in large-scale deployment 5. For example, vast tracts of monoculture plantations for bioenergy with carbon capture and storage (BECCS) can threaten biodiversity 6. No single option can sustainably meet the climate targets, so multiple NETs at moderate scales are needed 7. However, most research papers still focus on individual NETs, with few taking a system-level outlook. The global and regional potential capacities of NETs have been evaluated individually rather than as part of a carbon management portfolio 8,9. Integrated assessment modelling (IAM) studies have focused mainly on BECCS and afforestation and reforestation (AR) analyzed separately 10. Only one IAM study to date has attempted the optimization of a NETs portfolio consisting of BECCS, direct air carbon capture and storage (DACCS), enhanced weathering, and AR 11. The results of the study show that a full portfolio supports the attainment of net-zero faster and with a better balance of regional CDR contributions. Instead of assuming a predefined set of NETs for decarbonization, portfolio optimization approaches will be essential for enabling rapid scale-up to meet net-zero targets (Fig. 1). New models are urgently required to determine the optimal mix of NETs to maximize CDR within cumulative cost, natural resource, and sustainability limits while accounting for the trade of carbon credits. Process integration techniques originally developed to improve the energy and resource efficiency of industrial plants can be adapted for this purpose 12. In the previous decades, various mathematical models and algorithms from process integration have been extended to industrial decarbonization through the optimum allocation of renewable energy resources and optimal planning of CO2 capture and storage 13. Other variants have also been developed to optimize research and development portfolios of low-carbon technologies 14. Such techniques have only been recently developed for the optimal deployment of NETs considering impacts on background energy systems 15 as well as cost and environmental footprints 16. These early works demonstrate the promising potential of this class of models, but much more research remains to be done so they can be used to accelerate the scale-up of NETs in the coming decades.