Negative carbon emissions are proved necessary to limit global temperature increase below 1.5C compared to pre-industrial levels. (a) Reducing carbon sources and (b) increasing negative emissions are two effective ways to achieve this goal. (a) involves the introduction of renewable energy sources, improved efficiency of energy technologies, and changes in human lifestyles. For (b), the reinforcement of carbon sinks can be realized either in the industrial domain, i.e. by Carbon Capture, Utilization, and Storage (CCUS), or in nature, i.e. by afforestation and reforestation. However, these measures will entail fundamental changes in the structure of the energy system, involving the conversion, transmission, distribution, and storage of energy, as well as changes on the demand side, such as the transformation from a pure consumer to a prosumer (producer and consumer). The question of how to "achieve the goal of negative carbon emissions rapidly and stably whilst guaranteeing the security of energy supply" has become a hot topic in international academic, political, and economic activities in recent years.

With the aim of optimizing energy systems, this study develops a theory for modelling the increasingly complex carbon cycle from a linear to a circular economy. On this basis, a theoretical and methodological approach to parameterize the energy transition is proposed, quantifying and discussing the impact of the carbon emission rate, the timing of the transition on the cumulative carbon emissions, and the cumulative system costs. The results show that (1) Switzerland can achieve negative emissions by 2050, where the energy system can generate -6 million tonnes of annual carbon emissions; at the same time, (2) Switzerland can achieve energy independence, with a 92% penetration share of renewable energies; (3) the difference in cumulative carbon emissions between the different transition pathways over the next 30 years can reach up to 7 times the 2020's emissions subject to the same negative-emission target, while (4) the difference in cumulative costs is only 10%. These results highlight the necessity to shift from target dominance to process orientation for energy planning. For the European Union (EU), Switzerland and UK, the model results estimate the carbon emission could achieve lowest to -900 MtCO2-eq/year, but it necessitates the joint effort of the whole society to realize a radical revolution in lifestyle (diet, clothing, living, and travel), technology development (in particular energy and manufacturing sectors), and smart management in land utilization including agriculture and forestry. The results indicate that different pathways of the EU can lead to a global temperature fluctuation of approximately 0.025°C in 2050.

Furthermore, the Life Cycle Impact Assessment (LCIA) results show that the decarbonization process can significantly diminish the damages caused by human activities to the health and ecosystem (e.g. biodiversity). Nevertheless, low-carbon emissions are demonstrated as merely one necessary but not sufficient condition for the sustainability of the society. For the EU, decarbonization might exacerbate the food shortage, water stress, and depletion of mineral resources. Following the COP26, these side-effects are likely to be amplified, thus calling for joint efforts of the whole society to meet the sustainable demands of human beings.