## 1 Introduction  
Water vapour, carbon dioxide, methane, nitrous oxide and ozone are active gases naturally present in the Earth's atmosphere. They are commonly known as greenhouse gases (GHGs) because they let most of the incoming solar radiation heat the Earth's surface while they prevent some of the emitted thermal infrared (IR) radiation from escaping into space, thereby trapping some of the surface heat energy. As a result, the average surface temperature of the Earth's air is about 30 \({}^{\circ}\)C higher than it would be without the absorption and reradiation of infrared energy by the atmosphere [1]. In addition to global warming, a paper published by [PERSON] et al. [2] shows that polluting emissions such as particulate matter, ozone, carbon monoxide, sulphur dioxide and nitrogen dioxide caused by human activity increase the amount of hospitalisations due to respiratory diseases. Over geological timescales, greenhouse gas concentrations have fluctuated cyclically. These fluctuations are caused by unclear mechanisms; however, Earth's temperature fluctuates between ice ages and warmer interglacial conditions [3]. In recent decades, concentrations of greenhouse gases in the atmosphere have changed rapidly due to human activities. In particular, the amount of carbon dioxide (CO\({}_{2}\)) has almost doubled since preindustrial times (from about 270 molecules of CO\({}_{2}\) per million molecules of air in 1850 to 420 parts per million today) [4]. Human-induced warming reached 1 \({}^{\circ}\)C above preindustrial levels in 2017 and will reach 1.5 \({}^{\circ}\)C by 2040 if the current rate of warming (0.2 \({}^{\circ}\)C per decade) continues [5]. Global CO\({}_{2}\) emissions hit a record high of 37 billion metric tons in 2019 [6]. In 2020, restrictions imposed by the COVID-19 pandemic caused a 5% drop, but, as lockdowns eased, global CO\({}_{2}\) emissions rebounded again to 37 billion metric tons in 2021 [6]. Since the Industrial Revolution, global emissions have predominantly come from Europe and the United States [7]. However, from the second half of the 20 th century, there has been a significant increase in emissions from the rest of the world: currently, China alone is responsible for almost a third of global emissions [7]. At the same time, 2.73 billion metric tons of CO\({}_{2}\) were released into the atmosphere in the European Union in 2021, which is significantly lower than the peak in 1979 (3.99 billion metric tons) [8]. As of 2021, the main emitter is Germany, mainly due to its energy mix based on coal, whose combustion produces 50% more CO\({}_{2}\) than natural gas [8]. The first legally binding climate treaty was adopted in Kyoto in 1997 and entered into force in 2005. It committed industrialised countries to reduce their emissions to 5% below 1990 levels, but it did not compel developing countries like China or India. The 2015 Paris Agreement committed all countries to set emissions reduction targets to prevent the global average temperature from rising 2 \({}^{\circ}\)C above preindustrial levels and to continue efforts to keep it below 1.5 \({}^{\circ}\)C. Global net-zero emissions, or carbon neutrality (i.e., when the amount of GHGs emitted equals the amount captured from the atmosphere), is expected to be met in the second half of this century [9]. In 2018, as part of the \"Clean energy for all Europeans package\", the European Commission (EC) agreed to update the efficiency target of at least 32.5% relative to the 2007 modelling projections for 2030 [10]. With the 2021\({}^{\prime}\)s European Green Deal, the EC also set a target to reduce GHG emissions by 55% from 1990 levels by 2030 (Fit For 55), aiming to be carbon neutral by 2050 [11]. This follows the EU's 2020 target of a 20% reduction in emissions compared to 1990 and a 20% share of renewable energy in gross energy consumption. Some member states (MSs), such as Austria and Germany, have transposed the directive with more ambitious net-zero targets by 2040 and 2045, respectively.