Even though the contribution of the aviation sector to the global economy is very notable, it also has an adverse impact on climate change. Improvements have been made in different areas (i.e., technology, sustainable aviation fuel, and design) to mitigate these adverse effects. However, the rate of improvement is small compared to the increase in the demand for air transportation. Hence, greenhouse gas emissions in the aviation sector are steadily increasing and this trend is expected to continue unless adequately addressed. In this context, this study examined the following: (i) the factors that affect the growth of aviation, (ii) trends in greenhouse gas emissions in the sector, (iii) trends in energy demand, (iv) mitigation pathways of emissions, (v) mitigation challenges for the International Civil Aviation Organization, (vi) achievements in mitigating emissions, (vii) barriers against mitigating emissions, and (viii) approaches of overcoming barriers against emissions mitigation. This study finds that continued research and development efforts targeting aircraft fuel burn efficiency are crucial in reducing greenhouse gas emissions. Although biofuels are promising for the reduction of aviation emissions, techniques to reduce NOx emissions could enhance large-scale deployment. Pragmatic market-based mechanisms, such as the Emissions Trading Scheme (ETS) and/or carbon tax must be enforced on a global scale to capitalize on a collective stakeholder effort to curb CO₂ emissions. The findings of this study will help in understanding the emissions and energy consumption scenarios, which will provide a comprehensive package of mitigation pathways to overcome future emissions reduction challenges in the aviation sector.

The use of large proportions of reclaimed asphalt pavement is necessary to meet the increasing demand for road construction materials in a sustainable way. One of the challenges of using a greater percentage of reclaimed asphalt pavement (>30%) is the greater stiffness of mixes incorporating it. While this stiffness problem is usually resolved by using different commercial rejuvenators, there are circumstances in which commercial rejuvenators are not available. Therefore, this study evaluates the potential of using waste engine oil as a substitute for commercial rejuvenators for the higher percentage of reclaimed asphalt pavement that could meet the increasing demand in a more sustainable way. To assess the possibility of using a higher percentage of reclaimed asphalt pavement in road construction, different percentages of reclaimed asphalt pavement (30%, 40%, and 50%) are used. Following the property of the aged binder, three different percentages (7%, 13%, and 20%) of waste engine oil are considered. Each percent of waste engine oil is incorporated with one of the three mixes. The mixes (with the minimum required Marshall criteria) are evaluated for different properties, namely, their indirect tensile strength, resilient modulus, and durability. Finally, those properties are compared to those of the mixes rejuvenated by commercial rejuvenators. It is observed that, for the aforementioned properties, 7% to 13% of waste engine oil is identical to the commercial rejuvenator for the mixes with 30% to 40% of reclaimed asphalt pavement.

In road construction, different types of waste oil have been recommended to overcome the stiffening effect of reclaimed asphalt pavement content. However, the selection of an effective rejuvenator based on a comparative study can lead to using the resources more efficiently. In this study, waste cooking oil and waste engine oil are used to rejuvenate three different percentages (30%, 40%, and 50%) of reclaimed asphalt pavement following the current maximum industrial adaptability. The waste oil rejuvenated mixtures are compared to the fresh mixture, and mixtures rejuvenated with commercial rejuvenator. The moisture sensitivity, indirect tensile strength, and resilient modulus of the various asphalt mixtures are analyzed. Based on the statistical analyses and overall ranking, it is concluded that 7% of waste engine oil performs better till 40% of reclaimed asphalt pavement, whereas 13% of waste cooking oil can be used till 50% of reclaimed asphalt pavement.

This study focuses on the ability of waste cooking oil (WCO) to recycle old asphalt pavement into new pavements. The application of WCO was contemplated in the pavement materials to reduce the stiffness effect of reclaimed asphalt pavement (RAP). Following the Marshall's method, a detailed laboratory investigation was conducted to evaluate some of the properties of the WCO rejuvenated mixture and compared with the mixture without any RAP (SM) and the mixtures rejuvenated by a commercial rejuvenator (CR). For this comparison, laboratory tests of indirect tensile strength (ITS), percentage of ITS loss and resilient modulus ( ) were conducted following the respective standard method. Three different percentages (13%, 20% and 27%) of WCO (by RAP binder content) were evaluated for three different percentages of RAP mixture (30%, 40% and 50%). The statistical analysis of variance (ANOVA) was applied to quantify the effect of the WCO and the percentage of RAP on the mentioned properties of the rejuvenated mixture. It is observed that the values for ITS, percentage of ITS loss and of the WCO rejuvenated mixtures are compatible with those of the mixtures rejuvenated by CR and SM. However, the compatibility exists till a certain extent of RAP and WCO, and the increase in the percentage of RAP (>40%) and WCO (>13%) in the mixtures results in a deviation from the observed properties.

The road transportation sector in Saudi Arabia has been observing a surging growth of demand trends for the last couple of decades. The main objective of this article is to extract insightful information for the country’s policymakers through a comprehensive investigation of the rising energy trends. In the first phase, it employs econometric analysis to provide the causal relationship between the energy demand of the road transportation sector and different socio-economic elements, including the gross domestic product (GDP), number of registered vehicles, total population, the population in the urban agglomeration, and fuel price. Then, it estimates future energy demand for the sector using two machine-learning models, i.e., artificial neural network (ANN) and support vector regression (SVR). The core features of the future demand model include: (i) removal of the linear trend, (ii) input data projection using a double exponential smoothing technique, and (iii) energy demand prediction using the machine learning models. The findings of the study show that the GDP and urban population have a significant causal relationship with energy demand in the road transportation sector in both the short and long run. The greenhouse gas emissions from the road transportation in Saudi Arabia are directly proportional to energy consumption because the demand is solely met by fossil fuels. Therefore, appropriate policy measures should be taken to reduce energy intensity without compromising the country’s development. In addition, the SVR model outperformed the ANN model in predicting the future energy demand of the sector based on the achieved performance indices. For instance, the correlation coefficients of the SVR and the ANN models were 0.8932 and 0.9925, respectively, for the test datasets. The results show that the SVR is better for predicting energy consumption than the ANN. It is expected that the findings of the study will assist the decision-makers of the country in achieving environmental sustainability goals by initiating appropriate policies.