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This paper aims to define the effects of military expenses and renewable energy consumption on carbon dioxide emissions for the ten countries with the highest military expenses, namely, Saudi Arabia, Israel, Russia, the USA, South Korea, India, France, Australia, China, and the UK from 1993 to 2017. The research applied the common correlated effects mean group estimator (CCEMG), dynamic CCEMG, and cross-sectional augmented autoregressive distributed lag (CS-ARDL) approaches. These dynamic techniques elucidate slope heterogeneity and cross-sectional dependency and solve the problem of unit root bias. It is found that the environmental Kuznets curve (EKC) hypothesis does not apply for this region. The findings demonstrate that military expenses increase carbon dioxide emissions; thus, the treadmill theory of destruction is valid for the panel of these countries, and it is also found that the consumption of sustainable energy decreases CO2 emissions. This suggests that a reduction in pollution can be achieved by increasing sustainable energies in the use of military vehicles to decrease emissions. Further important policy implications for the 10 countries with the highest military expenses are provided at the end of the paper.

Energy stocks have become an essential segment of the investment portfolios of both households and institutional investors. This study investigates the dynamic aspect of evolving weak-form efficiency in six energy stock markets: those of the United States (US), Canada, China, Australia, India, and Saudi Arabia. The generalized autoregressive conditionally heteroskedastic in the mean GARCH-M(1,1) method is applied, alongside the state-space time-varying approaches with the Kalman filter estimation, to detect the evolving efficiency for periods ending in November 2019. The empirical results reveal that the studied markets undergo various extents of time-varying efficiency, containing periods of efficiency enhancement as well as periods of deviation from efficiency. Meanwhile, the 2007-2009 global financial crisis and the 2015 changes in the energy sector-in addition to other contemporaneous crises-have a profound influence on the timeline of market efficiency evolution. Overall, all of the markets gradually became more efficient, apart from India's energy market as a result of the current energy crisis in India. Amid the energy markets explored in this study, the US energy market was found to be the most efficient.

In this paper, we revisit the environmental Kuznets curve (EKC) hypothesis by using estimations that account for cross-sectional dependency (CSD) and asymmetry effect in 76 countries for the period 1971-2014. Our results lend moderate support to the EKC hypothesis. The country-specific results unfold that a total of 16 out of 76 countries support the EKC hypothesis using CCEMG estimator. Results from AMG reveal that the EKC hypothesis holds in 24 out of 76 countries. It is worth highlighting that 11 countries (Australia, China, Congo Dem. Rep., Costa Rica, Gabon, Hong Kong, India, Korea, Myanmar, Turkey, and Uruguay) exhibit an inverted U-shaped curve regardless of whether CCEMG or AMG is used. The asymmetry analysis using PMG is also able to support the EKC hypothesis. We conclude that the EKC hypothesis does not fit all countries. Policy implication and recommendation in designing appropriate energy and economic policies are provided.

In recent years, climate change and carbon sinks have been widely studied by the academic community, and relevant research results have emerged in abundance. In this paper, a scientometric analysis of 747 academic works published between 1991 and 2018 related to climate change and carbon sinks is presented to characterize the intellectual landscape by identifying and revealing the basic characteristics, research power, intellectual base, research topic evolution, and research hotspots in this field. The results show that ① the number of publications in this field has increased rapidly and the field has become increasingly interdisciplinary; ② the most productive authors and institutions in this subject area are in the USA, China, Canada, Australia, and European countries, and the cooperation between these researchers is closer than other researchers in the field; ③ 11 of the 747 papers analyzed in this study have played a key role in the evolution of the field; and ④ in this paper, we divide research hotspots into three decade-long phases (1991-1999, 2000-2010, and 2011-present). Drought problems have attracted more and more attention from scholars. In the end, given the current trend of the studies, we conclude a list of research potentials of climate change and carbon sinks in the future. This paper presents an in-depth analysis of climate change and carbon sink research to better understand the global trends and directions that have emerged in this field over the past 28 years, which can also provide reference for future research in this field.

The oxidation of sulfide ores is a common phenomenon. To better understand the current development and status of oxidation studies of sulfide ores (OSSO), a bibliometric analysis of OSSO was conducted by mapping the spatiotemporal distribution of the knowledge domain and the research focus using VOSviewer and Citespace tools. The data were derived from the Web of Science (WOS) core collection database from 2000 to 2018. Study emphases covered publication outputs, countries/regions, organizations, top journals, research focus and keyword co-occurrence network, and theme development. The results include the following findings: (1) The line of the 3-year moving average of publications (3-year MAP), h-index (3-year MAH), and authors (3-year MAA) increased from 2001 to 2018. Conversely, the h-index continuously declined. (2) Asia had the most publications, with 1052, followed by Europe, with 923, and America, with 767. China, the USA, and Australia are the most active countries. (3) The top 10 organizations with the most publications are five Chinese organizations and one organization from each of the following countries: Russia, Australia, France, and India. (4) Hydrometallurgy, Minerals Engineering, Ore Geology Reviews, Geochimica Et Cosmochimica Acta, and Economic Geology are among the top 10 journals that researchers are most concerned about. (5) Cooperation among different organization or different countries is the most effective way to produce the most influential papers. (6) The OSSO is still focused on the process of oxidation by using different methods and techniques. In future work, it is necessary to progress new methods to understand the process of self-heating and prevent spontaneous combustion disaster of sulfide ore which result from OSSO.

Biochar has been paid great attentions during the last two decades, because of its resources potentials and environmental benefits. A bibliometric analysis was applied to assess the publications regarding the keyword biochar from the Web of Science database during the period of 1999 to 2018. A total of 8629 publications were obtained with a rapid increase of annual citations and number of papers. The research topics were diversified, which were mainly divided into "Environmental Sciences and Ecology," "Agriculture," and "Engineering." Bioresource Technology was the journal which published most of the relevant papers. China ranked first in the number of published papers, followed by the USA, Australia, UK, and Germany. Especially, China established close collaboration with the USA in joint publication. Analysis of the keywords indicated that biochar production, comparative sorption, soil-applied black carbon, and soil management were the main research hotspots of biochar. The burst detection reflected that the innovation of biochar production and the new application field of biochar was the future research trends. These results can provide insight into the research progress regarding biochar.