1. Introduction

The importance of the environment is not new to anyone. Humans have known for millennia that they are inseparable from the environment in which they live. More recently, the media has been inundated with stories about environmental degradation and the havoc that humans as a species are wreaking. Not a day goes by without headlines about the melting ice caps and the seemingly inevitable warming of the Earth. Nonetheless, good news regarding the environment, both on an individual and on an industrial or governmental scale are also common.

Yet through all the news, it is important to acknowledge the tireless efforts of scientists toiling in laboratories and staring into computer screens for long hours, trying to figure out how to ameliorate the environment, or at least, trying to find ways to diminish the negative impacts. Research on environmental science and engineering might be among the most important on any university campus, and the scientists who have devoted their lives to this cause should be proud.

But what truly constitutes environmental research? One could argue that the answer depends on who you ask and with what lens you look at the problem. In a way, everything can somehow be relevant to the environment. It is -after all- all around us. The origins of the word environment come from Middle English (also as inviron), from Old French environ meaning “surroundings”. The word environ itself is derived from en “in” and viron “circuit” [1].

The environment includes both living (biotic) and non-living (abiotic) components. In other words, it is a complex system including physical, chemical, biological and even social and cultural elements, all linked together. More specifically, the “natural environment” is often defined via four interlinking spheres known as the atmosphere, hydrosphere, lithosphere and biosphere, which will be discussed as follows [2]:

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  The atmosphere is the cover of gases which envelope the planet. Although our ancestors believed that air was a single entity, it is now well known that it is a mixture of various gases. Volumetrically, the air is composed of 78.084% Nitrogen, 20.946% Oxygen, 0.934% Argon, 0.033% Carbon Dioxide, and 0.003% of all other gasses such as Neon, Helium, Methane, and Krypton (in order of most to least abundant). In terms of mass composition, 75.51% and 23.15% of the air are Nitrogen and Oxygen respectively. It should be noted that due to the different mass of the air's constituents, gas concentrations will vary with height. Heavier gases move to the bottom and lighter gases rise up. That is why the mean molecular mass of air is 29 g/mol at the surface of the planet, and decreases to about 18 g/mol at a height of 300 km. Due to varying properties, the atmosphere is subdivided into layers. The lowest and most dense layer containing 80% of all the atmospheric gas is called the troposphere (up to about 10 km). This is where nearly all weather phenomena such as tornados and hurricanes occur. Beyond the troposphere, are the stratosphere (11–30 km), mesosphere (31–100 km), thermosphere (100–400 km) and exosphere (above 400 km) [2].

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  The hydrosphere, as its name suggests, includes all water such as oceans, rivers, and aquifers. More than 70% of the Earth's surface is covered with oceans which act as temperature regulators of the planet. About 97% of water is salty (not suitable for drinking), about 2% is in the ice caps, and less than 1% is readily accessible to humans in the form of freshwater lakes, rivers, and groundwater. This lack of accessible water is the source of water stress on a global scale, leading to discussions regarding better water use and management, as well as tapping into unconventional sources such as the desalination of seawater [3]. Water is constantly transforming and moving in what is known as the hydrological cycle. When water comes into contact with contaminants that render it unsuitable for use, it is referred to as wastewater. Wastewater treatment is an area of widespread investigation and intensive research, both for domestic and industrial wastewaters.

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  The lithosphere consists of the rigid exterior of our planet, including all the mountains, rocks, soils, and land, down to a depth of about 100 km including the crust and the upper mantle. The crust of the Earth which is the outermost layer has varying thickness. It is thinnest under the ocean (5–10 km thick) and several times thicker under the main land mass. If one were to drill a hole through the crust, the temperature would increase by about 30 °C for every km of depth. After drilling through the curst, the Mohorovičić Discontinuity (Moho) is reached which is the boundary layer between the crust and the mantle at about 1500 °C. The Lithosphere–Asthenosphere boundary lies between the cooler, rigid lithosphere and the warmer, ductile asthenosphere which is not considered as being part of the classic four spheres of the natural environment. As one moves deeper within the mantle (2900 km thick), it becomes partially molten as temperatures increase to 2000 °C and beyond. Further down is the outer core with temperatures exceeding 3000 °C. The thickness and temperature of the inner core consisting of heavy elements such as iron and nickel, are approximately 2500 km and 7500 °C respectively [2]. Years ago, I heard a scientist on TV claim that we know far less about the interior of our planet than we do about the surface of the moon, and to date, this bit of trivia still fascinates me.

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  Finally, the biosphere consists of all plants, animals and living organisms in the environment. It spans from the smallest unicellular bacteria living on the cusp of a volcano, to the largest whales deep inside the ocean. The biosphere extends from the troposphere down into the lithosphere to a relatively shallow extent. It is estimated that it contains nearly 400,000 species of plants, millions of animal species, as well as millions of species of microorganisms and fungi. The biosphere can be further broken down into smaller ecosystems. As Encyclopedia Britannica puts it: “an ecosystem can be categorized into its abiotic constituents, including minerals, climate, soil, water, sunlight, and all other nonliving elements, and its biotic constituents, consisting of all its living members. Linking these constituents together are two major forces: the flow of energy through the ecosystem, and the cycling of nutrients within the ecosystem.” Living organisms in ecosystems can be categorized as producers (such as green plants), consumers (such as antelopes or humans), and decomposers (such as bacteria feeding on a corpse).

So far, specific bibliometric studies on subsegments of environmental science and technology have been published, including on climate change [4], plastic pyrolysis [5], green buildings [6,7], disinfection byproducts [8], water-energy-food nexus [9,10], microbial fuel cells [11], groundwater remediation [12], emergy [13], ecosystem services [14], landfill design [15], environmental damage [16], low carbon electricity [17], food waste [18], nano and microplastics [[19], [20], [21], [22]], biodiesel [23], incineration ashes [24], Fenton oxidation [25], electronic wastes [26], soil remediation [27], construction and demolition waste [28], environmental taxes [29], CO2conversion technologies [30], wetlands [31], biomass energy [32], and industrial wastewater [33] just to name a few. Our research team has also published bibliometric studies, specifically on landfill leachate treatment [34] and the removal of heavy metal ions from wastewater [35]. However, for anyone trying to make sense of the totality of the scholarly landscape in environmental science and engineering, an encompassing document is non-existent.

Considering the incredible extent and complexity of the environment, it seems daunting to provide a summary of the most recent scholarly work including all subsegments of the field. Indeed, how shall one go about summarizing such a huge area of research? And what tools are at one's disposal for taking on such a task? For this, a bit of background information is necessary.

2. Methodology

Although Table 2 provides a formidable summary of the most important publications in the field, it suffers from two main drawbacks. First, due to the sheer number of publications it will be extremely difficult to analyze the publications from the entire table. Second, some multidisciplinary publications with peripheral relevance to Environmental Science and Engineering have also made their way to the list.

Admittedly, the selection process for the journals was somewhat subjective. For example, publications specifically focusing on medicine, energy or biology were considered out of scope, and needing an entirely separate manuscript for their investigation. It should also be noted that here, the CiteScore was used to rank the journals. If an alternative metric (such as the SNIP) had been used, the selected list would have been different.

Once the journals to be investigated were selected, it was time to choose the proper tools and methods with which their data was to be analyzed [37]. Fortunately, there are numerous bibliometric software available, many of which are free to use. Each software has its own advantages and disadvantages. The list of some popular bibliometric software is provided below, the first two of which will be further introduced and utilized herein: