3.1. The Earth as a system

As discussed in the beginning of this course, the Earth can be seen as a closed system of matter, comprising a myriad of different, interdependent and interacting subsystems.

The science that studies the most large-scale processes on Earth is called Earth System Science.

Earth System Science studies the processes underlying the circulation of matter and energy on a planetary scale - including changes in the radiant energy from the Sun to the Earth (this was covered in the Introduction to Planetary Well-being course) - and the phenomena that result from these processes. Particularly relevant are changes in the composition of the atmosphere and the Earth's energy balance, which have resulted in large variations in the Earth's temperature over time.

Components of the Earth System

How the Earth system is modelled depends on the phenomenon or process being studied. There is no single 'right' model of the Earth system, but it is advisable to choose a model that includes the relevant components/elements and their interactions for the phenomenon or process in question. Nevertheless, the main components of the Earth system are generally considered to be the lithosphere, hydrosphere, biosphere and atmosphere. In addition to these, the models may also include the cryosphere (frozen water) and the anthroposphere/technosphere (the part of the environment built and shaped by humans). 

The lithosphere is the Earth's outermost solid sphere, consisting of the Earth's crust and the upper part of the mantle beneath it. The thickness of the lithosphere is about 50-140 km for oceans and 40-280 km for continents. Beneath the rocky crust is a partially molten mantle. The movement of the molten rock in the mantle causes the continental plates to move slowly, which over long periods of time affects the positioning of continents and oceans on the Earth. The movement of continental plates also causes earthquakes and volcanic eruptions. 

The lithosphere contains large amounts of carbon bound in minerals, but also stocks of fossil carbon (coal, oil and natural gas). Lithospheric processes are generally very slow, but the use of fossil fuels has increased the amount of carbon transferred from the Earth's crust to the atmosphere each year approximately 50-fold. (If you wish to put this into systems jargon, you could say the process of the anthroposphere have increased the flow of carbon from the lithosphere to the atmosphere approximately 50-fold).

The hydrosphere consists of all liquid water on Earth, including oceans, lakes, rivers and groundwater. Water has a high specific heat capacity, so the hydrosphere can store large amounts of heat energy. As shown in the figure below, the oceans have absorbed about 90% of the increase in global thermal energy caused by humans over the past decades. The thermal expansion of the oceans is a major contributor to sea level rise.

Figure. The increased concentration of greenhouse gases in the atmosphere has increased the Earth's heat energy by 358 zettajoules from 1971 to 2018. 89% of this heat energy has been transferred to the oceans (52% to depths of 0-700 m, 28% to depths of 700-2000 m and 9% to depths greater than 2 km), 6% to the ground and 4% to melting glaciers. 1% is retained in the atmosphere. To balance the Earth's energy budget (i.e. equal amounts of incoming and outgoing energy), the atmospheric carbon dioxide content would have to decrease by 57 parts per million (in 2020 the atmospheric carbon dioxide content was 410 parts per million). Source Von Schuckmann et al. (2020).

The cryosphere refers to the ice and snow on Earth, in other words, the solid state of water. The cryosphere includes ice in seas, rivers and lakes, continental and mountain glaciers, snow, frozen ground and permafrost. The vast majority of the Earth's fresh water is contained in glaciers, and a complete melting of the glaciers would raise the level of the oceans by about 80 metres. Of the increase in global thermal energy over the last decades, 3-4% has been absorbed by melting glaciers (see figure above). Glaciers and the seasonal snow cover in the northern hemisphere also have a significant impact on the Earth's energy balance, as ice and snow are highly effective at reflecting solar radiation back into space.

The biosphere includes all living organisms and their interactions with non-living nature. The biosphere is spread all over the planet: life is found in the ocean depths, high in the atmosphere, in the soil, even in ice and bedrock. Living things have a major influence on the cycling of many elements on Earth, and life has, for example, shaped the composition of the atmosphere very significantly over the course of Earth's history. Soil, consisting of minerals, dead organic matter, water, gases and living organisms, can also be considered part of the biosphere. 

Although humans are living beings and therefore part of the biosphere, their impact on the rest of the biosphere and the Earth system as a whole is so enormous that it is also justified to speak of the anthroposphere, which includes human constructs and impacts on the rest of the planet. As we saw in the Introduction to Planetary Well-being course, man-made structures are already larger in mass than the entire biosphere and human impact on the circulation of matter and energy on the planet is so enormous that we are already talking about a new geological period, the Anthropocene.

The atmosphere is the gaseous envelope around the Earth, consisting of nitrogen (78%), oxygen (21%), argon (0.9%), carbon dioxide (0.04%) and other gases. The atmosphere also contains water vapour (about 0.4%), but the amount of water vapour varies greatly depending on time and place. The upper layers of the atmosphere protect the Earth from harmful ultraviolet radiation. Greenhouse gases in the atmosphere (including carbon dioxide, methane and nitrous oxide) heat the Earth's surface by trapping energy from the Earth's thermal radiation. Atmospheric oxygen and carbon dioxide are essential for existing life forms on Earth. 

Carbon in the Earth system

If the Earth system is to be described in terms of the cycles and stocks of one substance, that substance is carbon. (There are, of course, also other very important cycles of substances, such as water, nitrogen and phosphorus, but we will not cover them in this course.) Carbon is the main building block of all living organisms, since all organic molecules contain carbon. Carbon dioxide in the atmosphere warms the Earth - without carbon dioxide, the average temperature of the Earth would be about 30 degrees lower than it is today. Carbon dioxide is also the main raw material for photosynthesis in plants and is therefore essential for the existence of the entire biosphere. Fossil carbon reserves are the main source of energy for humanity; without the exploitation of fossil fuels, the industrial revolution would not have taken place and societies would be very different from what they are today.

The figure below shows carbon stocks and flows in the Earth system during 2011-2020. The vast majority of all carbon on the planet is sequestered in sedimentary rocks in the ground and seafloor. The cycle of carbon sequestered in sedimentary rocks is very slow and its stocks or cycles are not included in the figure (however, carbon dioxide released in cement production and sequestered in concrete is included). The second largest carbon stock is inorganic carbon compounds dissolved in the ocean, such as carbonate (CO3-2) and bicarbonate (HCO3-). Other carbon stocks are two to three orders of magnitude smaller - their magnitude is shown in the figure.

Figure. Global annual averaged carbon cycle 2011-2020. The spheres represent the size of carbon stocks. Thick arrows represent anthropogenic effects on the carbon cycle and thin arrows represent the cycle in natural processes. Emissions from fossil energy use were 9.5 Gt (gigatonnes) and emissions from land use were 1.1 Gt. Of these emissions, 3.1 Gt were sequestered in terrestrial ecosystems and 2.8 Gt in the oceans. The atmospheric carbon stock increased by an average of 5.1 Gt per year. Source Friedlingstein et al. (2021).

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