Cycles on Earth
Chapter 2 - Society
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Welcome to the Cycles on Earth page
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Key takeaways
- Earth cycles refer to the continuous, interconnected processes that circulate energy and matter between different parts of the Earth system.
- These cycles, such as the energy cycle, carbon cycle, nitrogen cycle, and water cycle, involve the movement and transformation of elements and compounds between living organisms, the atmosphere, the oceans, and the Earth's crust. They are crucial for sustaining life and maintaining the planet's habitability.
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Core ideas
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Cycles are always and everywhere
| Cycles - Daniel Dennett |
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| Everybody knows about the familiar large-scale cycles of nature: day follows night follows day summer-fall-winter-spring-summer-fall-winter-spring, the water cycle of evaporation and precipitation that refills our lakes, scours our rivers and restores the water supply of every living thing on the planet. But not everybody appreciates how cycles —every spatial and temporal scale from the atomic to the astronomic — are quite literally the hidden spinning motors that power all the wonderful phenomena of nature. Nikolaus Otto built and sold the first internal combustion gasoline engine in 1861, and Rudolf Diesel built his engine in 1897, two brilliant inventions that changed the world.Each exploits a cycle, the four-stroke Otto cycle or the two-stroke Diesel cycle, that accomplishes some work and then restores the system to the original position so that it is ready to accomplish some more work. The details of these cycles are ingenious, and they have been discovered and optimized by an R & D cycle of invention that is several centuries old. An even more elegant, micro-miniaturized engine is the Krebs cycle,discovered in 1937 by Hans Krebs, but invented over millions of years of evolution at the dawn of life. It is the eight-stroke chemical reaction that turns fuel — into energy in the process of metabolism that is essential to all life, from bacteria to redwood trees.
Biochemical cycles like the Krebs cycle are responsible for all the motion, growth, self-repair, and reproduction in the living world, wheels within wheels within wheels, a clockwork with trillions of moving parts, and each clock has to be rewound, restored to step one so that it can do its duty again. All of these have been optimized by the grand Darwinian cycle of reproduction, generation after generation, picking up fortuitous improvements over the eons. How did all those seasonal cycles, water cycles, geological cycles,and chemical cycles, spinning for millions of years, gradually accumulate the preconditions for giving birth to the biological cycles? Probably the first thousand "tries"were futile, near misses. But as Cole Porter says in his most sensual song, see what happens if you "do it again, and again, and again." |
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Dive deeper
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The energy cycle
Energy from the Sun is the driver of many Earth System processes. This energy flows into the Atmosphere and heats this system. It also heats the Hydrosphere and the land surface of the Geosphere, and fuels many processes in the Biosphere. Differences in the amount of energy absorbed in different places set the Atmosphere and oceans in motion and help determine their overall temperature and chemical structure. These motions, such as wind patterns and ocean currents, redistribute energy throughout the environment. Eventually, the energy that began as Sunshine (short-wave radiation) leaves the planet as Earthshine (light reflected by the Atmosphere and surface back into space) and infrared radiation (heat, also called longwave radiation) emitted by all parts of the planet, which reaches the top of the Atmosphere. This flow of energy from the Sun, through the environment, and back into space is a significant connection in the Earth system; it defines Earth’s climate.
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The water cycle (the hydrological cycle)
The water, or hydrologic, cycle describes the journey of water as water molecules make their way from the Earth’s surface to the atmosphere and back again, in some cases to below the surface. This gigantic system, powered by energy from the Sun, is a continuous exchange of moisture between the oceans, the atmosphere, and the land.
Water is practically everywhere on Earth. Viewed from space, one of the most striking features of our home planet is the water, in both liquid and frozen forms, that covers approximately 75% of the Earth’s surface. Geologic evidence suggests that large amounts of water have likely flowed on Earth for the past 3.8 billion years, most of its existence. Believed to have initially arrived on Earth’s surface through the emissions of ancient volcanoes, water is a vital substance that sets the Earth apart from the rest of the planets in our solar system. In particular, water appears to be a necessary ingredient for the development and nourishment of life.
In all, the Earth’s water content is about 1.39 billion cubic kilometres (331 million cubic miles), with the bulk of it, about 96.5%, being in the global oceans. As for the rest, approximately 1.7% is stored in the polar icecaps, glaciers, and permanent snow, and another 1.7% is stored in groundwater, lakes, rivers, streams, and soil. Only a thousandth of 1% of the water on Earth exists as water vapour in the atmosphere.
For human needs, the amount of freshwater on Earth—for drinking and agriculture—is significant. Freshwater exists in lakes, rivers, groundwater, and frozen as snow and ice. Estimates of groundwater are complicated to make, and they vary widely. (The value in the above table is near the high end of the range.) Groundwater may constitute anywhere from approximately 22 to 30% of fresh water, with ice (including ice caps, glaciers, permanent snow, ground ice, and permafrost) accounting for most of the remaining 78 to 70%.
Groundwater is found in two broadly defined layers of the soil, the “zone of aeration,” where gaps in the soil are filled with both air and water, and, further down, the “zone of saturation,” where the gaps are filled with water. The boundary between these two zones is known as the water table, which rises or falls as the amount of groundwater changes.
The amount of water in the atmosphere at any moment in time is only 12,900 cubic kilometres, a minute fraction of Earth’s total water supply. If it were to rain out completely, atmospheric moisture would cover the Earth’s surface to a depth of only 2.5 centimetres. However, far more water—in fact, some 495,000 cubic kilometres of it—is cycled through the atmosphere every year. It is as if the entire amount of water in the air were removed and replenished nearly 40 times a year.
Despite its small amount, this water vapour has a significant influence on the planet. Water vapour is a potent greenhouse gas, and it is a major driver of the Earth’s weather and climate as it travels around the globe, transporting latent heat with it. Latent heat is heat obtained by water molecules as they transition from liquid or solid to vapour; the heat is released when the molecules condense from vapour back to liquid or solid form, creating cloud droplets and various forms of precipitation.
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| Global Commission on the Economics of Water (GCEW) |
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| The world faces a growing water disaster. For the first time in human history, the hydrological cycle is out of balance, undermining an equitable and sustainable future for all. |
| https://watercommission.org/ |
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The nitrogen cycle
Plants and animals could not live without the essential element, nitrogen. It makes up many biological structures and processes, such as cells, amino acids, proteins, and even DNA. It is also necessary for plants to produce chlorophyll, which they use in photosynthesis to make their food and energy.
Nitrogen forms simple chemicals called amino acids, the essential building blocks of all proteins, enzymes, and especially DNA. It helps plants use carbohydrates to gain energy, just as certain foods we eat help us to gain energy. Nitrogen controls how plants take their form and how they function inside, and nitrogen helps plants make the protein that helps them grow strong and healthy. Humans and animals benefit from eating vegetables and plants that are rich in nitrogen because proteins are passed on to humans and animals when they eat vegetables and plants.
We might commonly think of Earth as having an oxygen-dominated atmosphere, but in reality, the molecule makes up a little less than 20% our air. Most of what surrounds us is nitrogen, at 78 per cent in the form of diatomic nitrogen gas. The gas itself is very unreactive. Plants and animals cannot absorb the gas directly from the atmosphere. Nitrogen, in the forms of Nitrates (NO3), Nitrites (NO2), and Ammonium (NH4), is a nutrient needed for plant growth. Plants take up nitrogen in the form of nitrate (NO3-) and ammonium (NH4+). Most plants thrive on equal amounts of these ions, but nitrates are more quickly available to plants because they move through the soil solution. In contrast, ammonium ions become fixed or held onto clay particles, called colloids, because of their positive charge.
The nitrogen cycle involves specific processes that change nitrogen into different forms. Unfortunately, these forms of nitrogen are not always used by plants because they either get onto clay particles in soil, leach into the groundwater because the soil cannot absorb them, or change into nitrogen gases that escape into Earth's atmosphere. So, how does nitrogen change states from N2 in the air to these other states so that they are accessible by the Biosphere? Luckily, there are specific kinds of microorganisms living in the soil that can convert gaseous forms of nitrogen into inorganic nitrogen that plants can use.
Specialised bacteria in soil (and certain types of algae in water) can fix nitrogen. These bacteria that cling to roots within the soil convert (or "fix") this inorganic nitrogen into organic forms (ammonia and nitrate ions) that plants can absorb. This process of converting nitrogen to a "biologically available" form - in other words, converting nitrogen gas to a form that plants can use - is referred to as nitrogen fixation. Lightning strikes also result in some nitrogen fixation by splitting the nitrogen molecule into free nitrogen, which immediately reacts with oxygen in the air to form nitrogen oxides. Some of these nitrogen oxide gases dissolve in rainwater and eventually percolate into the soil (Pedosphere). The nutrients needed for plant growth are drawn from the soil by the roots to the leaves. Therefore, any organism (including humans) that consumes the nuts, leaves, seeds, roots, tubercles, or fruits of plants can digest this organic form of nitrogen. The Nitrogen Cycle is the process of moving nitrogen among plants, animals, bacteria, the atmosphere, and soil. This cycle is continuous.
Plants that get too much nitrogen have much foliage (leaf) growth, but are not strong. Plants that are not strong can get diseases more easily, can be bothered more by bugs, and can eventually fall over and die. An excess amount of nitrogen in plants can affect the amount of sugar and vitamins in fruits and vegetables, making them taste different. More importantly, excess nitrogen can build up in plant tissues, causing toxicity (poisoning) in livestock and in small children who eat nitrogen-rich, leafy vegetables. As we produce synthetic fertilisers, burn fossil fuels, grow legumes such as soybeans as a crop (which fix nitrogen), and clear, burn, and drain wetlands, we release nitrogen in forms that plants use. We have made the amount of biologically available nitrogen through human activity much greater than the nitrogen fixed by bacteria, algae, and lightning.
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The carbon cycle
Carbon is a fundamental part of the Earth system. It is the backbone of life on Earth. We are made of carbon, we eat carbon, and our civilisations—our economies, our homes, our means of transport—are built on carbon. Forged in the heart of ageing stars, carbon is the fourth most abundant element in the Universe. Most of Earth’s carbon—about 65,500 billion metric tons—is stored in rocks. The rest is in the ocean, atmosphere, plants, soil, and fossil fuels. Carbon moves from the atmosphere to the land, ocean, and life through biological, chemical, geological and physical processes in a cycle called the carbon cycle. Any change in the cycle that shifts carbon out of one reservoir puts more carbon in the other reservoirs. Carbon flows between each reservoir in slow and fast cycles.
On very long time scales (millions to tens of millions of years), the movement of tectonic plates and changes in the rate at which carbon seeps from the Earth’s interior may change the temperature on Earth’s thermostat. Earth has undergone such a change over the last 50 million years, from the extremely warm climates of the Cretaceous (roughly 145 to 65 million years ago) to the glacial climates of the Pleistocene (approximately 1.8 million to 11,500 years ago).
The movement of carbon from the atmosphere to the Geosphere (rocks) begins with rain. Atmospheric carbon combines with water to form a weak acid—carbonic acid—that falls to the surface in rain. The acid dissolves rocks—a process called chemical weathering—and releases calcium, magnesium, potassium, or sodium ions. Rivers carry the ions to the ocean. Rivers carry calcium ions—the result of chemical weathering of rocks—into the ocean, where they react with carbonate dissolved in the water. The product of that reaction, calcium carbonate, is then deposited onto the ocean floor, where it becomes limestone. In the ocean, the calcium ions combine with bicarbonate ions to form calcium carbonate, the active ingredient in antacids and the chalky white substance that dries on your faucet if you live in an area with hard water. In the modern ocean, most of the calcium carbonate is made by shell-building (calcifying) organisms (such as corals) and plankton (like coccolithophores and foraminifera). After the organisms die, they sink to the seafloor. Over time, layers of shells and sediment are cemented together and turn to rock, storing the carbon in stone—limestone and its derivatives. Limestone, or its metamorphic cousin, marble, is a rock made primarily of calcium carbonate. These rock types are often formed from the bodies of marine plants and animals, and their shells and skeletons can be preserved as fossils. Carbon locked up in limestone can be stored for millions—or even hundreds of millions—of years. Only 80 per cent of carbon-containing rock is currently made this way. The remaining 20 per cent contains carbon from living things (organic carbon) that have been embedded in layers of mud. Heat and pressure compress the mud and carbon over millions of years, forming sedimentary rock such as shale. In exceptional cases, when dead plant matter builds up faster than it can decay, layers of organic carbon become oil, coal, or natural gas instead of sedimentary rock like shale. This coal seam in Scotland was originally a layer of sediment, rich in organic carbon. The sedimentary layer was eventually buried deep underground, and the heat and pressure transformed it into coal. Coal and other fossil fuels are convenient sources of energy, but when they are burned, the stored carbon is released into the atmosphere. This alters the balance of the carbon cycle and is changing Earth’s climate.
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| Content source |
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| NASA |
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What science can tell you
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| Preston Cosslett Kemeny - Balance and imbalance in biogeochemical cycles reflect the operation of closed, exchange, and open sets - EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES - 2024 |
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| Biogeochemical reactions modulate the chemical composition of the oceans and atmosphere, providing feedbacks that sustain planetary habitability over geological time. Here, we mathematically evaluate a suite of biogeochemical processes to identify combinations of reactions that stabilize atmospheric carbon dioxide by balancing fluxes of chemical species among the ocean, atmosphere, and geosphere. Unlike prior modeling efforts, this approach does not prescribe functional relationships between the rates of biogeochemical processes and environmental conditions. Our agnostic framework generates three types of stable reaction combinations: closed sets, where sources and sinks mutually cancel for all chemical reservoirs; exchange sets, where constant ocean–atmosphere conditions re maintained through the growth or destruction of crustal reservoirs; and open sets, where balance in alkalinity and carbon fluxes is accommodated by changes in other chemical components of seawater or the atmosphere. These three modes of operation have different characteristic timescales and may leave distinct evidence in the rock record. To provide a practical example of this theoretical framework, we applied the model to recast existing hypotheses for Cenozoic climate change based on feedbacks or shared forcing mechanisms. Overall, this work provides a systematic and simplified conceptual framework for understanding the function and evolution of global biogeochemical cycles |
| https://www.pnas.org/doi/10.1073/pnas.2316535121 |
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