Hydrosphere
“Water is life’s mater and matrix, mother and medium. There is no life without water.” ~ Hungarian physiologist Albert Szent-Gyorgyi
Calling this planet Earth is a misnomer. Ours is a watery world. Water’s abundance has been a most significant factor in shaping the planet’s geophysical history.
On early Earth a steamy atmosphere cooled and condensed. Oceans formed from reservoirs in the mantle; possibly by 4.4 BYA; certainly by 3.8 BYA. The water was quite hot, and richer in hydrogen than modern oceans. By 3.4 BYA, ocean waters had cooled to 37 ºC.
H2O on Earth now exists as atmospheric vapor, ice, running water on land, and salted seawater (3.5% salinity, on average). Water reservoirs include the oceans, glaciers, and groundwater.
Groundwater is the deepest and most massive water store. Sponged up in the Earth’s interior is at least 25 times the water in the oceans.
“Water is as crucial to the workings of Earth’s interior as it is to Earth’s surface processes. Among other things, it triggers magma generation beneath volcanoes, lubricates deep fault zones, and fundamentally alters the strength and behaviour of Earth’s mantle.” ~ American Earth scientist Donna Shillington
Rainwater makes its way into the deep crust where it is heated and pressurized, altering tectonic dynamics. Water sews together the Gaia gyre.
75% of the Earth’s surface is covered by water; overwhelmingly ocean (72%), equivalent to 1.5 billion cubic kilometers, which is 100 times the volume of terrestrial habitats.
In the continents’ current configuration, the Pacific Ocean covers nearly half the Earth’s surface. 81% of the southern hemisphere is ocean, but only 61% of the northern hemisphere.
“About half of the world’s ocean is extremely nutrient-poor.” ~ German oceanographer Jens Kallmeyer
2% of the 3% of Earth’s surface water that is not oceanic is locked away as ice: in glaciers, ice fields, and snow that coat the polar regions. Antarctica has locked away 75% of the planet’s fresh water; at least for now.
Releasing that frozen storage to further flood the oceans would raise sea levels enough to cover the majority of human populations, which predominantly reside in coastal areas. Ice melts at the poles over the past few decades render dramatic uplift in sea level a certainty. Global warming is a gyre with feedback loops that guarantee accelerating sea-level rise much more quickly than humans will be prepared for.
0.5% of the water left is groundwater. 0.01% lies in lakes and rivers, and aloft in the atmosphere. On the surface, lakes contain 20 times the freshwater that is in all the world’s rivers.
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Lovelock observed that the oxygenating work of cyanobacteria stabilized water’s role on Earth, preventing the planetary dehydration that would have otherwise occurred, and probably has on other planets. By switching Earth’s atmosphere from a heavy carbon dioxide component to oxygen, cyanobacteria managed the greenhouse effect by maintaining a cool surface. If the surface heated substantially, such as by volcanic emission of CO2, life flourished in the warm, wet world, restoring equilibrium.
Conversely, if too much carbon was sequestered, such as in limestones, or down subduction zones, the planet cooled to an ice age. Life takes a hit, whereupon volcanic return raises the CO2 level.
Dynamic environmental regulation afforded the abundant water that has been crucial to life on Earth via an intricate interweave of the geological and biological. Biota play an active part in shaping the atmosphere and hydrosphere. Microbes and plants played a prominent role in the air and water cycle before industrialized man began to tear Nature asunder.
Microbes in the Ocean
“A bewildering swirl of tiny creatures dominates life in the oceans. More numerous than the stars in the universe, these organisms serve as the foundation of all marine food webs, recycling major elements, and producing and consuming about half of the organic matter generated on Earth.” ~ American marine biologists Virginia Armbrust & Stephen Palumbi
Microorganisms permeate Earth’s oceans. Microbial communities both drive the world’s biogeochemical dynamics and respond to changes in the environment.
“Microbes are responsible for virtually all the photosynthesis that occurs in the ocean, as well as the cycling of carbon, nitrogen, phosphorus and other nutrients and trace elements. They literally run the oceans.” ~ American marine biologist Mary Ann Moran
In providing the fundamental productivity of the oceans, marine plankton are crucial to the planetary gyre of life. As well as playing an integral role in the global carbon cycle, phytoplankton provide half of the atmospheric oxygen generated, with land plants producing the other half.
“Marine bacteria influence Earth’s environmental dynamics in fundamental ways. These large-scale consequences result from the combined effect of countless interactions occurring at the level of the individual cells. At these small scales, the ocean is surprisingly heterogeneous, and microbes experience an environment of pervasive and dynamic chemical and physical gradients. Many species actively exploit this heterogeneity, while others rely on gradient-independent adaptations.” ~ American microbial ecologist Roman Stocker
The top 60 microns of the ocean is a lively layer of microbial life. The microlayer communities that literally cover the oceans are dominated by microbes that form biofilms. This creates a durable skin housing phytoplankton and others. Storms disrupt continuity, but the miniature mariners ride it out.
Phytoplankton do more than just ride out turbulence. They use it to congregate, going with the flow to swim to social interactions.
“Cell motility can prevail over turbulent dispersion to create strong fractal patchiness, where local phytoplankton concentrations are increased more than 10-fold.” ~ English aquatic microbial ecologist William Durham et al
The microbial microlayer is crucial to the ocean absorbing carbon dioxide. Pollutants, such as pesticides, can become trapped there, disrupting the microbial congregation and changing its local composition. Unseen ocean ecosystems are more fragile than has been appreciated.
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Water is what enables a pliable crust to form and allows the tectonic plate system to operate. This includes subduction zones, along with granitoid intrusions and andesite volcanoes, which maintain the continents.
If the water evaporated, carbon dioxide would degas and the plate system would be replaced by a plume-led dynamic. Earth would be a much different world, like Venus or Mars.
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Seawater constantly sways to imposed rhythms. Surface waters respond to the drag of prevailing winds and the rotation of Earth by forming huge circulatory gyres. Just below are currents that reflect local and seasonal conditions. In the deep are abiding currents which span much of the globe.
Ocean Circulation & Climate
Ocean circulation impacts climate and vice versa in a worldwide gyre.
Eddies
Globally, ocean currents form loops and vortices, with eddies that can create variations in flow. Ocean eddies may be 500 km across at the surface and reach all the way to the bottom of the ocean. These massive eddies transport all manner of matter and thermal energy over long distances. The swirl of ocean eddies mirrors those of black holes.
“The boundaries of water-carrying eddies satisfy the same type of differential equations that the area surrounding black holes do in general relativity.” ~ American mathematician George Haller
Ocean eddies are weather makers: locally affecting near-surface winds, clouds, and rainfall patterns. A slackening cyclonic eddy takes the sail out of near-surface winds, which lessens cloud formation, and thereby reduces rainfall.
With climate change comes shifts in eddies. Altering these gyres will transport marine species to new areas. Reductions in polar sea ice will introduce life that originated in the Pacific and Atlantic into Arctic and subarctic oceans, thereby accelerating biogeochemical cycles. Severe disruption to marine ecosystems is inevitable with the current rapidly changing climate.
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Numerous dynamics affect currents in the oceanic gyre: Earth’s rotation and latitudinal variations; ocean basin and landmass configurations; water temperature, salinity, and density; and wind.
Ocean Conveyor Belt
Seawater density varies with salinity and temperature. Density changes drive current movement in the deep ocean; a phenomenon known as thermohaline circulation.
Seawater sinks as it becomes denser. On a global scale, this cooling and sinking create an ocean conveyor belt. The ocean conveyor belt is a continuous system. It does not begin or end anywhere.
As warm surface current gets colder and denser, it sinks: deep-water formation. Conversely, upwelling currents arise in the Southern Ocean, around Antarctica, as part of the Antarctic Circumpolar Current, where there is also deep-water formation.
The ocean conveyor belt that exists now was established 10–15 mya. In the absence of deep-ocean circulation, abyssal waters became stagnant or anoxic at times.
Stagnancy was common when continents were more concentrated, with less flow around land masses. Such instances led to marine mass extinction events.
If ocean temperature warms along the west coast of Africa, monsoon rainfall dramatically drops, affecting sub-Saharan Africa. Warming the cold North Atlantic causes the band of rainfall in the tropics to shift south, drying out northern South America and wetting the South Atlantic.
Sunshine warms the tropics more than near the poles, creating a thermal gyre. The oceans and atmosphere work together as a planetary thermostat, exporting heat from equatorial to polar regions.
Near the equator, the surface of the ocean is warm, floating over colder deep water. Warm currents, such as the Gulf Stream, porter the warmth toward the poles.
Warmth hastens the precipitation cycle, all else being equal. Whence tropical rainforests; though, once established, the forests themselves engender rain.
The northern hemisphere is warmer and much wetter than the southern half of the planet; this despite the southern hemisphere receiving more sunlight, and thereby basking in more solar energy. By atmospheric radiation alone, the southern hemisphere should be hotter and soggier.
The seeming temperature anomaly is because of the ocean conveyor belt. Warm surface water flows north from the Southern Ocean and sinks near Greenland as it cools. This cold water then travels along the benthic zone back to Antarctica. The cold-water upwells via the Antarctic Circumpolar Current.
Thus, the ocean conveyor belt regulates the world’s temperature. Europe is warmed by this circulation. A strong reduction in this gyre can cause widespread cooling by up to 10 ºC.
For their vicious winds, the rough waters around Antarctica are called the “Roaring Forties” and the “Furious Fifties” by sailors. The winds that cause such turbulent waves originate near the equator, where hot air rises and is pushed toward the poles by cooler air that rushes in to take its place. Earth’s rotation ensures that the heat transfer is spirally distributed rather than heading straight for the poles.
Near Antarctica, eddy-driven upwelling occurs, provoked by the wild winds. Water and wind join in a gyre that guides global climate.
A glitch in the ocean conveyor belt – weakening the flow in the northern hemisphere – would render dramatic changes in tropical rainfall patterns. Today’s global warming, especially polar deglaciation, makes this inevitable.
The ocean is the planet’s great heat sink. Having undergone prolonged cooling over millennia, temperatures in the deep Pacific Ocean are now rising faster than any time in the past 10,000 years.
Water Cycle
As water is the medium for all cellular chemical reactions, clean water is essential to all life.
Surface water on Earth is held in a massive aquatic reservoir (the oceans), with a smaller reserve on land (glaciers, rivers, and lakes).
The upper mantle holds at least 3.5 times as much water as the oceans, subducted in deep-sea fault zones.
“The subduction zones at which the tectonic plates beneath the sea thrust into the deep Earth act as gigantic conveyer belts, carrying water, fluids, and volatile compounds into our planet. Water in Earth’s interior is released back into the oceans and atmosphere by volcanoes. These inputs and outputs constitute a global deep-Earth water cycle.” ~ Donna Shillington
Earth’s water is incessantly in motion, and constantly mixed. Water evaporates from the ocean, then precipitates over land as rain, sleet, or snow; returning to the ocean through runoff and river flows.
The hydrological cycle (water cycle) circulates water through the biosphere, most actively the atmosphere. Via atmospheric exchange of water vapor, the water cycle is primarily a gaseous cycle. Water lingers in the atmosphere as vapor for 9–10 days.
Atmospheric bands of water vapor flow like rivers in the sky. These atmospheric rivers develop because of the temperature differences between Earth’s poles and tropics.
At any given time, 3 to 5 atmospheric rivers ferry water in each hemisphere. These rivers are over 1,000 km long and often no wider than 400 km, carrying in water vapor the equivalent of what flows at the mouth of the Mississippi river.
Water cycles from the atmosphere by precipitation: rain, sleet, snow, and hail. Each form has its own dynamics.
A single drop of water from a raincloud multiplies as it descends. Air resistance on an accelerating drop increases until the cohesive forces holding the drop together are overcome. The drop bursts into a shower of droplets.
That same friction that tears raindrops apart is lessened by the rain. The friction of falling raindrops dissipates wind. Rainfall soothes the atmosphere.
The power of precipitation is considerable. Globally, raindrops rip friction at a rate of 1015 watts. The rate of energy dissipation from worldwide precipitation is 100 times that of human energy consumption.
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Water cycles into the atmosphere via evaporation and transpiration. Evaporation is a taking, transpiration a giving.
Evaporation is the kinetic energy excitement of surface water molecules busting a move into vapor. As with many energetic transactions on Earth, the inspiring culprit is the Sun.
Oceanic evaporation is the source of most rainfall. But all surfaces are sources of evaporation: soils, bodies of water, and the bodies of organisms.
Evaporation and precipitation over land and ocean differ. Excess ocean evaporation is carried by the wind onto land, where more precipitation occurs than evaporation. 20% of global rainfall is over the oceans; 80% lands on land, where it is taken up by vegetation, stored in terrestrial reservoirs, or runs off into the oceans.
Transpiration comes from plant release of water; especially leaves, but also flowers, stems, and roots. Plant leaves are dotted with stomata: pores whose opening is regulated by guard cells.
Plants move water from the soil to leaves for photosynthesis. 90% of the water that plant roots absorb transpires.
Plants flow mineral nutrients from roots to shoots. In the finale of transpiration, plants let go of water to cool themselves. Only 1% of the transpired water that passes through a plant is used in the growth process.
A mature oak tree, fully leafed, transpires 4 tonnes of water a day in warm, dry weather. Globally, 10% of all water evaporation comes from transpiration.
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Over 35% of the forests in the world are in the humid tropics. Forests, especially tropical forests, promote rain. Tiny aerosols put out by rainforest inhabitants – especially trees – seed raindrop formation. Further, 25–56% of the rainfall in a tropical rainforest is recycled within the ecosystem.
Tropical rainforest is a gyral truism. Deforestation reduces rainfall for thousands of kilometers beyond the forest.
From 1970 to 2012, 20% of the Amazon rainforest was cleared. By 2050, a further loss of at least 40% is likely. Under that scenario, rainfall across the Amazon basin will drop over 20%.
Such disruption by itself would shift global rain patterns and promote aridity on other continents. The effect is accelerated with global warming.
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On land, gravity pulls water down into the ground, until the soil is saturated: the spaces between soil particles full of water. The level of ground saturation is the water table; the water found there: groundwater.
Groundwater provides 95% of the world’s supply of fresh water. If the land lies below the water table, surface water is present as wetlands, lakes, bogs, swamps, and streams.
Water is a restless medium: constantly moving. Groundwater recharges lakes, streams, and rivers. Some groundwater goes deep, recharging geologic pools of water: aquifers. Water running off to the sea forms watersheds, also termed catchments or drainage basins. In flowing back to the sea, river runoff closes the water cycle.
Owing to water’s restless nature, groundwater pollution quickly diffuses. If a part of a watershed becomes polluted, the entire watershed is debased in short order.
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Changes to the global water cycle alter precipitation patterns and redistribute freshwater on a geographically large scale. Such global water cycle changes have been apparent since 1950. The upshot is more extreme weather in many areas.
Since the mid-1990s, melting ice near the poles has changed the shape of the Earth: putting a bulge of ocean near the equator. This is merely an acceleration of a trend that begun in the mid-19th century, as industrialization got underway.
“The water cycle is intensifying quickly under global warming – twice as fast as climate models have been predicting.” ~ American science writer Richard Kerr
Continental Shelves
The land meets the ocean on continental shelves. Continents are surrounded by shelves that gently slope at an angle of 1° to 3°. These shelves are generally formed of young sediments from the land.
Continental shelves are covered by shallow seas, creating a zone rich in marine life. The North Sea and Persian Gulf overlie continental shelves.
Continental shelves cover 7% of the total seafloor. Averaging 80 km, they may be much wider. The Siberian Shelf in the Arctic Ocean stretches to 1,500 kilometers. The South China Sea lies over the Sunda Shelf, upon which Borneo, Sumatra, and Java sit; another expansive shelf.
Wide, shallow shelves, such as the Atlantic coasts, are situated on passive continental margins. These are made of thick sedimentary wedges, derived from extended erosion of a neighboring continent.
In contrast, active continental margins have narrow, steep shelves, owing to frequent earthquakes that shovel sediment into the deep sea. Off the coast of Peru, the Nazca plate subducts into the Peru-Chile trench.
The outer limit of a continental shelf is marked by a sudden slope change, from 3° to 30°. This where the continental slope plunges toward an abyssal plain, home of the deepest oceans.
Continental slopes cover 9% of the ocean floor. Such slopes are dark places of high pressure, marked by various terrain, including sediment slumps and canyons, carved from sediment-laden turbidity currents, which are sometimes an extension of a river system.
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Abyssal plains occupy most of the ocean floors: where pitch black reigns over high pressure and minimal sedimentation, which chiefly consists of fine clay particles and marine snow: the organic detritus of oceanic microorganisms.
Mid-ocean ridges are long, linear, oceanic mountain chains. These regions are subject to intense geological activity from tectonic plates moving apart (divergent boundaries). In contrast, trenches are deep troughs, typically at the margins of continents, where a crustal slab is subducted into the mantle.
At every depth, the sea floor has varied terrain and geological features, such as seamounts and reefs, as well as trenches and other seabed carvings. The same forces that shape terra firma work in the dark on the ocean floor; only water does all the work there, whereas wind lays its hand on the land.
