The Earth is made up of a series of components – the hydrosphere, atmosphere, lithosphere, and biosphere. These function as systems that are constantly interacting and adjusting to both internal and external factors. It is the continuous alterations to these cycles that produce the environmental conditions that we experience. This section explores the key characteristics and dominant drivers of these Earth system cycles.
The hydrosphere is a defining characteristic of the Earth. It is the only planet in the Solar System with a hydrological cycle. This includes: oceans; freshwater (rivers, lakes, and groundwater); and the cryosphere (where water exists as a solid – ice or snow – such as within ice sheets, glaciers and permafrost/frozen ground. These are also major stores of freshwater). The hydrosphere and cryosphere are also frequently recognised as two separate ‘spheres’. Together, all of these water sources are vital for almost all forms of life on Earth, and they are the reason that Earth is often termed the ‘water planet’. For more detailed information regarding the extent and distribution of the cryosphere visit the Snow and Ice Data Center.
The atmosphere is the thick layer of gaseous material which surrounds the Earth. Most of the atmosphere lies within 97 km of the Earth’s surface, but it has no definite boundary. The atmosphere is divided into four layers, the Troposphere (0-10 km altitude), the Stratosphere (10-45 km altitude), the Mesosphere (45-80 km altitude), and the Thermosphere (80-300 km altitude). The atmosphere comprises: 78.09% nitrogen, 20.95% oxygen, 0.93% argon, 0.039% carbon dioxide and very small amounts of other gases. These gases are often termed ‘greenhouse gases’ as they are delicately balanced to maintain the Earth’s temperature at a level suitable to sustain life. Without these gases, and the natural greenhouse effect, the average temperature of the Earth would be approximately -18°C (0°F) instead of its current 14°C (57°F) (see NOAA). Earth is one of the few planets in the solar system which has an atmosphere. Venus, one of our closest neighbours has an atmosphere made of 96.5% carbon dioxide (CO2) and 3.5% nitrogen (N2), which forms a very toxic atmosphere, allowing it to maintain a sweltering surface temperature of 467°C!
The lithosphere is the term given to the rock and minerals which form Earth’s outer crust and its tectonic plates. This is an important part of the Earth’s system as these rocks become eroded and weathered to provide important minerals to the other Earth systems. Through Earth’s history, the entire lithosphere has been recycled approximately six times. At the outermost layer of the lithosphere, the ‘pedosphere’ (meaning soil sphere) exists at the interface between lithospheric, atmospheric, biospheric and hydrospheric processes. The combination of these processes leads to soil formation, which is essential for sustaining life on Earth (the biosphere).
The biosphere refers to all types of life on Earth, including plants, animals, and bacteria. Over the history of the Earth the biosphere has changed considerably with a great number of species evolving, adapting, and becoming extinct. The diversity of the biosphere varies greatly across the Earth, as species can be highly vulnerable to even minor variations in climate. A slight change in air temperature or moisture, for example, can alter the type of vegetation (flora) which is able to grow in a given location. This may in turn affect the distribution of animals (fauna) as they move or adapt in tune with vegetation availability.
Ocean-atmosphere: the Earth’s great conveyor belt
For scientists studying climate change, the ocean (hydrosphere) and atmosphere are important parts of the Earth system and are very closely related. For this reason, the ocean-atmosphere components are often likened to an energy ‘conveyor belt’. The interactions between the ocean and atmosphere distribute incoming energy from the sun and drive climate patterns across the globe. This has important impacts upon the distribution of ice within the cryosphere, species within the biosphere, and spatial variations in weathering of the lithosphere.
A vast, and ever increasing, body of research continues to investigate the nature and complexity of ocean-atmosphere interactions (Webster, 1994; Kurtz et al., 2011). These studies have developed detailed monitoring networks of present-day changes. Perhaps the largest of these networks is the National Oceanic and Atmospheric Administration. Many studies also use palaeo records and computer models to assess long-term variability of ocean-atmospheric cycles during Earth’s history (Bush and Philander, 1998; Divine et al., 2010; Rahmstorf, 2002). Together, using what we understand about past and present events, these approaches are used to help predict future ocean-atmosphere dynamics under projected climate scenarios (Collins et al., 2010).
Oceans and their role in climate
Oceans cover 71% of the Earth’s surface and are able to absorb twice as much solar energy as the atmosphere and land combined. The strong ocean currents within them act as a ‘conveyor belt’ of water and energy, distributing heat and moisture around the globe. Solar radiation heats the ocean waters which are then transported by currents around the world – in fact, oceans move a similar amount of energy around the Earth as the atmosphere does. This conveyor belt is termed the global thermohaline circulation (Figure 1) and is driven by a combination of: heat, density, tides, wind, and salinity. These currents can have important atmospheric and biospheric implications.
For example, the North Atlantic Drift current, which originates in the Caribbean, transfers warm water across to the north east Atlantic. This current maintains a temperate climate in Britain (data from the UK Met Office indicates that in 2012 the mean annual temperature was 8.8°C), despite the fact that Britain is situated at the same latitude as Newfoundland in Canada, which has an average annual temperature of 0.5°C. The warm North Atlantic Drift current reaches the colder, higher latitudes, cools, sinks within the oceanic water column, and is returned south by the great conveyor system (Figure 1). There are many of these warm-cold and cold-warm conveyors operating across the Earth’s oceans at any given time, and all are in some way, interlinked.
Research suggests that changes in climate will impact the ocean conveyor system and may lead to adjustments, or the complete shutdown, of certain currents. Under one scenario, the melting of the Greenland Ice Sheet will introduce large quantities of cold, freshwater (Greenland contains enough water in its solid, frozen, state to raise global sea level by 7m – Ice2Sea) into the North Atlantic. This will dilute the warm, saline waters of the North Atlantic Drift current and weaken its insulating effect on the British Isles.
Atmosphere and its role in climate
Like the ocean, the atmosphere also transfers heat energy and moisture across the Earth. Incoming solar radiation (insolation) is redistributed from areas in which there is a surplus of heat (the equator) to areas where there is a heat deficit (the North and South Pole). This is achieved through a series of atmospheric cells: the Hadley cell, the Ferrel cell and the Polar cell (Figure 2). These operate in a similar way to, and indeed interact with, the ocean conveyor. For example, as the oceans at low latitudes are heated, water evaporates and is transported poleward as water vapour. This warm air eventually cools and subsides. Changes in temperature and CO2 concentrations can lead to: changes in the size of atmospheric cells (in particular, the Hadley cell is susceptible to these alterations); warming in the troposphere; and disproportionately strong warming in Arctic regions. The strong interactions between ocean and atmospheric dynamics, and the significant feedback mechanisms between them, mean that climate researchers must consider these Earth components as interlinked systems. The necessity to assess ocean-atmospheric changes at the global scale has implications for the way in which research is conducted. It is only by integrating palaeo evidence of past changes, with present day monitoring, and projected models, that we can begin to understand such a complex system.