What Is The Hydrosphere?

The hydrosphere is the water (H2O) component of Planet Earth. We explain how it’s affected by climate change and vice versa. We also describe how the water cycle works and how it moves H2O around the planet. We also detail how pollution, eutrophication, ocean warming, and acidification have caused serious problems for marine ecosystems and the health of the seas.
Fisherman catches fish in coral reefs
Photo: Arc Centre of Excellence for Coral Reef Studies/Coral Reef Image Bank

The “hydrosphere” includes (a) All water vapor in the atmosphere. (b) All liquid freshwater and seawater, in streams and rivers and oceans, as well as subterranean groundwater in wells and aquifers. (c) All solid water (ice and permanent snow) in the cryosphere, held in ice sheets, ice caps, glaciers and frozen ground (permafrost).

Water itself is life’s most basic building-block: one reason why the search for extra-terrestrial life always focuses on first locating water. 1

It is also a key influence on Earth’s climate system in the air and on land, as well as in the sea. For example, it’s a major carbon reservoir and absorbs the vast majority of all excess heat. Unfortunately, as global warming intensifies, there are concerns that the hydrosphere will be unable to continue in this supportive role.

What Does The Hydrosphere Do?

The hydrosphere regulates the temperature of the planet, is an essential ingredient in photosynthesis, and transports life-giving nutrients around the biosphere for the benefit of all living things. The hydrosphere is also home to many plants and animals, including phytoplankton that live near the water’s surface, where they photosynthesize sunlight and produce oxygen as a byproduct. Scientists believe that these tiny organisms may contribute as much as 85 percent of the oxygen in Earth’s atmosphere. In addition, as well as removing atmospheric pollutants like aerosols from the atmosphere, through rainfall and through the actions of its wetland swamps, the hydrosphere provides for the purification of salt water into the fresh water that we (and all other living creatures) depend upon. 2

Water Moves Around In The Water Cycle

Water Cycle: 3 Stages Diagram
The three stages of the water cycle (also called the hydrologic cycle), is the journey water makes from one state to another. As the word ‘cycle’ suggests there is no starting point. Image: © NASA

Thanks to the mechanism of the “water cycle“, involving the three processes of evaporation, condensation and precipitation, water is continuously moving around the planet.  It works like this. Water evaporates from the ocean or some other water body, before rising on warm updrafts into the atmosphere. Here, it condenses into clouds of liquid droplets and is blown by the wind, before falling back to the Earth as precipitation. On land, water is recycled through the tiny pores (stomata) of trees and plants in a process called transpiration, or evaporated from the surface of freshwater sources. The collective term for both these two processes is evapotranspiration. 3

The water cycle (also called the hydrologic cycle) helps to transfer heat and energy, and nutrients, from the surface of the Earth to the atmosphere, and from there to different places on the planet. In this way, the water cycle connects the hydrosphere with the other sub-systems of the Earth: namely, the atmosphere (air), pedosphere (the soil layer), lithosphere (rocks, source of volcanic out-gassing) and biosphere (all living things).

The Oceans Dominate

Planet Earth is known as the blue planet because of the startlingly blue colour of its oceans. These life-giving waters (average depth 2.5 miles) make up the vast bulk of the hydrosphere, covering 361,132,000 sq km (139,434,000 sq mi), equivalent to 71 percent of the Earth’s surface. Roughly 97 percent of all water on Earth is found in the oceans.

How Much Water Is There On Earth?

Hydrologists estimate that the Earth contains a total of 1,337 million cubic km (321 million cu mi) of water, of which roughly 97.5 percent is saline, and 2.5 percent freshwater. Roughly 68.7 percent of freshwater is found in glaciers and ice sheets; 30.1 percent in groundwater; and 1.2 percent in a variety of categories, which divide up as follows: 69 percent in permafrost and ground ice; 21 percent in lakes; 3.8 percent in soil moisture; 3 percent in the atmosphere; 2.6 percent in swamps and marshes; 0.5 percent in rivers and 0.26 percent in living things.

The Distribution of Earth’s Water

Water may be the most plentiful natural resource on the planet, but notice how much of the world’s total water supply is saline (average salt content 3.5 percent). Notice in particular, how little freshwater is located in easily accessible sources such as rivers, reservoirs and lakes – about 1/150th of one percent of total water. What’s more, this freshwater is not evenly distributed across the world. A number of heavily populated countries are located in regions of low rainfall where fresh water is scarce.

How Does The Hydrosphere Help Our Climate?

  • First, water vapor in the atmosphere boosts the greenhouse effect, which prevents some of the planet’s radiant energy from being returned to deep space. This keeps Earth’s temperature at a cosy 15 °C (59 °F), instead of the minus 18°C (0°F) it would otherwise be. But water vapor is not a cause of today’s global warming. It’s a climate feedback, not a climate forcing.
  • Water vapor also forms clouds which have a high albedo (ability to reflect sunlight), which reduces the amount of solar energy reaching Earth. At the same time, they also stop heat escaping near the surface of the Earth. The former action is stronger than the latter, so on balance clouds have a small net cooling effect. For more on this topic, see: How Do Clouds Affect Climate?
  • Polar ice and high-altitude ice caps lock up huge amounts of liquid water that might otherwise enter ocean basins and raise sea levels. A major fragmentation of the Greenland or West Antarctic ice sheet would cause catastrophic sea level rise around the world.
  • Snow and ice have a high albedo which redirects roughly 80 percent of all the sunlight they receive back into space.
  • The ocean acts as a massive buffer against climate change by absorbing more than 90 percent of the excess heat in the climate system. 4
  • Ocean waters are constantly being moved about by powerful currents, of the deep water thermohaline circulation, such as the ‘ocean conveyor belt’. Surface currents tend to be wind-driven, although the physical presence of continents, rotation of the earth, and the oceans’ internal processes also play a part. In contrast, deep-ocean currents are regulated by differences in density, caused by temperature and salinity. Clouds exacerbate these density differences by blocking the warming rays of the sun (cooling the sea temperature) or by raining (diluting surface salinity).
  • Oceans are a direct and indirect source of renewable energy. Direct energy comes from tidal power and wave power, while indirect energy derives from seawater after it evaporates and falls to earth, where its power is harnessed in the form of hydropower (hydroelectricity).

An important function of the thermohaline circulation is to move heat from the tropics to the poles, where it is more easily lost to deep space. For example, the Gulf Stream – which happens to be a necessary part of the global conveyor belt (to close the loop), although it’s actually a wind-driven surface current – carries water from the tropical and subtropical Atlantic into the northern Atlantic, where it bathes the shores of Western Europe, producing a climate that is abnormally mild for the latitude. 5 For more, on the relationship between the hydrosphere and climate, see our in-depth article: How Do Oceans Influence Climate?

Has The Hydrosphere Been Affected By Human Activities?

Water Eutrophication Pollution
Fertilizers from farms runoff into nearby water, causing an increase in nutrient levels and the overgrowth of certain plants. These plants use up all the oxygen in the water, sucking life out of the ecosystem. This is known as marine eutrophication, or simply water pollution. Photo: © europa.eu

Yes. Human activities are having a major impact on the hydrosphere.

  • The water cycle is being severely compromised by the discharge of toxic chemicals, radioactive substances, and other industrial wastes, as well as the seepage of herbicides, fertilizers, and pesticides into aquatic systems. Improper sewage disposal is another major pollutant.
  • Another problem is eutrophication – the enrichment of a lake, stream, river or other freshwater aquatic systems with an excessive amount of nutrients – typically caused by the discharge of nitrate or phosphate-containing detergents, fertilizers, or sewage – resulting in abnormally high levels of algae or phytoplankton. 6 Ecological effects of eutrophication include: decreased volumes of water due to the accumulation of organic detritus; decreased biodiversity; the possible invasion of new species; and toxicity leading to disease and fatalities in the food chain. Estuarine ecosystems are also vulnerable.

How Does Climate Change Affect The Hydrosphere?

  • Over the last 40 years, the discharge of increasing levels of so-called greenhouse gases – caused by the combustion of fossil fuels – has led to serious problems for marine ecosystems and the health of the seas.
  • Previously, there was a net shift of CO2 from the oceans through the atmosphere to the land, where the gas was used in photosynthesis and the chemical weathering of rocks. Because of fossil-fuel burning and damaging land use practices, the net transfer from ocean to land has been reversed, and the ocean is now an extremely important sink of carbon dioxide.

In September 2019, the United Nations’ Intergovernmental Panel on Climate Change (IPCC) released a Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC). The 1,300-page report was written by 104 authors and editors from 36 countries.

Ocean Warming

In its Summary for Policymakers, the report stated: (1) it is virtually certain that the global ocean has warmed continuously since 1970; (2) the ocean has absorbed more than 90 percent of the excess heat in the climate system; (3) Since 1993, the rate of ocean warming has more than doubled. (4) Acidification of the ocean surface has increased, while ocean deoxygenation has spread from the surface to a depth of 1,000 m (3,300 feet). For an in-depth review, see: Effects of Global Warming on the Oceans.

Ocean Currents Weakening

The report also highlighted the weakening (by 15 percent) of the ocean current system known as the Atlantic meridional overturning circulation (AMOC), part of the so-called “global conveyor belt”. 7 This current system, driven by thermohaline circulation, carries up to 25 percent of the northward atmosphere-ocean heat transfer in the northern hemisphere. 8 The AMOC is the largest carbon sink in the Northern Hemisphere, removing 700 million tonnes of CO2 from the atmosphere annually. 9

Polar Ice Melt

In its summary of the IPCC’s Special Report, Carbon Brief said that the melting of the Greenland ice sheet is “unprecedented in at least 350 years.” When combined with the Antarctic ice sheet, the pair have contributed “700% more to sea level rise” than in the 1990s. Furthermore, a rise of 2 metres (6.6 ft) by 2100 is possible if greenhouse gas emissions continue to increase strongly. 10

Reduction In Polar Sea Ice

Arctic sea ice is also disappearing. The report warned that the Arctic Ocean could be ice free in September “one year in three” if global warming reaches 2°C. Before the Industrial Revolution it was ice free “once in every hundred years”. 11

Reduction In Polar Albedo Effect

Not only is climate change melting polar ice, but also, it’s reducing the region’s albedo effect. Thus, less sunlight is reflected back into space, leading to an increase in solar energy striking the surface of the planet. 

Thawing Permafrost

In its summary of the IPCC Report, Carbon Brief said that if greenhouse gas emissions remain unchecked, 30-99 percent of Arctic permafrost will disappear releasing tens or hundreds of billions of tonnes of heat-trapping methane as well as carbon dioxide.

The IPCC report admitted that laboratory soil incubation studies show that substantial quantities of carbon could potentially be transferred from the permafrost carbon pool into the atmosphere under the Representative Concentration Pathway (RCP) 8.5″ scenario, laid out in the IPCC’s Fifth Assessment Report (2013). 12

Low Lying Islands Could Be Submerged

In its section on low-lying islands and coasts (LLIC), the report warns that cities and megacities – including New York City, Jakarta, Mumbai, Shanghai and Cairo – will incur a serious risk of climate-related flooding. Furthermore, if CO2 emissions stay high, some low-lying islands are likely to become “uninhabitable” by 2100. 13

For more about the timeline of our planet and its oceans, see: History of Earth in One Year (Cosmic Calendar).


  1. “Environmental Science.” Barbara Murck. John Wiley & Sons Inc. Hoboken, New Jersey. 2005. []
  2. “Swamp power: how the world’s wetlands can help stop climate change.” Arthur Neslen. The Guardian. 20 Jul, 2015. []
  3. North Carolina Climate Office. “The Water Cycle.” climate.ncsu.edu/edu/WaterCycle []
  4. IPCC. Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC) (2019) Summary for Policymakers. []
  5. Rahmstorf, S., 2006: Thermohaline Ocean Circulation. (PDF) In: Encyclopedia of Quaternary Sciences, Edited by S. A. Elias. Elsevier, Amsterdam. []
  6. Eutrophication: Causes, Consequences, and Controls in Aquatic Ecosystems”. Nature Education Knowledge. Chislock, M.F.; Doster, E.; Zitomer, R.A.; Wilson, A.E. (2013). []
  7. Chapter 6: Extremes, Abrupt Changes and Managing Risks (PDF). IPCC (Report). Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC). September 25, 2019. []
  8. “Ocean heat transport.” International Geophysics. Vol. 77. Academic Press, 2001. 455–474. Bryden, Harry L., and Shiro Imawaki. []
  9. Interannual variability in the North Atlantic Ocean carbon sink.” Science 298.5602 (2002): 2374–2378. Gruber, Nicolas, Charles D. Keeling, and Nicholas R. Bates. []
  10. In-depth Q&A: The IPCC’s special report on the ocean and cryosphere”. Carbon Brief. September 25, 2019. []
  11. IPCC (Press release). Special Report on the Ocean and Cryosphere in a Changing Climate. September 25, 2019. []
  12. Chapter 3: Polar Regions (PDF). IPCC Special Report (SROCC). September 25, 2019. p. 173. []
  13. Integrative Cross-Chapter Box: Low Lying Islands and Coasts (PDF). IPCC Special Report (SROCC). September 25, 2019. []
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