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What is the hydrosphere?

The hydrosphere is the sum of all water on Earth and the water cycle that distributes it around the planet. Earth is unique in the solar system for its abundant surface waters. Our orbital distance from the sun, in addition to our unique atmosphere, gives Earth the right temperature in our middle-aged solar system to have water as a liquid, and lots of it. Venus is too hot, Mars is too cold. Earth is just right. Noted astronomer Carl Sagan described Earth as seen from distant space as a "pale blue dot," signaling our planet as an outpost of life. It's because of the hydrosphere that life flourishes on Earth.

Just as important as the existence of water is the hydrologic cycle that moves water around the globe. Driven by solar energy, surface waters evaporate into the atmosphere, condense, and fall back to the surface as precipitation, shaping continents, creating rivers, and filling lakes. This process has eroded billions of tons of surface material from the continents to the oceans, forming the major river deltas. By far, most of the hydrosphere is salt water - around 97 percent - but the 3 percent that is fresh is critical for terrestrial and fresh water species.

Water Distribution Water on Earth. Most of the water on Earth is either salty or inaccessible to humans. Only 3% is fresh, and of that only about 32% is unfrozen.

The variable hydrosphere

Precipitation around the globe is highly variable – from deserts (0 to 50 cm per year) to wet rainforests (125-660 cm per year). This variability is a key attribute of productive terrestrial ecosystems. While most precipitation evaporates from and falls onto the oceans, precipitation on land dominates as a key determinant of terrestrial biological zones of the Earth. While some organisms called extremophiles have found ways to adapt to very dry, hot, frozen, or low or high pH environments, the most abundant ecosystems on Earth exist where temperatures are tropical to temperate, nutrients are plentiful, and water is available.

Global Precipitation This animation illustrates the variability of precipitation around the globe. Tropical areas in South America and Africa, for instance, may receive up to 15 mm of rain each day, while arid regions like the Sahara receive little to none at all.(Source: NASA)

How is the hydrosphere changing?

Human contributions to greenhouse gases in the atmosphere are warming the earth's surface - a process which is projected to increase evaporation of surface water and accelerate the hydrologic cycle. In turn, a warmer atmosphere can hold more water vapor. Some evidence suggests global warming is already responsible for more extreme precipitation events. Precipitation in a warming world is also projected to lead to departures from current timing and patterns of rainfall distribution.

The Florida Keys connected by Highway One. The landscapes we enjoy today were once quite different. For example, 125,000 years ago, the Florida Keys were an underwater coral reef. Then, as the Earth settled into an ice age, sea levels fell, leaving the islands up to 120 m above sea level. Today, the low-lying islands are now at risk of re-submergence due to sea level rise that is the result of human-caused global warming.

While the exact changes are difficult to project, it is highly likely that some places will get drier while other places get wetter over the course of the 21st century as a result of global warming. For example, current climate models indicate that with global warming, high latitudes in the Northern Hemisphere are likely to see more precipitation.

Changes in types of precipitation may occur as well. Some precipitation may shift to rain rather than snow. This would decrease mountain snowpack and affect the timing and quantity of seasonal runoff. Changes in the patterns of spring runoff from major snow-fed river systems, such as those that flow from the Himalayas, will impact the lives and livelihoods of upwards of a billion people who depend upon snowmelt-fed rivers for domestic, agricultural, and industrial use.

As the Earth warms, so too will the ocean. As water warms, it expands. Expansion of warming water makes up about half of the present rise in sea level. The rest of the sea level rise we are currently witnessing is the result of land-based snow and ice melting into the ocean. The melt water component of sea level is expected to make up a more significant component of sea level rise as this century unfolds. The 2013 Intergovernmental Panel on Climate Change report found a 0.19 m rise in sea level between 1901 and 2010 and projected an additional 0.52-0.98 m of sea level rise over this century. However, recent research suggests that the amount of glacier melt could be significantly greater, raising sea level a meter or more. Island nations with little elevation above sea level are in peril, as are many countries with large coastal populations, such as the United States. Lowland settlements will be faced with a choice: whether to hold the line with engineered structures or retreat to higher ground.

IPCC 2013 sea-level rise projections: Climate change is more than just global warming. Forecasts predict shifts in precipitation and run-off patterns that will affect agricultural practices and human livelihoods. (Source: IPCC 2014)


    Materials: Notebook, paper and pencil or dedicated computer file where you can keep your work.

  1. Pick one or more of the Hydrosphere Questions.
  2. Write down or sketch your answers based on your own understanding without looking anything up.
  3. Ask a family member, friend, or teacher the same question(s) and write down or sketch their answers.
  4. What are the common ideas in the answers you've collected? Write or sketch the common themes/ideas.
  5. Come up with your own strategy for digging deeper (ask a scientist, check out university and government agency websites like NASA and NOAA, go to the library, design and conduct an experiment, etc.) until you're satisfied the answer makes sense to you.
  6. Summarize what is known and unknown about the subject of the question(s). Also note what evidence there is in supporting what is known and how the evidence was obtained.
  7. Rate the answer you've come up with on a scale of 1 to 10, 1 being weak with lots of uncertainty, 10 being perfect.

Earth Systems