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, lots of it. Venus is too hot, Mars 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.
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. Click to expand.
Just as important as the existence of water is the hydrologic cycle (animation) 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 erodes 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, some 97 percent, but the 3 percent that is fresh is critical for terrestrial and fresh water species.
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 ) – and is a key attribute of productive terrestrial ecosystems. While most precipitation evaporates from and falls on 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 to high pH environments, the most abundant ecosystems on Earth exist where temperatures are tropical to temperate, water is not a limiting factor, and nutrients are plentiful.
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, which in turn is projected to accelerate the hydrologic cycle since a warmer Earth would enhance the evaporation of surface water. 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 depart from the current pattern of distribution and seasonality.
Climate change is more than just warming.
Forecasts predict shifts in precipitation and run-off patterns that will affect agricultural practices and human livelihoods. Click to enlarge. (Source: IPCC 2007)
While the exact changes are difficult to project, it is highly likely that some places will get drier and others wetter over the course of the 21st century as a result of global warming. For example, current climate models indicate that with global warming, more precipitation is likely at higher latitudes in the Northern Hemisphere.
Climate change is more than just warming.
Forecasts predict shifts in precipitation and run-off patterns that will affect agricultural practices and human livelihoods. (Source: IPCC 2007)
As the Earth warms, more precipitation will fall as rain, which decreases mountain snowpack and affects the timing and quantity of seasonal runoff. Change in the pattern 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. This 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 2007 Intergovernmental Panel on Climate Change report estimated about 0.6 m of sea level rise this century, but 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 the 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.
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