The hydrosphere, the collective designation of all water on Earth, significantly affects Earth's greenhouse effect and global temperatures generally. Atmospheric water vapor is the major GHG, and the oceans are major storage sites for water, heat, and CO2.

The range of Earth's surface temperatures and atmospheric pressures permits water to occur naturally as a solid, liquid, and vapor. Water is present in all Earth's climate subsystems. In broadest terms, the hydrosphere includes liquid water at the Earth's surface, water in the atmosphere in all its forms, and water in the cryosphere stored as ice. It extends several kilometers below Earth's surface and reaches 12 kilometers into the atmosphere.

The hydrosphere contains an estimated 1.36 billion cubic kilometers of water, and liquid water covers about 71 percent of the Earth's surface. Most of the world's water is contained in the oceans as salt water; only 2.5 percent is freshwater. Ice sheets and glaciers concentrated on about 10 percent of the Earth's land area, mainly in Antarctica and Greenland, constitute less than 2 percent of all water, but they account for 69 percent of freshwater. Groundwater and soil water constitute the subsurface water storage, which is important as a water supply for humans in arid regions and as a moisture source for plants. Groundwater and soil water together account for 0.5 percent of all water, but 30 percent of freshwater. Rivers, lakes, and streams contain 0.02 percent of all water and 0.3 percent of freshwater.

Water vapor and cloud droplets in the atmosphere account for 0.0001 percent of all water and 0.04 percent of freshwater, but water vapor and cloud droplets contribute between 66 and 85 percent of the greenhouse effect, which traps heat in the atmosphere, preventing it from escaping into space (Barnett, 2001). The hydrosphere's formation required a series of steps occurring over millions of years to reach the sequence of energy and moisture transfers currently linking the hydrosphere and Earth's climate.

Water from outgassing by volcanic eruptions and possibly from incoming comets collected in the early atmosphere as water vapor along with other gases. Eventual cooling of the Earth's surface and the atmosphere caused the water vapor to condense into droplets and fall as rain to the Earth's surface. The rain collected in topographic depressions in the Earth's crust and formed rivers, lakes, and oceans. Some water drained vertically through the rocks and collected in large underground reservoirs, and some water was held by the soil that developed as rocks disintegrated.

The Earth's original hydrosphere probably was freshwater exclusively, with the hydrologic cycle as the centerpiece. Water cycling was sustained by evaporation mainly from the oceans that returned water vapor to the atmosphere to fall as rain. Dissolved gases fell with the rain from the early atmosphere, and the oceans absorbed large amounts of atmospheric carbon dioxide (CO2). The transition to today's salt-dominated oceans spanned hundreds of millions of years and involved crustal differentiation, life processes, and a series of mineral equilibriums. Rain on the land dissolved minerals from rocks; rivers and streams carried the minerals to the oceans; chlorine leached from the oceanic crust accumulated in seawater; and dissolved carbonates were gradually removed. During periods of global cooling, snow accumulated on the continents, forming glaciers and ice sheets, and sea levels were lowered.

Evidence of warmer temperatures exists in all the hydrosphere's components, but it is most apparent in the oceans. Oceans store more than 90 percent of the heat in the Earth's climate system, and they buffer the effects of climate change (Bigg, 1996). Sea surface temperatures since 1860 show essentially the same trends as land temperatures, and ocean temperature changes contribute about one-half Earth's net temperature gain. Ocean circulation transfers energy from warm tropical regions toward the poles and brings colder water back to the tropics. Ocean circulation is accomplished by powerful, wind-driven surface currents coupled with deep-ocean currents responding to density differences produced by temperature and salinity variations. British oceanographers have reported that circulation in a section of the Atlantic Ocean has slowed 30 percent in the past five decades, which raises concern that the global transfer of energy by the oceans may be weakening.

Warmer oceans produce thermal expansion, which is a major contributor to rising sea levels of 15-20 centimeters over the last century. Observed twentieth century melting of glaciers and ice sheets contributed to sea-level rise, but the volume of melting accounts for about 20 percent of the observed increase. An observed 10 percent per decade decrease in Arctic sea ice probably contributes more to radiation balance changes and warming the air and water than to sea-level rise. Oceans are the source for 85 percent of the water vapor transferred into the atmosphere, and warmer temperatures increase evaporation. Increased atmospheric water vapor supports greater precipitation, an intensification of the greenhouse effect, increased cloud cover, and increased global albedo. Changes in the timing, amount, and location of precipitation and runoff are occurring as global temperatures increase, and these changes alter water availability and quality and the duration and intensity of floods and drought.

CO2 in the oceans increases as atmospheric CO2 increases, but warm oceans are a less efficient absorber of CO2. About half of the known CO2 produced by fossil fuel consumption over the past fifty years has been absorbed by the oceans. Water is essential for humans and for natural ecosystems, but the relatively small proportion of Earth's total water represented by freshwater is unevenly distributed globally. Approximately one third of the world's population lives in regions where the freshwater supply is less than the recommended per capita minimum, and 70 percent of all freshwater withdrawals are used for crop irrigation. The freshwater resource relies on precipitation delivered by the hydrologic cycle. Precipitation is sustained through the conversion of ocean water into freshwater by a complex system responding to global-scale climatic influences. Hydrological processes, related energy exchanges, and interactions of these exchanges with clouds and the radiation balance are highly variable at the regional scale where the global warming impact on the freshwater resource is greatest. Quantifying regional temperature and precipitation patterns and the effects of altered evaporation, transpiration, soil moisture storage, and runoff on the freshwater supply is a great challenge facing scientists as they strive to understand global warming at the regional scale.

References

1. Barnett, Tim P., David W. Peirce, and Reiner Schnur. "Detection of Anthropogenic Climate Change in the World's Oceans." Science 292, no. 5515 (April 13, 2001): 270-274.

2. Bigg, Grant R. The Oceans and Climate. New York: Cambridge University Press, 1996.

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