from Assessment of Major Ecosystem Services from the Marine Environment (Other than Provisioning Services)
Published online by Cambridge University Press: 18 May 2017
The interactions between the seawater and freshwater segments of the hydrological cycle
The global ocean covers 71 per cent of the Earth's surface, and contains 97 per cent of all the surface water on Earth (Costello et al., 2010). Freshwater fluxes into the ocean include: direct runoff from continental rivers and lakes; seepage from groundwater; runoff, submarine melting and iceberg calving from the polar ice sheets; melting of sea ice; and direct precipitation that is mostly rainfall but also includes snowfall. Evaporation removes freshwater from the ocean. Of these processes, evaporation, precipitation and runoff are the most significant at the present time.
Using current best estimates, 85 per cent of surface evaporation and 77 per cent of surface rainfall occur over the oceans (Trenberth et al., 2007; Schanze et al., 2010). Consequently, the ocean dominates the global hydrological cycle. Water leaving the ocean by evaporation condenses in the atmosphere and falls as precipitation, completing the cycle. Hydrological processes can also vary in time, and these temporal variations can manifest themselves as changes in global sea level if the net freshwater content of the ocean is altered.
Precipitation results from the condensation of atmospheric water vapour, and is the single largest source of freshwater entering the ocean (∼530,000 km3/yr). The source of water vapour is surface evaporation, which has a maximum over the subtropical oceans in the trade wind regions (Yu, 2007). The equatorward trade winds carry the water vapour evaporated in the subtropics to the Intertropical Convergence Zone (ITCZ) near the equator, where the intense surface heating by the sun causes the warm moist air to rise, producing frequent convective thunderstorms and copious rain (Xie and Arkin, 1997). The high rainfall and the high temperature support and affect life in the tropical rainforest (Malhi and Wright, 2011).
Evaporation is enhanced as global mean temperature rises (Yu, 2007). The water-holding capacity of the atmosphere increases by 7 per cent for every degree Celsius of warming, as per the Clausius-Clapeyron relationship. The increased atmospheric moisture content causes precipitation events to change in intensity, frequency, and duration (Trenberth, 1999) and causes the global precipitation to increase by 2-3 per cent for every degree Celsius of warming (Held and Soden, 2006).
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