E9 Global Hydrological Cycle

Topic

The global hydrological cycle describes the continuous movement of water among four reservoirs: the atmosphere, containing nearly thirteen thousand cubic kilometers of water; the oceans, containing nearly 1.4 billion cubic kilometers; ice, containing nearly twenty-six million cubic kilometers; and the continents, containing nearly one million cubic kilometers of water (excluding ice). Water constantly moves between these four reservoirs of the global hydrological cycle, and this continuous movement is what defines the cycle itself—distinguishing it from a mere collection of four static, independent reservoirs.

There are three primary flows connecting these four reservoirs of the global hydrological cycle. Evaporation removes nearly 434,000 cubic kilometers of water from the ocean annually, transferring it to the atmosphere. Precipitation returns that same water from the atmosphere to the surface: nearly 391,000 cubic kilometers fall as precipitation over the ocean each year, while nearly 107,000 cubic kilometers fall over the continents. Finally, runoff returns nearly 44,000 cubic kilometers of water to the ocean annually—water that originally fell as precipitation on the continents and is eventually drained back to the ocean by rivers and groundwater, thereby closing the global hydrological cycle among the ocean, the atmosphere, and the continents.

Global warming intensifies this hydrological cycle because the atmosphere's capacity to hold water vapor increases by approximately seven percent for every degree Celsius of warming. The greater the atmosphere's capacity to hold water vapor, the more intense—on average—is the precipitation produced by the cycle when it finally releases that accumulated vapor. However, this intensification of the global hydrological cycle is not evenly distributed across Earth's surface: regions that are already wet tend to become even wetter, while regions that are already dry tend to become even drier; thus, the intensification of the global hydrological cycle itself amplifies existing contrasts between wet and dry regions rather than distributing the effects equally among them.

The global hydrological cycle moves not only water but also energy, given the intimate link between the two. Evaporation accounts for approximately eighty-five watts per square meter of the global latent heat flux—the energy absorbed from the surface to convert liquid water into water vapor—and serves as the dominant mechanism for transferring energy from the surface to the atmosphere in the tropics, playing a more significant role there than the direct transfer of heat through contact between the surface and the air. Since the evaporation, precipitation, and runoff processes of the global hydrological cycle transport both water and the energy associated with it, any change in the cycle—such as the intensification associated with global warming described earlier—directly implies a shift in the climate system's energy distribution, rather than merely altering where and how much it rains.

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