In today's climate the oceans are mainly liquid; only about 2% of the water on the planet is frozen. Most of the frozen water is in the ice sheets of Antarctica (with 89% of the world's ice, and an average depth of about 2 km) and Greenland (8%, 1.5 km deep). The volume of sea ice, formed by the freezing of seawater, is far less than that of land ice because typically it is only a few meters thick. Its extent also varies considerably by season. However, the importance of land ice and sea ice for climate is in some ways comparable because their areal coverage is similar: about 10% of land is covered with ice year round and about 7% of the ocean. Ice on both land and the ocean has a high albedo, up to 70% when fresh compared to about 10% for seawater, and both make a noticeable difference to the climate at high latitudes.

Seawater itself is made of water plus a collection of minerals, or "salts," mainly chloride (19%o, or 19 parts per thousand by weight) and sodium (11%o). The total average concentration of such salts in the ocean is about 34.5%o or 34.5 g/kg.2 Thus, when 1 metric ton (about 1 m3) of seawater evaporates, it yields almost 35 kg of salt (and 1 ft3 yields 2.2 lb of salt). The origin of the salt is weathering and erosion of rocks, as well as the outgassing of chloride from Earth's interior and, when the oceans formed, the leaching of sodium from the ocean floor. The overall level of salinity in the open ocean is now almost constant in time— at least on millennial and shorter timescales—but varies spatially, from a high of about 37%o in surface waters of the subtropics, where evaporation removes freshwater, to a low of about 32%o at high northern latitudes, where rain brings freshwater and evaporation is small.

Variations in salinity and temperature bring about corresponding variations in the density of seawater, and these variations play a large part in ocean circulation. Density decreases as temperature increases and increases as salinity increases (although freshwater, but not salty seawater, has anomalous behavior in that its density increases as the temperature rises between 0°C and 4°C). For small variations of temperature, salinity, and density (T, S, and p, respectively) around a reference state (T0, S0, and p0), density varies according to the formula p p0 [1 bT (T T0) + bs (S S0)], (2.1)

with p0 = 1.027 X 103 kg m-3, T0 = 10°C, S0 = 35 g kg-1. The parameters bT and bS are the coefficient of thermal expansion and the coefficient of haline contraction, respectively. Their values are not in fact constant throughout the ocean but vary with temperature and pressure, with bT varying from about 1 X 10-4 K-1 (at very low temperatures) to about 2.5 X 10-4 K-1 (at high temperatures). The value of bS varies less and typically is about 8 X 10-4 kg/g. The fact that density varies less with temperature when the water is very cold means that salinity plays a greater role in determining density at low tem-peratures—and so at high latitudes and in the cold climates of the past—than does temperature itself.

The specific heat of seawater is about 4,180 Jkg-1 K-1, which is about four times that of air per unit mass, so overall the heat capacity of the ocean is more than 1,000 times that of the atmosphere. In fact, the top 3 m of an ocean column have about the same heat capacity as the corresponding atmosphere above. This heat capacity is responsible for one of the largest effects of the ocean on climate—its moderating effect in bringing milder winters and cooler summers to locations near the ocean, especially on the western edges of the continental land masses, and in slowing down the progression of global warming, as we discuss in later chapters.

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