16. HOH: The Keystone of Earth’s Climate

Water has the structure HOH, whereby the hydrogen atoms are separated by about 104°; as a result the water molecule is more electrically positive on one side, negative on the other. This makes water molecules ‘stick’ and gives this common substance many unique properties. Water exists in three phases, gas (vapor), liquid (water), and solid (ice) on and near the surface of the Earth. Very large amounts of energy are involved in the change from one phase to another. Most of the water on Earth contains salt. Fresh and sea water behave differently as their temperature changes. Fresh water reaches its maximum density at about 4 °C, and becomes less dense as it approaches the freezing point. At a salinity of 27.4 ‰ the temperature of maximum density and the freezing point coincide. The average salinity of ocean water is about 34.5 ‰, so it gets denser and denser as it approaches the freezing point. Sea ice forms by snowflakes diluting a tiny amount of seawater so it can float and freeze. Ice, the sold phase, is less dense than water, the liquid phase. Ice covers the polar regions of Earth and is a powerful stabilizer of climate. Ice cores from Greenland and Antarctic contain not only an isotopic record of climate change, but bubbles containing the ancient atmosphere. Ice cores in Greenland go back about 100,000 years, in Antarctica about 700,000 years.