The Importance of Water
Introduction
Water vapor is the main greenhouse gas. The reason why the doubling of carbon dioxide results in an
estimated warming by 4 - 9°F is the amplifying power of water vapor. The carbon dioxide alone would
only account for a modest fraction of this increase. Water vapor is a gas that is ubiquitous in the
atmosphere, but greatly varies in concentration, unlike the other greenhouse gases. Water
concentration depends on the temperature of the air (which sets the capacity of the air to hold water)
and on the history of a given parcel of air (that is, whether it had a chance to take up water according to
its capacity).
Water Vapor & Deserts
Dry air is not good at intercepting infrared radiation. For example, the air moving over coastal
mountains into the rain shadow desert on the other side loses its water in the mountains and arrives dry
in the desert. This dry air does little to keep heat radiation from escaping into space. Also, a lack of
clouds over the desert means that the path to space is rather open. Thus, upon the setting of the Sun,
the heat of the ground is rapidly dissipated and it gets chilly quickly. In a sense, the poetic notion that
the cold of space is invading the desert at night is not so far from the truth.
Water and Heat
On a global scale, falling winds are associated with the great desert belts of the Earth, centered
between 25 and 30 degrees of latitude. These winds are dry, having been stripped of moisture in the
updrafts of the tropics, where rain is the rule. Because of the great importance of water as a greenhouse
gas, the availability of water and its temperature is crucial to the workings of climate in any given region.
Also, great amounts of energy can be taken up (or released) by water when it changes phase, that is,
when it changes its state between gas, fluid and solid. This property of water, and others, makes the
hydrosphere a planetary air conditioner of great efficiency.
Even without change of phase, water can do amazing things with heat storage and transport. It
turns out that water is much more efficient in storing heat than any other common substance on Earth.
To increase the temperature of fluid water by one degree centigrade takes one calorie of heat energy
for each gram of water. By comparison to create a one degree centigrade increase in a rock takes 0.2
calories per gram and for petroleum requires 0.5 calories per gram.
But the change of water from one state to another is the process to focus on, when
contemplating the heat budget of Earth. To change a gram of ice into water (without increasing the
temperature) takes 80 calories. Conversely, when water freezes, it releases this amount of heat into the
environment. To change a gram of water into vapor takes 580 calories! This means that evaporation is
the best cooling mechanism around, and our body takes advantage of this fact when cooling itself by
sweating. The heat expended in turning water into vapor is not lost, but is contained in the vapor as
latent heat. When the vapor condenses, this heat is freed for warming the surrounding air.
The phase changes of water, combined with its unique heat-related properties, are intimately
involved in all aspects of climate and weather. Water transfers and stores heat on an immense scale,
and thereby evens out the temperature differences between day and night, summer and winter, tropics
and polar areas. Besides moist winds, ocean currents transfer heat from the warm tropics to the cold
polar regions. A familiar example is the relatively mild climate of Norway, which depends upon the
warm waters brought north along the Norwegian coast through an extension of the Gulf Stream System.
Excepting the poles, the most severe climates are in the interior of continents, far from the sea with its
benign influence on seasonal contrast.