The Ice on Our Planet
The temperature conditions that govern our planet are such that water can exist in three forms (vapor, liquid, and solid) in proportions that vary in different climate conditions. All of the water on the Earth makes up what is called the hydrosphere. Most water exists in liquid form, 97% of which is in the seas and oceans, which cover 360 million km2, or more than 70% of the planet’s surface. Freshwater, a vital resource found in the ground, lakes, rivers, and above all aquifers, represents only a small proportion of the total, a bit more than 0.5%; water vapor in the atmosphere amounts to .001%; and the water of the living world, the biosphere, represents much less.
Low temperatures are found in high altitudes and high latitudes; this is where ice forms. In current climate conditions, this ice represents only around 2% of the hydrosphere. How is that 2% distributed? How is it formed? What surface does it cover? What climatic role does it play? Before looking to the past, we will describe our white planet as it is now. Table 1.1 shows its main characteristics.
When we think of ice, we immediately think of 0°C (or 32°F). This is the temperature at which water freezes and ice melts. It is a reference point for the temperature scale that we use in our everyday lives. In nature, it is a bit more complex because that temperature of reference involves only freshwater. In fact, because it contains salt, seawater freezes only below – 1.8°C. And in certain circumstances, even very pure water does not easily become solid. This is the case with very small cloud droplets: they form from the condensation of water vapor and can remain in liquid form. Up to temperatures of –20°C, or even below, we then speak of supercooling. We should add that