365 Days of Climate Awareness 51 - The Cryosphere

The frozen regions of earth comprise the cryosphere. The term comes from Greek, "kryos" = cold, and "sphaira" = sphere. It includes not only permanently frozen regions like Greenland and Antarctica, but seasonally frozen areas such as the upper-mid-latitude band across North America and Eurasia.

The cryosphere includes glaciers, both terrestrial and marine, sea ice, permafrost-covered tundra, and the snowy, seasonally frozen regions. One of the most important checks on the global energy balance is the cryospheric albedo. Snow and ice reflect 80-90% of incoming radiation. Roughly 12% of the earth is permanently covered in ice and snow. During the northern hemisphere's winter, global snow and ice cover increases to roughly 33% of the globe. Both figures are diminishing because of global warming.

Frozen regions of earth.  Larger version

Glaciers are masses of ice tall enough that the weight causes the ice to flow, as an extremely slow fluid. Most fluids we're used to, like air, water or gasoline, are "Newtonian" fluids, meaning their viscosity is constant. Because of its constant viscosity, the fluid's resistance to motion is independent of the amount of force (inducing some kind of flow) used on it. (Viscosity of a Newtonian fluid can change with temperature, however--generally, higher temperature means lower viscosity: it becomes more runny.)

Non-Newtonian fluids, however, become more or less viscous--that is, more or less likely to flow--depending on the amount of stress applied to them. (Stress is force per unit area. This is why the tiny heel of a high-heeled shoe can dig into a wooden floor, where a full-soled shoe is much less likely to.) Ketchup is one example of a non-Newtonian fluid: it flows when shaken. So are mayonnaise and motor oil. Corn starch in water by contrast thickens when stirred. And ice flows, slowly, when it becomes thick enough and there is sufficient pressure on lower layers.

Sea ice is the frozen surface of the ocean, but much thinner, ranging from a film less than a centimeter in sheltered areas to more than 5 m thick. Permafrost occurs in permanently frozen ground called tundra, which locks soil, gases, and nutrients into a solid layer. The surface can occasionally melt, but the water does not drain through the ice below, resulting in a cold bog known as muskeg. And where winter is sufficiently cold, it creates temporary frozen conditions which include frozen ground and snow cover. All of these elements contribute to the cryosphere.

Even in an interglacial period, as we now are, a significant portion of the earth is frozen. For this reason we say we are in an icehouse climate, as opposed to a greenhouse climate, where no permanent ice exists outside of the mountains. An extreme version of the icehouse is snowball earth, where frozen conditions reach the equator. It is possible, but not consensus, that this has happened at least once.

Tomorrow: properties of ice.

Be well!