Chapter Summary

The functioning of Earth as a system is seen in the global cycling of energy, water, and carbon, and in other biogeochemical cycles. This chapter introduces the fundamental scientific concepts of energy, water, and biogeochemical cycles that regulate climate and link the atmosphere, hydrosphere, cryosphere, biosphere, pedosphere, and anthroposphere. Heat flows between materials due to temperature differences. Heat is transferred in the atmosphere by radiation, conduction, and convection. These flows of heat determine the balance of energy gained, lost, or stored. For the planet as a whole and averaged over the year, the solar radiation absorbed by Earth is equal to the longwave radiation emitted to space. That is, the net radiation absorbed by Earth is zero in the annually averaged planetary mean. The hydrologic cycle describes the cycling of water among land, ocean, and air, principally in terms of precipitation, evaporation, and the runoff of freshwater from land into oceans. The hydrologic cycle regulates the amount of water vapor in the air, which is a key greenhouse gas. The increased capacity of air to hold moisture as temperature increases is an important thermodynamic principle that affects climate. In addition, the change of water among its solid, liquid, and vapor states requires considerable energy. These phase changes provide energy to fuel storms. Atmospheric gases interact with radiant energy flowing through the atmosphere to determine the planetary energy budget. Principal among these are carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). These gases cycle among the atmosphere, ocean, and land, regulated in part by biological and geochemical processes. Human activities modify their natural cycles, which can be seen in the rising concentration of these greenhouse gases in the atmosphere.

Scientific Units

All units of measurement are derived from four basic units (Table 3.1): length, measured in meters (m); mass, measured in kilograms (kg); time, measured in seconds (s); and temperature, measured in kelvin (K). An additional quantity, mole (mol), is used in chemistry to measure the amount of a substance. Mass and moles are related by the molecular mass of the material (kg mol–1). Force is a quantity that accelerates an object (force = mass × acceleration). The scientific unit for force is the newton (N).