Climate is simply the average weather of some area, so you might think of the global climate in terms of average global temperature (about 15°C these days). The global climate system then is the set of related, coordinated processes and reservoirs that determine the energy balance of the surface environment, which determines the temperature. A simplified drawing of what this system involves is shown in FIGURE 1.1, which is worth taking a close look at. The global climate system involves the flow of energy as well as matter through the geosphere, the atmosphere, the hydrosphere, and the biosphere. All of these parts of the larger system play important parts in determining how the solar energy, in the form of visible and ultraviolet light, is absorbed, reflected, transformed to infrared (heat) energy, how that heat is emitted and absorbed, ultimately determining how much heat is stored in the system and how that heat sets various parts of the system in motion.

The incoming solar energy, called insolation (not to be confused with insulation) shines down on the Earth -- most intensely at the equator -- and is either absorbed or reflected. Clouds are highly reflective and so is ice; land is generally more reflective than water, and deserts are more reflective than forests. So, the locations of clouds and continents and the nature of land surface are important to the climate system. Why is location important? A land mass at the equator reflects more energy than an identical land mass near the pole because the intensity of the insolation is much greater at the equator. As we will see, land masses do move quite a bit over periods of many millions of years, so this is something to keep in mind.

The insolation that is not reflected is absorbed at the surface, and warms the surface, which then radiates a different kind of energy, infrared radiation -- heat -- back up from the surface. Much of the heat is actually transported by water; as it evaporates, it takes heat energy from the surface, and when it condenses up in the atmosphere to form a cloud or a rain drop, it releases that heat. Plants are important in this process since they absorb water from the soil and release it to the atmosphere. Fortunately for us, not all of that heat escapes to outer space. Some of that heat is absorbed by gases like carbon dioxide, water, and chlorofluorocarbons (CFCs) in the atmosphere and those gases then return some of that heat back to the surface -- this is the famous greenhouse effect, and without it, the Earth would be intolerably cold, about 33°C colder than the present. To put this in perspective, the global temperature during an ice age is about 8°C colder than the present. These greenhouse gases can be thought of as a global blanket, keeping out planet warm.

Because the Earth's surface receives varying intensities of solar energy, temperature differences occur and sets winds and ocean currents in motion. The basic job of these fluids is to transfer heat from the equatorial region to the poles. While winds are the primary driving agents of surface ocean currents, variations in temperature and salinity of the ocean water produce currents that stir up the deeper parts of the oceans. Polar ice plays an important role in generating these cold, deep currents that sweep along the ocean floor for great distances; regions of the oceans with little rainfall lead to water that is higher in salinity and therefore denser than normal ocean water that can also lead to deep currents. These deep ocean currents appear to be very important in switching our global climate from a glacial age to an interglacial age (the present).

The right-hand side of Fig. 1.1 shows a mountain range where weathering and erosion are occurring, two processes that may not seem obviously connected to the global climate system. But in fact, these processes are quietly involved in moderating the balance of CO2 in the atmosphere. Weathering of rocks includes several processes that break the rocks down into smaller pieces; some of these processes involve physically breaking the rock, while other involve chemical alteration and decomposition. These chemical reactions proceed very slowly and often use weak acids to do the job. One of the most important of these acids is called carbonic acid and is formed from CO2 and water, which come from the atmosphere. Thus, chemical weathering uses up atmospheric CO2 and erosion sweeps the by-products of weathering aside, exposing fresh rocks to the surface so that they can be weathered. The by-products of weathering are transported by rivers to the oceans, where some of them are used by plankton to make skeletons of carbonate (CaCO3) that end up being deposited on the ocean floor, forming sedimentary rocks. The rates of chemical weathering are sensitive to the temperature, which is related to the amount of CO2 in the atmosphere. This means that on a global scale, when the Earth is hotter, weathering is faster and has a cooling effect since it removes CO2 from the atmosphere -- it stabilizes our planet's temperature, but it acts slowly, allowing for dramatic short-term changes.

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