Identify the major parts of the climate system and distinguish between climate forcing factors and climate feedbacks.

There is a saying, “Climate is what you expect, but weather is what you get.” Climate is the long-term average of weather and the average frequency of extreme weather events. Weather is the state of the atmosphere at any given moment and comprises ever-changing events on time scales ranging from minutes to weeks. Sunshine, rain showers, heat waves, thunderstorms, and clouds all are aspects of weather.

Table 6.1 summarizes events that represent weather and climate. These events occur along a time continuum ranging from hours to tens of millions of years.

Weather observations such as temperature, precipitation, wind, and humidity are averaged to represent the climate of a given region. Simple annual averages of temperature and precipitation, however, do not fully describe the climate of a region. Take, for example, the average annual temperature and precipitation for San Diego, California, and Tucson, Arizona (Figure 6.2). Judging by their annual averages, these two cities appear to have similar climates—but they do not. Remember that climate also includes the frequency of extreme events. For example, much of Tucson’s rainfall comes from thunderstorms in July and August, but San Diego gets winter precipitation from midlatitude cyclones and almost no summer rain. Tucson has a greater annual temperature range, with colder winters and hotter summers, than San Diego. Hard freezes and snow in winter are extremely rare in San Diego, but below-freezing winter temperatures do sometimes occur in Tucson.

Geographers have identified and named many different types of climates, ranging from wet equatorial rainforest climates to dry interior desert climates. Several different classification systems are used to identify and classify Earth’s many types of climates. The one used in this book is called the Köppen climate classification system.The emphasis in this chapter, however, is on the average state of Earth’s climate as a whole, rather than on climate types in different geographic regions. The Köppen system is presented in Section 8.1 in the context of global vegetation patterns.

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Climate is a result of the interaction between Earth’s major systems: the atmosphere, biosphere, lithosphere, hydrosphere, and cryosphere. Energy and matter move through these systems and form the climate system. The first four systems were introduced in Section GT.2. The cryosphere is the frozen portion of the hydrosphere, which includes glaciers and sea ice. We live in the atmosphere and are affected by it in a direct way, but the other systems are equally important in determining how Earth’s climate functions.

In addition, the climate system involves long-distance connections between different geographic regions. El Niño (see Section 5.5) is a good example of the role of such long-distance connections, called climate teleconnections. We will examine all of these aspects of the climate system later in this chapter.

The subject of anthropogenic climate change is often in the news. Weather stations, orbiting satellites, and ocean buoys have recorded a gradual creeping upward of temperatures in the troposphere. Since 1880, the average temperature of the lower atmosphere has increased 0.83°C (1.5°F). The surface of the oceans has warmed by about 0.56°C (1°F) in the last century as well. These temperature trends are climate change.

Climate change occurs when the long-term average of any given meteorological variable, such as temperature or precipitation, changes. Individual extreme weather events do not change the long-term average. Think of putting a single drop of water in a glass half filled with water. One drop does not change the water level. If enough drops are added, however, the water level will gradually rise.

One question that frequently comes up is whether a single extreme event, such as a single heat wave or storm, was caused by climate change. Scientists do know that the long-term average number of heat waves worldwide is increasing. The “extra” heat waves are a result of Earth’s changing atmosphere. Yet separating the heat waves or storm events that would have occurred naturally from those that were caused by increased atmospheric temperatures is scientifically challenging. Picture This explores this topic further.

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The behavior of Earth’s climate is controlled by forces that are unaffected by the climate system, called climate forcing factors. Earth’s climate is also controlled by factors that arise within the climate system and are changed by the climate system, called climate feedbacks. A climate feedback enhances or diminishes climate change that has already been set in motion (see Section 3.6).

As an example of a climate forcing factor, the Sun, if it were to shine more intensely, would force climate into a warmer state through solar forcing. Similarly, volcanic forcing occurs when volcanoes erupt aerosols into the stratosphere, where they reflect sunlight and cool the planet’s surface.

Unlike climate forcings, climate feedbacks involve interacting parts of the climate system that affect one another. We learned in Section 3.6 that negative feedbacks maintain a system’s stability and that positive feedbacks destabilize a system. There are many feedbacks in the climate system, some of which can support climate stability and others that can destabilize the climate system and cause climate change.

An example of a destabilizing positive feedback in the climate system is the ice-albedo positive feedback. When the temperature of the atmosphere increases, more snow and ice are melted. Bare ground and ice-free water absorb more solar energy than snow and ice and cause more warming, creating a positive feedback loop:

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The ice-albedo positive feedback destabilizes the climate system and causes climate change by enhancing the warming trend that was already taking place. But the ice-albedo positive feedback can cause cooling as well. If, for whatever reason, there were a cooling trend in Earth’s atmosphere, the ice-albedo positive feedback would enhance that cooling trend:

In this positive feedback loop, it would get colder as more snow and ice reflected sunlight. But positive climate feedbacks do not go on forever. They are kept in check by negative feedbacks that function to stabilize a changing system. We will return to the important role of climate forcing factors and feedbacks as we move through the remainder of this chapter.