6.1 The Climate System

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.

climate

The long-term average of weather and the average frequency of extreme weather events.

weather

The state of the atmosphere at any given moment, comprising ever-changing events on time scales ranging from minutes to weeks.

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.

Table : TABLE 6.1 Weather and Climate

PHENOMENA

TEMPORAL SCALE

WEATHER OR CLIMATE?

Cloudiness, rain shower, rainbow, sea breeze, tornado

Hours

Weather

Night-and-day temperature difference

Days

Weather

Hurricane, midlatitude cyclone

Weeks

Weather

Winter, hurricane season, drought

Months

Climate

Asian monsoon

One year

Climate

El Niño and La Niña

Years to decades

Climate

Younger Dryas*

1,000 to 10,000 years

Climate

Quaternary* glacial and interglacial cycles

10,000 to 1,000,000 years

Climate

Cenozoic* cooling

Millions of years

Climate

*These terms will be defined in Section 6.2.

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.

Figure 6.2

Climate diagrams for San Diego and Tucson. Although average annual temperatures and amounts of precipitation for these two cities are similar, the climate diagrams, which show average monthly temperature (red line) and precipitation (blue bars), reveal that their climates are quite different.

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.

194

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.

cryosphere

The frozen portion of the hydrosphere.

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.

Climate Change

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.

Picture This

Extreme Events and Climate Change

The year 2012 was a year of extreme events. (A) The United States experienced a series of record heat waves in June 2012. In the same month, the western United States saw record-breaking wildfires. This satellite image shows smoke aerosols on June 26, 2012. (B) In October, Superstorm Sandy, which had the lowest barometric pressure (940 mb) ever recorded in the North Atlantic Ocean, brought the highest storm surge New York City had ever experienced. Here, a Seaside Heights, New Jersey, roller coaster lies stranded in the sea after Sandy. (C) In March, the United Kingdom experienced its fifth worst drought, followed immediately by its wettest April on record. (D) As a whole, the year 2012 stood as the ninth warmest in recorded history and the warmest ever for the United States. This map shows 2012 surface temperatures measured by satellite, above or below the annual average for the period 1951–1980.

Question 6.1

Was Superstorm Sandy a result of anthropogenic climate change?

No single weather event can be definitively attributed to climate change. Scientists do know that the intensity of this storm would have been less had it not been for climate change.

Scientists want to know which, if any, of these extreme events were caused by climate change. In September 2013, a study published in the Bulletin of the American Meteorological Society took significant steps toward answering this question. Seventy-eight researchers from the National Oceanic and Atmospheric Administration (NOAA) and the British Met Office found that half (6 of 12) of the extreme weather events that occurred in 2012 could be statistically linked to the increased average global temperature. For example, they found that the extreme rainfall in April 2012 in the United Kingdom was caused by natural climate variation. On the other hand, the record flooding from Superstorm Sandy and the June 2012 U.S. heat waves were attributed to human-caused climate change. This study does not say that Superstorm Sandy and the heat waves would not have occurred without climate change. It finds, instead, that they would not have been as intense if not for anthropogenic climate change.

Consider This

  1. Question 6.2

    bcBxi+Ig5Cl+FuG2yVoJieH7e2a2M9UhghtVS0SO694xuDijvqEjvGLOwEpgfXt021aN/3O5SMH2GYAi7rksCF0ulWrE/BCbh9hTTemssz/aOQzH1qbBty9gMs7XmJP/9v+nKg==
  2. Question 6.3

    YweY3ogTjY2FASvs3wqfDDbfVNcBPaMvClggJcPCkleNewLtAoARSYY07Au4x/udc1KAQSU5jfsKJdSRL/wkafmywjMz6+iN8tgpTI+wkBXsS8goflW/o7toeM6fg7Jt5zVyR7vOuca310p63pdVfKMxJkivXfju

195

Climate Forcing and Feedbacks

Question 6.4

What factors cause Earth’s climate to change?

Climate changes in response to climate forcing factors from outside the climate system and climate feedbacks within the climate system.

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).

climate forcing factor

A force that can change climate and is unaffected by the climate system.

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:

ice-albedo positive feedback

A destabilizing positive feedback in the climate system in which the melting of ice and snow expose bare ground and ice-free water, which absorb more solar energy and cause more warming.

196

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.