Page 34 - Earth's Climate Past and Future

10 PART I • Framework of Climate Science

evaporation, and winds. These processes extend from the it occurs mainly by way of additions to the atmosphere
warm tropics to the cold polar regions and from the Sun of materials such as carbon dioxide (CO ) and other
2
in outer space down into Earth’s atmosphere, deep into greenhouse gases, sulfate particles, and soot. Anthro-
its oceans, and even beneath its bedrock surface. All these pogenic effects will be covered in Part V.
processes will be explored in this book.
The complexity of the top part of Figure 1-5 is sim-
plified in the bottom part to provide an idea of how the 1-6 Climate System Responses
climate system works. The relatively small number of The components of Earth’s climate system vary widely:
external factors shown on the bottom left force (or global mean and regional temperatures, the extent of ice
drive) changes in the climate system, and the internal of various kinds, the amounts of rainfall and snowfall, the
components of the climate system respond by changing strength and direction of the wind, the circulation of
and interacting in many ways (bottom center). The water at the ocean’s surface and in its depths, and the types
result of all these interactions is a number of observed and amounts of vegetation. Each of these parts of the cli-
variations in climate that can be measured (bottom mate system responds to the factors that drive climate
right). This complexity can be thought of as the opera- change with a characteristic response time, a measure of
tion of a machine: the factors that drive climate change the time it takes to react fully to the imposed change.
are the input, the climate system is the machine, and the Consider the example shown in Figure 1-6: a beaker
variations in climate are the output. of water above a Bunsen burner. The Bunsen burner

1-5 Climate Forcing
Three fundamental kinds of climate forcing exist in the
natural world:
Temperature of water
• Tectonic processes generated by Earth’s internal heat (response)
affect its surface by means of processes that alter the
basic geography of Earth’s surface. These processes
are part of the theory of plate tectonics, the unifying
theory of the science of geology. Examples include
the slow movement of continents, the uplift of
mountain ranges, and the opening and closing of
ocean basins. These processes operate very slowly Source of heat
over millions of years. The basic processes of plate (forcing)
tectonics are explained in Part II of this book.
A
• Earth-orbital changes result from variations in Earth’s
orbit around the Sun. These changes alter the
Heat
amount of solar radiation (sunlight and other turned on
energy) received on Earth by season and by latitude Heat maintained
(from the warm, low-latitude tropics to the cold,
high-latitude poles). Orbital changes occur over tens Warm
to hundreds of thousands of years and are the focus Water temperature
of Parts III and IV. 50%

• Changes in the strength of the Sun also affect the
amount of solar radiation arriving on Earth. One
example appears in Chapter 5: the strength of the No heat Cool
Sun has slowly increased throughout the 4.55 Byr Response Time
of Earth’s existence. In addition, shorter-term B time
variations that occur over decades or longer are part
of the focus of Part V. FIGURE 1-6 Response time Earth’s climate system has a
response time, suggested conceptually by the reaction of a
A fourth factor capable of influencing climate, but not beaker of water to heating by a Bunsen burner. The response
in a strict sense part of the natural climate system, is the time is the rate at which water in the beaker warms toward
effect of humans on climate, referred to as anthropogenic an equilibrium temperature. (Adapted from J. Imbrie, “A
forcing. This forcing is an unintended by-product of Theoretical Framework for the Ice Ages,” Journal of the Geological
agricultural, industrial, and other human activities, and Society (London) 142 [1985]: 417–32.)