Page 125 - Environment: The Science Behind the Stories

Earth’s Environmental Systems Systems involve feedback loops

A system is a network of relationships among parts, elements,
Understanding the rise and fall of the oyster industry in the or components that interact with and influence one another
Chesapeake Bay, as with many other human impacts on the through the exchange of energy, matter, or information.
environment, involves comprehending the complex networks Earth’s environmental systems receive inputs of energy, mat-
of interlinked systems that comprise Earth’s environment. ter, or information, process these inputs, and produce outputs.
These include physical systems ranging from matter and As a system, the Chesapeake Bay receives inputs of freshwa-
molecules up to magma and mountains (Chapter 2). They ter, sediments, nutrients, and pollutants from the rivers that
include biological systems ranging from organisms and pop- empty into it. Oystermen, crabbers, and fishermen harvest
ulations (Chapter 3) to communities of interacting species some of the bay system’s output: matter and energy in the
(Chapter 4). In ecosystems they involve the interaction of liv- form of seafood. This output subsequently becomes input to
ing creatures with the nonliving entities around them. Earth’s the nation’s economic system and to the digestive systems of
systems encompass cycles involving rock, air, and water that people who consume the seafood.
shape our landscapes and guide the flow of chemical ele- Sometimes a system’s output can serve as input to that
ments and compounds that support life and regulate climate. same system, a circular process described as a feedback loop.
We depend on these systems for our very survival. Feedback loops are of two types, negative and positive. In a
Assessing questions holistically by taking a “systems
approach” is helpful in environmental science, in which so negative feedback loop (Figure 5.1a), output that results from
a system moving in one direction acts as input that moves
many issues are multifaceted and complex. Such a broad and the system in the other direction. Input and output essentially
integrative approach poses challenges, because systems often neutralize one another’s effects, stabilizing the system. For
show behavior that is difficult to predict. The scientific method instance, a thermostat stabilizes a room’s temperature by turn-
(pp. 28–30) is easiest when researchers can isolate and manip- ing the furnace on when the room gets cold and shutting it off
ulate small parts of complex systems, focusing on manageable when the room gets hot. Similarly, negative feedback regulates
components one at a time. However, environmental scientists our body temperature. If we get too hot, our sweat glands pump
are rising to the challenge of studying systems holistically, out moisture that evaporates to cool us down, or we move into
helping us to develop comprehensive solutions to complicated the shade. If we get too cold, we shiver, creating heat, or we
problems such as those faced in the Chesapeake Bay.
move into the sun. Most systems in nature involve negative

Figure 5.1 Negative feedback loops
Brain
(control center) exert a stabilizing influence on sys-
tems, whereas positive feedback
Seek shade Body
Too hot loops have a destabilizing effect. The
Sweat cools
human body’s response to heat and cold
(a) involves a negative feedback loop that
Wear more clothes Body keeps core body temperatures relatively
Too cold
Shiver warms stable. Positive feedback loops, in contrast,
push systems away from equilibrium; for
example, when Arctic glaciers and sea ice
melt because of global warming (b), darker
(a) Negative feedback surfaces are exposed, which absorb more
sunlight, causing further warming and
further melting.
1 In cool climate, sunlight reflects off 2 As climate warms, sunlight is 3 Light absorption speeds warming,
white surfaces absorbed where dark surfaces are exposing more dark surfaces
exposed

Solid surface Glacier completely Sea ice Glacier More water More land
of sea ice covers land melting melting exposed exposed
124 (b) Positive feedback

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