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Guide
to Reading 
You’ve read 18 chapters about the physical
makeup of Earth and the actions and interactions of its component
parts. This last chapter presents some new ideas but also reviews
material previously presented, to stress the idea that Earth has
always been and will continue to be a dynamic planet—ever changing
in its physical and biological features. Some new vocabulary is introduced
to further develop this concept of change, and several topics from
previous chapters are presented in the context of how they will shape
Earth’s future.
The chapter begins by reminding the reader that Earth
is a unique planet for three basic reasons: it has a mobile asthenosphere
that allows plate tectonics phenomena to occur, gravity influences
the flow of air and water across its surface, and its surface temperature
straddles the freezing point of water, a fact that strongly influences
surface processes. Numerous internal and external processes interact
and create constant change on the planet. The term “Earth system”
is commonly used to embrace this worldwide interconnecting web of
physical and biological phenomena, and “global change”
refers to transformations or modifications of the components of this
system. These changes may be gradual or catastrophic, unidirectional
or cyclical, biogeochemical or anthropogenic.
The unidirectional changes (transformations that
progress in one direction and never repeat the same steps) you read
about involve the evolution of the solid Earth, the atmosphere, the
oceans, and life on Earth (the biosphere).
Cyclical changes involve the same steps, repeated
over and over again, that may or may not produce similar results.
Three physical cycles are discussed:
- The supercontinent cycle: You’re reminded
that geologists believe all continental material has been merged
into one giant continent at least three different times in Earth’s
history.
- The sea-level cycle: The sea’s transgressions
and regressions have left a physical record of major sedimentary
sequences and their minor subdivisions called cyclothems.
- The rock cycle: Earth’s internal processes
(including rifting, mantle plumes, subduction, sea-floor spreading,
and convergence) and external processes (including weathering,
erosion,
and deposition) cycle the atoms of minerals through the three basic
rock types (igneous, sedimentary, and metamorphic).
Two biogeochemical cycles are examined:
- The hydrologic cycle: Water may exist in any
of its states (gas, liquid, or solid) as it cycles through the
oceans, atmosphere, surface water, groundwater, glaciers, soil,
and living organisms.
- The carbon cycle: Emphasis is placed on the role
of greenhouse gases as they interact with rocks and influence
climate.
The chapter continues with a discussion of the many
facets of global climate change. There are long-term and short-term
changes and global warming and global cooling to be considered when
trying to predict climate change. Geologists look at past climates
(paleoclimates) in order to predict future climates. They study paleoclimates
by examining the stratigraphic record, paleontological evidence (including
fossil pollen), oxygen-isotope ratios in ice and in plankton shells,
air bubbles in ice, growth rings of trees (dendrochronology) and
of corals and shells, and human history.
Long-term climate changes can result in greenhouse
periods or ice-house periods (with or without ice ages). They’re
caused by changes in the positions of the continents, volcanic activity,
uplift of land surfaces, and formation of coal, oil, and limestone.
Short-term climate changes (such as the Younger Dryas, the Holocene
climatic optimum, the medieval warm period, and the little ice age)
may be explained by fluctuations in solar radiation, changes in Earth’s
orbit (Milankovitch’s ideas), changes in the reflectivity (albedo)
of Earth, and changes in the ocean currents.
Catastrophic climate changes seem to be linked to
mass-extinction events, when large percentages of existing species
disappeared and the biodiversity on Earth was greatly diminished.
You read about two such mass-extinction events, one at the Permian-Triassic
boundary and the other at the Cretaceous-Tertiary (K-T) boundary.
Before getting into a discussion about how humans
affect Earth, the author presents some basic facts about human population.
It is obvious that at first human population increased slowly, but
now the population and the population growth rate are high enough
to significantly impact Earth. Human activities have modified landscapes
and ecosystems, and quite possibly they’re starting to modify
the global climate by contributing chemicals that enhance global
warming.
There’s general agreement some human-caused
(anthropogenic) changes are undesirable. Slash-and-burn agricultural
practices and the introduction of contaminants that cause pollution
(smog, photochemical smog, water contamination, acid runoff, acid
rain, radioactive materials, and ozone depletion) are detrimental
to both Earth and humankind. Whether or not Earth is truly warming
and whether or not human activities are playing a significant role
in the process are both hotly debated issues. Individuals and governments
worldwide are concerned because any changes will have political ramifications
and will affect the welfare of the physical Earth and all life on
it.
The chapter (and text) quite appropriately conclude
by addressing the question of Earth’s future. All through the
text you’ve been reminded that if it’s happened on Earth
before, it will probably happen again, but what does this really
mean? There’s general agreement that human activity will play
a significant role in the geologic near-term future and that over
the long-term plate tectonics activity will slowly but inexorably
change the look of Earth’s surface. Other long-term scenarios
are less certain, and some of them range from unpleasant to disastrous.
Severe inland flooding has happened and could happen again. Earth
has been damaged in the past by bolide impacts and could be again.
The creator of Superman probably had the right idea, that planets
can be destroyed by large enough impacts, and that could be Earth’s
fate. Scientists do believe that in 5 billion years the Sun will
run out of fuel and “die,” and if Earth is still around,
it will be engulfed and vaporized by the expanding Sun in its “death
throes.” Remember all of these predictions are not certainties,
but they are the best scientific guesses possible today. Are they
really going to happen? Only time will tell.
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