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Chapter 23: Global Change in the Earth System
Guide to Reading
You've read twenty-two 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 two basic reasons: it has a mobile asthenosphere that allows plate tectonics phenomena to occur, 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. Change within the Earth system 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).
- 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.
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 and cosmic rays, changes in Earth's orbit and tilt (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 (KT) 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 that 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. Most climate researchers believe that global warming is real, and that human activities have played a significant role in causing it. Just how significant a role, and what we can and should do about it, are still 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 has 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 future and that, over the long term, plate tectonics activity will slowly but inexorably change the appearance 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 asteroid and comet 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. « Return to Chapter 23 Study Plan