Origin and Evolution of Earth Research Questions for a Changing Planet (2008) / Chapter Skim
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3 A Habitable Planet
3 A Habitable Planet
Pages 71-94
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From page 71... ...
However, despite the many ways that natural forces can change Earth's QUESTION 7: WHAT CAUSES CLIMATE climate, substantial geological evidence suggests that TO CHANGE -- AND HOW MUCH CAN IT Earth's overall climate, although it has oscillated be- CHANGE? tween relatively warm and relatively cold states many times, has somehow been maintained in a reasonable, Among the systems of planet Earth addressed in this and quite narrow, range that is conducive to the pres- report, climate is the most widely discussed in public ervation of life.
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From page 72... ...
The overarching issue is the fraction temperature, sea surface temperature, and mean annual of Earth's carbon that is present in the atmosphere in precipitation -- vanished thousands or millions of years the form of CO2 or other greenhouse gases like CH4. ago, along with the climate they shaped.
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From page 73... ...
by geological and biological processes. Over the past Over the past century and through the next, century, fossil fuel burning has overwhelmed natural changes in the greenhouse gas content of the atmoprocesses, quickly transferring a large amount of bur- sphere are the most important factor affecting climate, ied carbon (in the form of organic matter, petroleum, although changes in atmospheric particulates and coal, and natural gas in sedimentary rock formations)
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From page 74... ...
affect how canoes, for example, tend to move CO2 from the deep solar energy is distributed around the globe and lead to Earth to the atmosphere, whereas erosion of mountain changes in mean annual temperature, precipitation, and ranges and the associated chemical weathering of seasonality. Earth's orbital cycles are responsible in part minerals tend to remove CO2 from the atmosphere for the oscillations between ice ages and interglacial pe- and ocean and bury it as calcite and organic matter in sediments on the ocean floor.
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From page 75... ...
Fifty million years ago, bottom water temperatures were about 10°C to 12°C, which corresponds to a global mean surface temperature of about 25°C. From the early Oligocene to the present, about 70 percent of the variability in the δ18O record reflects changes in Antarctica and northern hemisphere ice volume.
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From page 76... ...
It is this process The geological record suggests that climate has stayed within that produced the rock formations that we now exploit these extremes throughout Earth's history, except for geologically for fossil fuel. brief "snowball Earth" episodes in the Precambrian.
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From page 77... ...
, creating a "snowball Earth." This hypothesis geological record occurred in the Cretaceous period, is vigorously disputed (e.g., Hyde et al., 2000) -- not the about 120 million to 90 million years ago (Barron and anomalous cold but its cause, duration, and severity.
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From page 78... ...
Mean annual temperature, Millennia Speleothems (O isotopes) precipitation Sea surface temperature Millennia Corals (O isotopes, Sr/Ca, and U/Ca)
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From page 79... ...
required if the snowball Earth hypothesis is to become The warm conditions following the snowball Earth an established chapter in Earth's climate history. Nevperiod may have arisen because volcanism would have ertheless, even the most moderate of interpretations of continued through the snowball period, contributing the Neoproterozoic evidence for glaciation suggest that CO2 to the atmosphere that could not be removed by it was the coldest period in the past 2 billion years.
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From page 80... ...
Examples of abrupt climate recorded in Greenland ice cores. A number of mechachange include the Permian-Triassic boundary (see nisms have been invoked to explain them, including Question 8)
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From page 81... ...
Other key questions include whether other greenhouse gases were important in the more distant occurred during earlier glacial periods, although high- geological past, and whether other causes of climate resolution ice core and marine sediment records are not change besides greenhouse gas forcing can be inferred available to confirm this. from the geological record.
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From page 82... ...
Seafloor generation rates are calculated from plate tectonics reconstructions and ridge or trench lengths or from global sea level determined from shoreline markers. However, uncertainties are large and results vary.
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From page 83... ...
Because of this importance, there relate weathering rates to erosion rates and mountain is a major effort to reduce its uncertainties. building, and to evaluate how the age dependence of Weathering rates of ancient rock are not well weathering rates affects models for the regulation of known because of basic uncertainties about the pro- global climate.
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From page 84... ...
Geological time periods, natural geological processes control the processes and astronomical events have strongly and greenhouse gas content of the atmosphere, and other repeatedly influenced the story of life on Earth and geological and astronomical factors are influential. We often determine the kinds of life that can survive and have a good qualitative understanding of the factors flourish.
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From page 85... ...
also shines its actions. a strong new light on natural biochemical processes, bringing the possibility of calculating from quantum How Does Life Affect Geological Processes?
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From page 86... ...
O2 (% of present day) 0 FIGURE 3.11 The history of life, based on geological evidence, along with long-term oxygen, ice ages, and mass extinctions.
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From page 87... ...
For eral skeletons and tough organic materials preserved example, we would like to know whether rates of ero- as conventional fossils, and this is particularly true sion will change with changing climate and whether of the microorganisms whose metabolic capabilities climate-induced variations in vegetation will reduce or define much of the interface between the physical and enhance the response of erosion rates to climate change. biological Earth.
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From page 88... ...
. Copyright 2006 by Elsevier Science and Technology Journals.
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From page 89... ...
Whether the diversity of early photosyn thetic bacteria included the oxygen-producing cyanobac 1 2 3 4 5 teria remains uncertain. Geological evidence indicates 500 that whether or not oxygen-generating photosynthesis 400 evolved in Archean oceans, O2 did not become significant deep oceans in the atmosphere and surface oceans until about 2.4 mO (µmol)
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From page 90... ...
. Copyright 2003 by Elsevier Science and Technology Journals.
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From page 91... ...
So far, we know that these greatest mass extinction at the end of the Permian peevents coincide with perturbations in the carbon cycle, riod (252 Ma) when marine ecosystems collapsed (e.g., as deduced from records of the isotopic composition see below)
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From page 92... ...
Problems as diverse as the influence of rainforests them. But what specific events or environmental on Earth's hydrological cycle, the role of vegetation in changes precipitated the great mass extinctions, and stabilizing the land surface, the relationship between what aspects of biology influenced the patterns of nutrient availability and diversity, and the oceanwide survival and recovery, are not known.
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From page 93... ...
Erosion rates, climate, and weathering rates long-term history of life? The answer certainly requires affect the habitability of specific regions of Earth, and macroecological insights from biologists, but the ques- the ecosystems themselves in turn affect erosion rates, tions are necessarily framed by paleontologists.
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From page 94... ...
94 ORIGIN AND EVOLUTION OF EARTH interrelationships between surficial processes that shape lectual challenge. Meeting this challenge will help the land and the life that inhabits it presents a critical us understand how life will respond to present-day challenge for managing land resources and becomes environmental change, but Earth scientists will have even more important as we attempt to forecast the ef- to develop new research and educational partnerships fects of future climate change.
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