U4735x Environmental Science for Decision Makers
Lecture 3: Atmospheric evolution, iron deposits, carbon dioxide trends and recent glacial climate
Jim Simpson
1 - Key concepts about time scales and orders of magnitude
2 - Earth history: PreCambrian & Cambrian
3A - Abundance of major gases in atmosphere
3B - Abundance list of gases in atmosphere, including green house gases
4 - Greenhouse gas warming: net increase in global mean temperature due to trace gases in the atmosphere
5A - History (schematic) of free oxygen in atmosphere (log-log plot)
5B - History (schematic) of free oxygen in atmosphere (linear plot)
6 - Histogram of ages of deposition of large banded iron formations
7 - Solubility of dissolved iron in water as a function of oxidation state of Fe
8 - Locations of PreCambrian banded iron formations
9 - Iron (Fe) available for mining and human rates of current use: order of magnitude estimates of time to deplete global Fe ore reserves (& resources)
10 - Reserves & resources: definitions
11 - Global amounts of reserves & resources vs cost of production per ton: plentiful vs scarce non renewable resources
12 - Public policy options for “plentiful” vs “scarce” non renewables: examples representative of Fe & liquid petroleum
13 - Northern hemisphere ice sheet today: Greenland much smaller than Antarctic Ice Sheet
14 - Northern hemisphere ice sheet 18,000 years before present: huge new ice sheets over N American & N Eurasia
15 - Sea level changes equivalent to continental glacier ice volumes
16 - Location of Mauna Loa Observatory (Hawaii): central North Pacific
17 - Atmospheric carbon dioxide at Mauna Loa Observatory: 1958-1974
18 - Atmospheric carbon dioxide at Mauna Loa Observatory: 1958-1994
Presentation of material will largely be from the viewpoint of earth science, rather than economics, engineering, or policy regulatory framework. However, these latter viewpoints are clearly crucial to development of effective management policies and will be discussed in terms of explicit examples during the course. Several features characteristic of the earth sciences include (Projection 1):
Our primary goal will be to outline some aspects of natural processes influencing available fresh water and energy, as well as global climate, especially as they are related to human activities. Examples of specific short-term, local management issues will be included, primarily to illustrate long-term, regional to global concerns. In all cases, it is essential to have sufficient understanding of the natural environment to be able to evaluate the likely effectiveness of particular policies. One of the first examples to be discussed here of a resource important to human activities is iron. Processes which led to formation of the most abundant iron ores are complicated and involve critical steps in the evolution of Earth’s atmosphere and ocean chemistry.
The earth is known to have been formed about 4.6 billion (109) years ago, and subsequently evolved an ocean and atmosphere prior to approximately 4 billion years ago. Enormous detail about the history of life on the earth for the most recent 0.6 billion years (less than 15% of geologic time) is known through study of the fossil record, especially for marine organisms. The geological period that began about 0.6 billion years ago is known as the Cambrian (Projection 2). The 87% of geologic time prior to the Cambrian is often described as the PreCambrian. Detailed understanding of geologic history that has evolved for the time since the PreCambrian came about primarily because the most recent 600 million years includes development of life forms with phases resistant to rapid breakdown after death, such as calcium carbonate shells, and internal mineralized skeletons. Prior to 0.6 billion years ago, very little of the soft organic tissues of early life has been preserved.
Our present atmosphere is more than three quarters nitrogen and about one fifth oxygen, with a number of lower abundance gases that play critical roles in the heat budget of the atmosphere and in other process (Projections 3A, 3B). Water vapor, which accounts for less than one percent of the mass of the troposphere, is the most important single component of the "greenhouse gases" that are responsible for maintaining average temperature conditions at the Earth’s surface (15°C or 59°F) that are well above the freezing temperature of water. Without their presence, assuming the same reflectivity of the present earth (albedo), the mean temperature of the planet would be -20°C (-4°F), well below the freezing point of water (0°C) (Projection 4) and nearly all water molecules would be in the form of ice as is now true for Antarctica and Greenland. The term "greenhouse gas" is often used as a short-hand description of the ability of some gases to permit visible radiation from the sun to pass through to reach the Earth’s surface, but to absorb longer-wave infrared radiation emitted from the surface before it can escape to space, somewhat analogous to the effect of the panes of glass in a greenhouse. Many of the important greenhouse gases other than water vapor are present in extremely small concentrations (orders of magnitude less than nitrogen and oxygen) yet have major effects on the heat balance of the earth due to their very high efficiencies at absorbing outgoing long-wave radiation. Note that all the greenhouse gas molecules have three or more atoms per molecule, which makes them far better able to interact with the low energy radiation emitted from the Earth's surface in the infrared part of the spectrum. Current reconstructions of the history of the atmosphere indicate that appreciable free molecular oxygen (at about one percent of the present atmospheric level) was not present until about 2 billion years ago, and accumulated to approximately one tenth of the present level sometime between 2.0 and 0.6 billion years ago (Projections 5A, 5B). Thus accumulation of an oxygenated atmosphere essential to all current animal life required several billions of years, a major fraction of earth history, and was the result of the evolution of single cell plant life capable of photosynthesis and survival in the absence of free O2. No other planet in our solar system has an atmospheric composition even remotely like that of earth because our planet was apparently the only one to develop life on a large scale. The evolution of living organisms and the composition of the atmosphere have been linked through almost the entire history of earth. Without the extremely long history of free oxygen accumulation in the atmosphere as the result of photosynthesis by primitive green single cell plants (prokaryotic cells), none of the higher life forms would have been able to evolve.
One of the effects of rising oxygen in the atmosphere was the formation of ozone in the stratosphere as high energy solar ultraviolet (uv) radiation impacted oxygen molecules at elevations of 15 to 30 kilometers above the earth’s surface. The subsequent presence of ozone in the stratosphere provided a shield to further penetration of uv radiation that is destructive to the organic compounds that comprise living organisms. Without that shield green plants would have been unable to live on the land surface exposed to direct sunlight. Again one of the critical steps in the evolution of life was linked to the interplay between the global environment and living organisms.
There is considerably evidence that the first half of earth history had an atmosphere largely free of oxygen. One of the most dramatic clues involves large masses of unique iron mineral deposits, known as banded iron formations, that accumulated at a number of sites around the world between 2.5 and 2 billion years ago (Projection 6). The discrete period for formation of these deposits indicates that very special environmental conditions were required for them to develop. They required very low, but not zero free oxygen in the atmosphere to form. Iron is a peculiar element that is slightly soluble in water in the absence of oxygen, but extremely insoluble with trace amounts of oxygen (Projection 7). The assemblage of minerals, especially siderite (FeCO3) and other iron carbonate minerals, that make up banded iron formations suggest that the iron was initially in solution, and then precipitated out with almost no associated detrital minerals such as clays and other aluminosilicate minerals. Such deposits could not form in estuaries or coastal waters of the modern ocean because of the abundance of clays and silts found in such environments today. The oceans of this mid PreCambrian world must have had appreciable areas where iron was a moderately abundant dissolved ion, in great contrast to today’s oxygen-rich ocean which contains essentially no dissolved iron. No mineral formations even remotely similar to these PreCambrian banded iron deposits are being accumulated anywhere in the world today. For our modern mineral-hungry human society, these banded iron formation represent the primary source of the element that is refined and transformed into steel, one of the key materials that underlies the modern industrial economy. In the USA, about 75% of our annual supply of iron ore is derived from the large banded iron PreCambrian deposits of the Lake Superior District (Projection 8). Other famous deposits of this iron ore type include western Australia (Hammersley Range), South Africa (Transvaal), and southern Brazil (Minos Gerais). The locations of these deposits include many of the oldest fragments of continental plates from early in Earth history. The unusual chemistry of dissolved Fe, influenced by the presence or absence of dissolved O2, will be discussed later in the course during consideration of arsenic contamination in Bangladesh groundwaters. The dependence of dissolved Fe behavior on reducing/oxidizing conditions is sometimes described in terms of a "redox" sensitive environmental cycle. The chemistry of many ions dissolved in natural waters can be strongly influenced by the presence or absence of dissolved oxygen.
One issue of interest concerning any material that plays a central role in industrial society is the order of magnitude of the total global amount available, compared to the amount consumed each year. Qualitatively, you would not expect that the world supply of iron ores was likely to be depleted in the near future (i.e. during the 21st century). Iron is one of the most abundant elements in the earth’s crust (5% by weight). World per capita consumption of steel, the predominant use of Fe in industrial human activity, is about 150 kg per year (Projection 9), equivalent to about 3 times the weight of an average human! Note that we will generally use metric units during this course, since they are most common in scientific reports and more convenient for calculations. Multiplying annual per capita steel consumption by current global population (about 6 billion), total steel consumption is about 1 billion metric tons per year. Dividing this rate of Fe use into recent estimates of Fe ore reserves (2.2 x 1011 T), indicates a supply life-time of these reserves about 240 years. The term “reserves” for mineral deposits usually refers to those ores which could be exploited economically at their current market price and cost of production. If we were to assume that the entire global volume of banded iron deposits, multiplied by 0.2 because they are only about 20% by weight Fe, could be eventually mined (global “resource”), the total time to deplete all such deposits at current consumption rates would be about one hundred thousand (105) years! Although it is not plausible that the entire volume of banded iron deposits in the world would be eventually mined, this order of magnitude calculation provides strong indication that all accessible Fe ores in the world are not likely to be depleted in the next few centuries.
The rate of iron ore consumption today is many orders of magnitude faster than the rate at which these ore deposits were formed several billion years ago. If we assume all the Fe in banded iron formations were deposited over a period of one billion years (formation time about 109 yrs), and that all were to be mined at the rate of current consumption (consumption time about 105 yrs), the rate of consumption would be about ten thousand times faster than the rate of original deposition. Hence iron ores clearly should be classified as "non renewable" but probably also in the category of “plentiful”.
Discussion for the timing and mode of deposition of banded iron formations provides some motivation relevant to public policies for non renewable resources such as iron ores. The term RESERVES in such a context usually refers to identified deposits that can be recovered and sold at a profit at current market prices (Projection 10). In the case of Fe, the 1990 estimate of global reserves was about 2.2 x 1011 T. The term RESOURCES generally includes known RESERVES plus all undiscovered deposits plus deposits already discovered but currently not economical (marginal and sub economic) to extract and sell at world market prices. In the case of Fe, the global resource (assuming all banded iron formations could eventually be mined and sold) is more than two orders of magnitude greater than global reserves (actually about a factor of 500 greater). It is not unreasonable to classify Fe as a “plentiful” non-renewable resource in the sense that moderate increases in costs associated with extraction and refining could probably expand the total available supply appreciably, perhaps by several orders of magnitude (Projection 11). Note that the graphical illustration of this simple classification uses a semi-log plot of total amount of material (i.e. Fe) which could be obtained (log scale) vs cost per metric ton (T) for supply (linear scale).
If estimates of the global resource of a material were only a modest amount greater than known global reserves (say a factor of 3), and the material were in short supply relative to current rate of consumption, then one could reasonably classify that material as “scarce”. Compared to Fe ores, liquid petroleum would be much more towards the “scarce” end of the spectrum. Estimating the size of global resources for any material is difficult and substantial differences using different approaches would be expected. However, enough is known about natural processes which led to formation of economic deposits such as banded iron formations and petroleum to provide strong constraints on the likely magnitudes of global resources. It is important that such insights be much more explicitly included in formation of public policies towards nonrenewable resources than currently occurs.
In the case of Fe (Projection 12), it might be plausible to let world market forces govern most prices at the producer and consumer level, since Fe ores (including those huge banded iron formations that are known but currently subeconomic to extract) are relatively “plentiful”, especially compared to many less abundant metals and liquid petroleum. One obvious issue relevant to Fe pricing is the degree to which recycling is stimulated. It has been estimated (Meyers, 1993, p. 105) that currently almost 1/2 of Fe used as input for new steel production is derived from recycling “old” steel, such as auto bodies. It is plausible that explicit policies to encourage such practices could appreciably reduce the proportion of finished Fe industrial products that were derived from new ore extraction and thus extend the ultimate life of these ore resources.
In contrast, materials which are relatively “scarce”, such as liquid petroleum, when considered on time-scales of a few decades to half a century (which is longer than is usually reflected in much of the current pricing of non-renewable resources) are plausible candidates for public policies that result in significantly higher effective prices on such materials. Substantial consumption taxes at the retail level, such as are almost universally imposed in Europe and most other industrial countries for gasoline fuels for personal automobiles, can be effective in reducing total demand, although they are generally quite unpopular with consumers and political leaders. Another possibility is taxes at the extraction phase. Federal tax policy in the USA (oil depletion allowance) for many years was the inverse of this policy. Thus one could argue that in the USA, central government policies in the past tended to stimulate both domestic production and consumption of petroleum products (through quite low consumer taxes relative to most other countries). The net effect has been to accelerate depletion of the total domestic USA reserves of petroleum.
Water is most commonly considered as a plentiful, renewable resource, except in arid areas. This general assumption can be quite misleading for fossil groundwaters as well as some surface water resources, indicating that much more careful assessment of the dynamics of human interactions with water resources is needed.
In the above discussion of resources of Fe ores relevant to steel production, no mention was made of other constituents of steel besides Fe. Steel production involves a wide range of types of metal from large beams for buildings, to automobile bodies, to machine tools for fabrication of other materials. Each of these has special requirements in terms of hardness, etc. In terms of general classes of total world steel production (information derived from Encyclopedia Britannica), the proportions of the three main types are approximately: 90% carbon steel, 10% low alloy steel and 1% high alloy steel (die stainless steel). Carbon steel contains from 0.08% to 1.2% C, derived from reduced C such as coke. Such large amounts of coke (derived from coal) are used as an energy source and also to remove oxygen from Fe oxides in refining of Fe ore to make steel, that the C ingredient in the final steel is not a significant additional resource demand. Thus reduced carbon resources will not limit the potential to produce steel. The largest amounts of other metals used in steel production are Ni and Cr, neither of which is currently in very short supply. A significant number of other elements are used to manufacture various types of steel production, including: V, Mo, B, W, Co Cu, Pb, Mn, Ti, Al, P, S, Si. Of these elements, molybdenum (Mo) and tungsten (W) are probably most likely to experience resource limitations in the coming century. Thus to the first approximation, the basic ingredients for steel production, including elements added in small proportions appear to be available at a reasonable cost of extraction for the next few centuries.
All of us living in the temperate zone at latitudes similar to New York City (40°N) experience fairly dramatic changes of mean daily temperature during the course of a year, and even temporarily over a few days during the spring and fall seasons that can experience rapid movement of air masses from quite different latitudes. However, we tend to assume that summer or winter temperatures in the New York City area have been similar over very long periods. However, eighteen thousand years ago, there was a massive sheet of ice covering a major fraction of North America which extended right to the current site of NYC! The climate here at that time must have been quite similar to that of southern Greenland today, with a huge continental-scale sheet of ice that was several kilometers thick just to the north. On the time-scale of earth history, 18,000 years ago is yesterday! Over the past 2.6 million years, there have apparently been more than 25 occurrences of such vast continental-scale ice sheets in North America that lasted for tens of thousands of years and then mysteriously disappeared to yield a mean climate similar to today. People are still struggling to understand how this came about, but there is no serious doubt that such massive climate shifts have occurred repeated in this most recent geological epoch, often referred to as the Pleistocene (last 2 million years of geologic time). The time since the end of the last glacial episode, about the last 10,000 years, is often referred to as the Recent or Holocene epoch. Note that this most-recent interglacial interval is roughly coincident with development of organized agriculture by humans.
The largest continental ice sheet on the planet today is in Antarctica. So even today, during an interval which is generally referred to as “interglacial”, there is a substantial amount of “permanently” frozen water. Thick layers of ice have been present continuously in the Antarctic for more than 30 million years. In the northern hemisphere today, the only significant volume of permanent ice is on Greenland (Projection 13), although the area covered by sea ice in the Arctic Ocean is also quite substantial. The total areal coverage and volume of permanent ice in the northern hemisphere today is much smaller than in high southern latitudes. However, during the last glacial maximum the situation was dramatically different (Projection 14). A major fraction of North America and northern Europe and Asia were covered with ice that had thickness approaching 4 kilometers near the center of modern day Canada. The effects of this “ice age” are still obvious today in features as dramatic as the St. Lawrence Great Lakes and the bed rock channel of the Hudson River, which was carved by a huge ice tongue, just as were the coastal fjords of Norway and British Columbia. Essentially all of modern Long Island is a huge sand pile produced as glacial till swept along by the continental glacier to the north to a zone at which melting balanced the southward flow of ice, to generate the low latitude margin of the huge glacier.
The volumes of ice stored on the land during glacial maximum periods were so large that the mean level of the ocean dropped substantially (Projection 15). Current estimates indicate the magnitude of this decline in sea level to be more than 100 meters at 18,000 years before present (BP). As the glaciers melted between 18,000 and 10,000 BP, sea level returned to approximately that of today. There is a very detailed record of past sea levels stored in the isotopic composition of small calcium carbonate fossils of marine organisms, which has recorded the very dynamic behavior of the world climatic system over the past two million years. Clearly the history of climate during the evolution of humans has been one of dramatic fluctuation. The hydrologic cycle during glacial intervals was very different from that of today, leading to groundwater recharge in some areas much greater than now. Exploitation of these “fossil” groundwaters is very likely to lead to depletion of some of these resources in the near future.
The planet on which we live has undergone major modifications over its history as the result of the existence of life, especially green photosynthetic plants. The atmospheric oxygen on which all animal life depends and precious supplies of fossil fuels such as coal, petroleum and natural gas, essential to modern industrial economies, were all derived from the integrated activities of photosynthetic plants interacting with geologic process over millions to billions of years. Over the past several centuries, the effects of human activities on the global atmosphere have begun to be readily measurable. As the rise in numbers of people and increased per capita demands on resources compound the integrated effects, we have become a force that impacts on the world environment on scales, including both rate of change and magnitude, that are almost unprecedented in geologic history. Probably the only other events in Earth history comparable in magnitude (and even more rapid in impact) to current human influences were the catastrophic collisions of huge meteorites and/or comets with Earth that led to large-scale extinctions of species, such as probably occurred at the end of the Cretaceous Period (65 million years ago) and resulted in the termination of the age of dinosaurs. One of the most important of human impacts since the beginning of the industrial revolution is alteration of the composition of the atmosphere due to combustion of fossil fuels.
In the late 1950’s one of the most important long-term environmental monitoring efforts of the 20th century was initiated near the top of a volcano on the Island of Hawaii (Projection 16). At Mauna Loa Observatory, originally a US Weather Bureau research station, measurements of the concentration of carbon dioxide in the atmosphere were initiated in 1958 and have been continued to the present. This site is located far from large urban regions with highest rates of combustion of fossil fuels, such as North America, Europe and eastern Asia, but is in the same hemisphere as those regions and the majority of other industrialized countries. Air samples collected from high elevation in Hawaii are thus excellent indicators of average northern hemisphere CO2 concentrations for derivation of long-term trends. A number of other sites now also have CO2 measurements in progress, but the longest continuous record is that from Mauna Loa. A portion of that record for the period of 1958 through 1974 illustrates some of the features that have been observed (Projection 17). Using average monthly CO2 concentrations (about 0.03% of total atmospheric gases), expressed in units of parts per million (ppm), concentrations increased over this 17 years from 312 to 333 ppm. The variations within a single year of about 5 ppm, around the mean trend from year to year, are believed to result primarily from uptake of CO2 by land plants in the northern hemisphere during the temperate and boreal latitude zone growing season, followed by decay of organic carbon detritus, mostly by bacteria and fungi during colder seasons having much lower rates of photosynthesis. Thus the Mauna Loa record responds to the integrated effects of the planetary-scale annual cycle of respiration and photosynthesis by the terrestrial biosphere in the temperate and boreal zones. Superimposed on these annual fluctuations is an increase in mean CO2 concentration on the order of 1 ppm per year, or about 0.3% per year.
The trend in atmospheric CO2 concentrations observed for the period 1958 through 1974 has continued through the present, with mean atmospheric concentrations of CO2 now averaging about 370 ppm, about 17% higher than levels measured during the late 1950s when direct monitoring of atmospheric CO2 began (Projection 18). Most scientists have concluded that the dominant cause of this increase in mean CO2 levels since the late 1950’s has been combustion of fossil fuels, (coal, oil, gas) much of which occurred in the industrialized nations. The record of atmospheric CO2 concentrations has been extended to earlier periods (pre 1950s), although with much less temporal resolution than from direct measurements of air samples. The gases from tiny air bubbles trapped in glacial ice obtained from drill cores from the Antarctic indicate that the mean level of CO2 in Earth’s atmosphere during the mid 18th century and earlier was only about 280 ppm. Thus the integrated impact of human activities over the past two centuries, much of which has involved combustion of fossil fuel, has been to increase the mean concentration of carbon dioxide in the atmosphere by more than 30%. This greenhouse gas is one of the most critical for the heat budget of the troposphere. Thus humans are already well into a major modification of Earth’s atmosphere with little understanding of the likely long term implications of this alteration.
The composition of Earth’s atmosphere is unique in our solar system, and has been greatly modified by the integrated effects of living organisms operating over most of geologic history. Free oxygen has accumulated in the atmosphere over more than three billion years as the result of photosynthesis, initially entirely through the biochemistry of primitive single-cell green plants. At an intermediate phase of oxygen accumulation, while atmospheric O2 concentrations were probably only about 1 to 2 percent of present levels, there was a period, lasting about half a billion years, of chemical precipitation of massive deposits of banded iron formations at many places around the world when much of the deep ocean probably contained large concentrations of dissolved iron and no dissolved oxygen. These unique deposits, which provide strong evidence of very low, but positive oxygen levels in the early planet atmosphere, together form the largest source of iron ore for modern industrial economies throughout the world.
Accumulation of atmospheric oxygen occurred because of storage of large amounts of reduced carbon produced by photosynthesis before it could be respired back to carbon dioxide (hence using up atmospheric oxygen). Most of this carbon is present in sedimentary rocks called shales, with very much smaller total amounts present as the fossil fuels coal, petroleum and natural gas. Combustion of a significant portion of these fossil fuels over the past century has substantially increased carbon dioxide concentrations in the global atmosphere, with consequences to long-term climate that are not well understood.
Earth’s climate has experienced dramatic shifts, especially during the past two million years, with alternating cycles of glacial and interglacial periods, with each cycle lasting on the order of 40 to 100 thousand years. These cyclic glacial changes are one of the most obvious indicators available that the global climate system is very sensitive, and is susceptible to major excursions with apparently relatively subtle changes in forcing processes. Human activities are now one of the major components of global atmospheric composition and global climate.
Updated
September 11, 2003
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