Abstract

Precise measurements of atmospheric O2 concentration can provide constraints on several aspects of the global carbon cycle. Seasonal variations in O2 concentration, driven in part by biological and physical cycles in the ocean, can be used to constrain seasonal net photosynthesis rates of marine biota. Interannual variations in O2 concentration, driven largely by O2 uptake by fossil fuel burning and O2 exchanges with land biota, can be used to partition the net global uptake of anthropogenic CO2 into oceanic and land biotic components. The latter application is potentially complicated, however, by interannual sources and sinks of O2 from the ocean. Model simulations are presented that suggest that interannually driven air–sea O2 exchanges may be several times larger on a mole-for-mole basis than interannually driven air–sea CO2 exchanges.

Introduction

It has recently become feasible to measure the atmospheric oxygen concentration to a degree of precision that allows for the detection of variations in the remote atmosphere (Keeling and Shertz, 1992; Bender et al., 1994; Bender et al., 1996; Keeling et al., 1996). These variations primarily reflect changes in O2 because N2 is constant to a very high level. Variations in O2 are caused primarily by production and consumption of O2 by photosynthesis, respiration, and combustion, and are thereby tied to the rate at which carbon is transformed between organic and inorganic forms.

Before Europeans brought the horse to the New World, Native Americans in the Great Plains hunted bison from foot. Their technique was ingenious: by making a fire or creating a ruckus near a herd of bison, they stampeded the skittish animals toward a bluff. The Indians lined the route to the bison “jump” with fallen trees or thicket and waved robes to shoo the beasts toward their destination. If the hunters dared to risk a general conflagration, they set more fires to direct the herd. Once they had stampeded the bison over the precipice, they peppered the crippled animals with arrows. For the bison, the stampede to the bluff was probably a disorienting experience. As they hurried toward their deaths, the more perceptive among them might have wondered, “Where is all this commotion leading me?” As readers of this book consider – among other things – grassland ecology, horses, smallpox, the fur trade, and gender roles in Indian and Euroamerican societies, they may find themselves pondering the same question. Nonetheless, just as pedestrian hunters herded their prey to their deaths, this book eventually leads to the destruction of the bison.

Why consider so many seemingly disparate subjects? Because, a host of economic, cultural, and ecological factors herded the bison toward their near-extinction. That diverse assembly of factors first emerged in the middle of the eighteenth century from ongoing encounters among Indians, Euroamericans, and the Great Plains environment.

In 1811, the naturalist John Bradbury paused near the juncture of the Grand and Missouri rivers and remarked upon the abundance of honeybees there. Bradbury, an English botanist accompanying a group of American fur traders up the Missouri, was astounded that a European insect could be found in great numbers in the trans-Mississippi West. He wrote, “Even if it be admitted that they were brought over soon after the first settlement took place” – indeed, bees arrived in Virginia in the 1620s – “their increase since appears astonishing, as bees are found in all parts of the United States; and since they have entered upon the fine countries of the Illinois and Upper Louisiana, their progress westward has been surprisingly rapid.” Bradbury believed that bees had not crossed the Mississippi River until 1797, although later in the nineteenth century some writers claimed that Madame Marie Thérèse Chouteau kept bees in her St. Louis garden in 1792. In any event, as Bradbury wrote, “They are now found as high up the Missouri as the Maha nation, having moved westward to the distance of 600 miles in fourteen years.”

Indians of the Missouri watershed such as the Omahas, or the Mahas as Bradbury called them, regarded honeybees as the vanguard of European expansion. Bradbury wrote, “Bees have spread over this continent in a degree, and with a celerity so nearly corresponding with that of the Anglo-Americans, that it has given rise to a belief, both amongst the Indians and the Whites, that bees are their precursors, and that to whatever part they go the white people will follow.”

In the middle decades of the nineteenth century, the nomadic societies of the western plains encountered a new wave of Euroamerican ecological and economic expansion. The eighteenth-century invasion had levered the mounted bison hunters to dominance in the western plains; the renewed incursion of the nineteenth century devastated both the nomads and the bison. In part, the social and environmental catastrophe of the mid-nineteenth century resulted from the scale of the invasion. The bison robe trade of the American Fur Company far exceeded the commerce in beaver pelts of the eighteenth-century voyageurs. The smallpox that ravaged the inhabitants of the western plains between 1837 and 1840 was more virulent than the epidemic of 1780 to 1782. Yet the extensive economic and environmental changes in the western plains in the mid-nineteenth century were not simply the product of exogenous forces, however powerful. Social changes that had attended the nomads' transition to mounted bison hunting in the eighteenth century contributed to their receptivity to trade in the mid-nineteenth century and therefore to the depletion of the bison and the spread of disease.

Steamboats began to ascend the Missouri River to tap the labor and resources of the western plains in the early 1820s. In 1832, an American Fur Company steamboat reached the mouth of the Yellowstone River in the northern plains.

As the Civil War approached its end, the citizens of the United States faced the prospect of an Indian war in the Great Plains. John Evans, the governor of the Colorado Territory, predicted in 1864 that the coming hostilities “will be the largest Indian war this country ever had, extending from Texas to the British line involving nearly all the wild tribes of the plains.” Contention over the control of natural resources was the root cause of the brewing conflict. In the southern plains, the nomads struggled to preserve their stewardship of the bison herds, while Euroamericans sought access to ranchlands and the gold mines of Colorado. For the southern plains nomads, the conflict reached its nadir on November 29, 1864, when Colonel John Chivington led two companies of Colorado volunteer cavalry in an attack on noncombatant Southern Cheyennes at Sand Creek. Chivington and his men killed one hundred fifty Cheyennes; two-thirds of the dead were women and children. The Commissioner of Indian Affairs and a Congressional committee rebuked those involved in the massacre, but Chivington won broad support among Euroamericans in Colorado, including Governor Evans.

In the northern plains, the conflict between Indians and Euroamericans over the control of natural resources also raged. The dispute of the northern plains nomads with the United States centered on the Bozeman Trail leading from Fort Laramie to the gold mines of Montana.

The Indian hunters who came to the western plains in the eighteenth century encountered not only the villagers who already inhabited the region but other interlopers like themselves. The complex and multifaceted encounters that ensued among nomads and villagers became still more complex when Euroamericans arrived in the grasslands. The encounters among Indians and Euroamericans were both intercultural and environmental. Immigrants to the western plains introduced new animals, systems of resource use, and microbes to the region. The newcomers confronted not only other peoples but the semiarid environment. As a result of these cultural and environmental encounters, villagers became nomads, farmers became bison hunters, and bison hunters became destroyers of a species.

The environmental history of the destruction of the bison thus unites two meta-narratives of eighteenth- and nineteenth-century history. First, imperial expansion brought Europeans, their economic system, and their biota into worldwide contact with non-Europeans. The conquest of indigenous peoples in North America was duplicated in South America, southern Africa, Australia, New Zealand, and elsewhere. Second, the migration of European people, plants, animals, and economies overseas occasioned a global decline in biological diversity. Ultimately, European domesticated animals supplanted indigenous wildlife in many parts of the world. Domesticated livestock's displacement of bison in the Great Plains was but one example of this pattern of ecological simplification.

The encounter between European and Indian societies was both a cultural and an ecological phenomenon, converging in the domestication of the plains environment: the transformation of the bison and the grasslands for human convenience.

The Great Plains, extending from the Missouri River valley in the east to the base of the Rocky Mountains in the west, and from Canada south to Mexico, is the largest biome in North America. Although flatness is popularly believed to be the distinguishing characteristic of the plains, its topography is quite varied. Between 50 and 70 million years ago, surging molten rock from beneath the earth's surface created the Black Hills of western South Dakota and several ranges in Montana, among them the Highwood, Bearpaw, Judith, and Crazy Mountains. During the last five to ten million years, geological forces have carved a multitude of hills and bluffs in the region, from the Badlands of South Dakota to the Flint Hills of Kansas. Generally, the area between the Rocky Mountains and the Missouri River slopes from 5,000 feet above sea level at the base of the mountains to 2,000 feet above sea level at the Missouri. An ubiquitous flatness exists only to the east of the Missouri and in the Llano Estacado, or Staked Plains of west Texas. In general, the region consists of many landscapes: primarily shortgrass and mixed-grass rolling plains but also wooded river valleys and high, forested hills.(See Map 1.1.)

The Mandan Indians, whose villages on the banks of the Missouri date from at least the thirteenth century, attributed the variety of the western Great Plains landscape to their chief god.

When the equestrian nomads rose to dominance in the Great Plains at the end of the eighteenth century, the bison was the largest mammal in North America, but it was not the largest animal ever to have inhabited the continent. Between twelve thousand and fourteen thousand years ago, during the last Ice Age, glaciers edged into the plains to the north and forests covered much of the plains to the south. In this period – the Pleistocene epoch – large herbivores now extinct such as the mammoth (mammuthus primigenius) and the giant bison (bison latifrons) foraged in mid-continental North America. As the Ice Age ended, global warming and human predation conspired to kill off the giant mammals (a lethal combination of anthropogenic and environmental pressures that foreshadowed the near-extinction of the bison thousands of years later). The extinction of the large herbivores coincided with the appearance of Paleoindian hunting societies in North America. The enormous glaciers of the Pleistocene epoch had lowered the sea level sufficiently to create a land bridge between Siberia and Alaska in what is now the Bering Strait. Paleoindians probably came from Asia to America between twenty-five thousand and twenty thousand years ago, and again between fourteen thousand and ten thousand years ago, while the Bering land bridge existed but glaciers did not block the migration.

The itinerant artist George Catlin reflected, while touring the Missouri River valley in 1832, that the bison was “so rapidly wasting from the world, that its species must soon be extinguished.” To save the herds, Catlin recommended that the federal government create “a nation's Park” in the grasslands. He imagined that both the bison and the Indians who hunted them “might in future be seen (by some great protecting policy of government) preserved in their pristine beauty and wildness, in a magnificent park.” For the next forty years, however, the United States government declined to pursue Catlin's vision. Rather, federal authorities welcomed the diminution of the herds because it forced famished Indians of the western plains to submit to the reservation system. Yellowstone, the United States' first national park, was located in the scenic Mountain West, far from the bison's range in the shortgrass plains. In the late nineteenth century, however, Yellowstone National Park, established in 1872, was the only public refuge for the bison apart from city zoos. By the 1880s, a few hundred bison – the largest group of survivors in the United States – had found refuge there from commercial hunters, drought, and the destruction of grazing lands by farmers and livestock.

This remnant herd and other scattered survivors might eventually have perished as well had it not been for the efforts of a handful of Americans and Canadians.

The variable

Just as air, warmed by contact with the ground, is transferred into the atmosphere by processes of diffusion, turbulence and convection, so too is the water vapour produced by evaporation. The ratio in which the net radiative energy is divided between heating the atmosphere, heating the ground and evaporating water is dependent on many factors, such as the amount of water actually available, the nature of the ground and the type of vegetation. Knowing the rate of evaporation of water is useful information in hydrology, meteorology and agriculture, but it is difficult to measure. However, the amount of water vapour in the air, i.e. the air's humidity, is easier to measure and this chapter looks at how it is done; Chapter 6 addresses the more difficult problem of how evaporation rates are measured.

Units and terminology

Hygrometry is the measurement of the water content of solids, liquids and gases; in environmental applications this usually means the water content of the atmosphere. Water vapour exerts a pressure, the vapour pressure (VP) of water, which can be measured in any of the usual units of pressure, such as millibars. Above a water surface in an air-filled container, the VP rises to a maximum level, the saturation vapour pressure (SVP), beyond which it cannot rise any further at that temperature (the rate at which water molecules are leaving the surface of the water being then the same as the rate at which they return due to molecular bombardment).

The need for measurements

Whether it be for meteorological, hydrological, oceanographic or climatological studies or for any other activity relating to the natural environment, measurements are vital. A knowledge of what has happened in the past and of the present situation, and an understanding of the processes involved, can only be arrived at if measurements are made. Such knowledge is also a prerequisite of any attempt to predict what might happen in the future and subsequently to check whether the predictions are correct. Without data, none of these activities is possible. Measurements are the cornerstone of them all. This book is an investigation into how the natural world is measured.

The things that need to be measured are best described as variables. Sometimes the word ‘parameters’ is used but ‘variables’ describes them more succinctly. The most commonly measured variables of the natural environment include the following: solar and terrestrial radiation, air and ground temperature, humidity, evaporation and transpiration, wind speed and direction, rainfall, snowfall, and snow depth, barometric pressure, soil moisture and soil tension, groundwater, river level and flow, water quality (pH, conductivity, turbidity, dissolved oxygen, biochemical oxygen demand, the concentration of specific ions such as nitrates and metals), sea level, sea-surface temperature, ocean currents and waves and the ice of polar regions.

The variable

The rate of evaporation is controlled by the relative humidity and temperature of the air, the amount of net radiation, the wind speed at the surface, the amount of water available, the nature of the surface (for example its roughness) and the type of vegetation. Open water presents another situation, as do ice and snow. The net incoming energy is apportioned to the three fluxes – sensible, latent and soil heat – according to the infinite variety and combination of circumstances.

It is much more difficult to sense the rate of loss of water from the surface through evaporation – the latent heat flux – than it is to measure the gain of water through precipitation. Nevertheless, the rate of evaporation is expressed in the same units as precipitation: it is the equivalent depth of liquid water lost into the atmosphere as water vapour, expressed in millimetres lost over an hour or a day.

Measuring and estimating evaporation

Evaporation can be measured either as the loss of liquid water from the surface or as the gain of water vapour by the atmosphere, but few of the methods involve a direct measurement, most inferring the amount by indirect measurement.

Reasons for telemetering data

Telemetry is the transmission of data from one point to another. If data are needed in real time they must be telemetered, for example for weather forecasting and flood warning. Telemetry also has two significant advantages over in situ data logging, even if the measurements are not required in real time: the cost of visiting field sites to collect data is saved and the failure of field stations can be detected – months of data could be lost if a logging station failed soon after a visit. Logging is best suited to applications where stations are within relatively easy access or where the loss of some data is not a serious problem.

The general process of telemetering data is sometimes referred to generically as system control and data acquisition (SCADA), although the term applies more strictly to management applications – where not only are data acquired from a remote location but remote control is also exercised back. A dam managed from a distant control-room, for example, is a more appropriate use of the term SCADA than is the one-way collection of environmental data.

The structure of a telemetry system

Figure 12.1 is a schematic of a telemetry system, showing its main subdivisions into sensors, logger, modem, communications link and a PC at the base station. This basic arrangement is similar for all telemetry systems although it will differ in detail, mostly depending on the communications link used.