Gases of both industrial and natural processes both define and directly effect the environment we inhabit. Tony Wheatley discusses this ever-evolving environment.
The human body is capable of surviving partly submerged in water for periods of time and seems naturally suited to living on dry land is critically dependant on the life sustaining properties of air.
This life sustaining environment is often evolving and the subject of much awareness-raising concern. Gases of both industrial and natural processes both define and directly effect the properties of the environment we live in. This includes the atmospheric, aqueous and terrestrial ‘environment’.
The atmospheric environment
All living organisms take in oxygen during respiration, which is then converted to heat and carbon dioxide. The oxygen we breathe allows us to extract the energy that our bodies need to function, from the food we eat. The oxygen content of the atmosphere is naturally replenished by growing plants that, through photosynthesis, take in sunlight and carbon dioxide to produce the substances they need to grow and thrive – while producing oxygen as a by-product.
Even plants (producers of oxygen during the day) begin to consume it after the sun goes down. Another significant demand for oxygen that is critically important to a life-sustaining environment, is that of the anaerobic bacteria that decompose both plant and animal remains after they die.
A delicate balance of gases in Earth’s atmosphere is unique in the universe and allows life to exist and thrive on our planet. Oxygen is the life sustaining component without which all plant, animal and human life would cease.
The Earth is surrounded by a gaseous envelope held in place by it’s gravitational field, which explains the stratification into concentric layers, with the least dense gases ‘floating’ high above denser layer of an oxygen/nitrogen/argon mixture commonly referred to as air.
Atmospheric pressure is a direct result of the total weight of the air above, the point at which the pressure is measured and therefore varies with location and time. As a result of variations in temperature, average molecular weight, gravity and compressibility, throughout the various atmospheric layers, the density of air reduces very significantly with altitude.
50% of the atmosphere by mass is below an altitude of 5.6km.
90% of the atmosphere by mass is below an altitude of 16km. The common altitude of commercial airliners is about 10km.
99.99997% of the atmosphere by mass is below 100km. The highest X-15 plane flight in 1963 reached an altitude of 108,000m.
Four distinct layers can be identified by their differential absorption of solar energy and little mixing occurs between these layers, because of the temperature gradient. Composed mainly of nitrogen and oxygen but including traces of carbon dioxide, water vapour, and as many as several hundred other gases, Earth’s atmosphere acts as a buffer between it and the sun.
It is an interesting and somewhat ironic fact that the chemistry of the atmosphere and the temperature of our planet are not controlled by the major gases, but rather by the trace gases. There is growing evidence that the trace-gas composition of the atmosphere is changing.
The environmentally significant trace gases are affected by both natural and human-produced gases. Among these gases are the chlorofluorocarbons, human-made gases such as CFC-11 and CFC-12 that are used as propellants in aerosol spray cans; halons used in fire extinguishers; and carbon dioxide, nitrous oxide, and methane, which are produced by the burning of fossil fuels and living and dead biomass.
These are also produced by the metabolic processes of micro-organisms in the soil, wetlands, and oceans of our planet.
Another change in the atmosphere that is affecting our climate is the increasing concentrations of greenhouse gases. This build-up of greenhouse gases results in a greater warming of the Earth’s surface. Greenhouse gases have the ability to absorb, or trap, upward-moving infrared radiation, heat energy, emitted by the Earth’s surface.
The greenhouse gases quickly re-emit, or release, the absorbed heat energy with approximately 50% of the re-emitted energy directed back toward the Earth’s surface. This heat energy would have been lost to space if it had not been trapped by greenhouse
Deforestation by burning, a widespread practice in the world’s tropical rainforests, adversely impacts the trace-gas composition of the atmosphere in two different ways. First, the burning of the tropical forests produces large amounts of carbon dioxide, carbon monoxide, methane, and other trace gases that are the combustion products of biomass burning.
Secondly, the tropical forest is an important sink, or repository, for carbon dioxide. Atmospheric carbon dioxide is incorporated into the living biomass via the process of photosynthesis, which in turn produces atmospheric oxygen.
The aqueous environment
Around 71% of the Earth’s surface, a total area of 362million km2, is covered by water and more than 97% of that water is derived from the ocean. The ocean has tremendous heat capacity, exchanges heat and moisture with the atmosphere and transports huge amounts of energy from the tropics, towards the polar-regions.
It hosts most of the biological activity on Earth and is a major component of the Earth’s biochemical cycles. Like the atmosphere, the ocean is a global-scale fluid on a rotating Earth but there are significant differences between the behaviour of the two systems:
The ocean is divided by continental barriers that impede fluid motion.
The ocean is heated by the sun on its upper surface.
The atmosphere absorbs heat from the earth at its lower surface.
The ocean is effectively opaque to electromagnetic radiation.
Moisture convection occurs in the
It is far more difficult to observe the ocean living as we do at its upper edge compared with our view of the atmosphere from its base.
Like terrestrial animals, fish and other aquatic organisms need oxygen to live. As water moves past their gills (or other breathing apparatus), microscopic bubbles of oxygen gas in the water, called Dissolved Oxygen (DO), are transferred from the water to their blood. Like any other gas diffusion process, the transfer is efficient only above certain concentrations.
In other words, oxygen can be present in the water, but at too low a concentration to sustain aquatic life. Oxygen also is needed by virtually all algae and all macrophytes and for many chemical reactions.
Oxygen enters the ocean waters from three sources:
Aquatic plants generate oxygen by photosynthesis, but only in the presence of sunlight and it can be absorbed depending on the saturation point.
Rivers and streams flowing into the ocean contain DO.
Where air and water meet the tremendous difference in concentration, causes oxygen molecules in the air to dissolve.
All gases are less soluble as temperature increases, particularly nitrogen, oxygen and carbon dioxide which become about 40-50% less soluble with an increase of 25°C. The gases dissolved in sea water are in constant equilibrium with the atmosphere, but their relative concentrations depend on each gas’ solubility at that temperature, which also depends on salinity.
As salinity increases, the amount of gas dissolved decreases and as water temperature increases, the increased mobility of gas molecules enables them to escape from the water. When water is warmed, it becomes more saturated, eventually resulting in bubbles leaving the liquid.
Inert gases like nitrogen and argon do not take part in the processes of life and are thus not affected by plant and animal life. But non-conservative gases like oxygen and carbon dioxide are influenced by sea life. Plants reduce the concentration of carbon dioxide in the presence of sunlight, whereas animals do the opposite in either light or darkness.
However, this cannot explain the abundance of carbon dioxide dissolved in sea-water when it is relatively scarce in the atmosphere.
The high solubility of carbon dioxide occurs because it reacts chemically with water (hydrolyses), releasing H+ ions to form anions Bicarbonate, Carbonate and Carbonic Acid. Ions are far more stable as dissolved species than neutral molecules like O2 and N2 are. This is referred to as the Dissolved Inorganic Carbon system and buffers changes in acidity and atmospheric carbon dioxide content.
Oxygen consumption is usually greatest in the bottom waters of a sea or lake due to the Biological Oxygen Demand (BOD). The decomposition of dead plant and animal matter is accomplished by bacteria that consume oxygen and turn organic molecules into organic nutrients and carbon dioxide, while pollution usually exacerbates this problem.
The terrestrial environment
Dry land constitutes 29% of the Earth’s surface and almost 40% of this area is under agricultural use as either cropland or pasture. Despite the rapid pace of urbanisation during the past 50 years the total area covered by cities, suburbs, factories, roads dams and other human developments is less than 2% of the total land area.
The World Atlas of Biodiversity, published by the United Nations in 2002, suggests that within the next 30 years the complex but delicate interaction between plants and animals will be threatened on almost 75% of the land surface.
According to the Atlas, half of the area of forest that had developed since the last ice age has since been cleared or degraded by man.
The decline is especially prevalent today in the tropical rainforests of South-East Asia, the Congo and parts of the Amazon. Around 22% of these areas are used for farming, towns and other kinds of human development. Humans have also degraded many of the natural shrub-land regions by burning and overgrazing, activities that have also led to the loss of valuable plant species.
The extinction of land plants is contributing to the loss of important genes for crops as well as new sources of medicines and pharmaceutical products. The Atlas suggests that we are losing one important new drug every two years because of the extinction of plants and animals and yet, less than 1% of the world’s estimated 250,000 tropical plants have been screened for potential pharmaceuticals.
Freshwater, and the lakes and rivers that provide it, as a vital resource for human survival, come under conflicting demands with increasingly adverse consequences for their biodiversity, according to the World Atlas on Biodiversity.
Only 3% of Earth’s water resource is available as freshwater and about 75% of this is unavailable, frozen in the polar ice caps and glaciers. This means that slightly less than one one-hundredth of 1% of the world’s total supply of water is easily accessible as fresh-water lakes, rivers, and shallow groundwater sources that are renewed by snow and rainfall.
Water scarcity is further compounded by the disparity between where human populations are located and when and where rainfall and run-off occurs. Viewed in this manner, 81% of total global runoff is within geographic reach for human use, but three-quarters of that comes as floodwater and therefore is not accessible on demand.
Dr Mark Collins, the Director of the United Nations Environment Programme World Conservation Monitoring Centre in Cambridge, emphasises that life on Earth cannot be considered in isolation.
“Biodiversity is not just about species. It’s about how they interact with one another. The conservation of the resources of the natural world is not a luxury any more but essential to the quality of human life,”