For many years the Flinders University of South Australia has offered a first year topic Earth Sciences in two parts. The first semester topic, Earth Sciences 1A, covered the place of the Earth in the universe, aspects of geology, and an introduction into geophysics and hydrology. Meteorology and oceanography were covered in the second semester topic Earth Sciences 1B.

Beginning in 2000 the two topics are delivered as Earth Sciences 1, which continues as the first semester topic with identical content, and Marine Sciences 1 as the second semester topic. Marine Sciences 1 still contains extensive material on meteorology and physical oceanography but also contains an elementary introduction to aspects of marine biology.

These notes represent the topic content for physical oceanography. In addition, two introductory lectures place the atmospheric and oceanographic aspects of the topic in the context of the exact sciences; they are an abbreviated version of the first two lectures given at the beginning of the semester.

The topic for today: The concept of cycles and budgets

Meteorology and oceanography are physical sciences which aim to understand processes in the environment and describe, analyze and predict them in a quantitative manner.

A common way of expressing processes quantitatively is through the concept of cycles and budgets.

On time scales of geological history, all processes on earth are based on a constant reservoir of materials.

The forms in which the materials are present change constantly. In a state of equilibrium this change has to be cyclic.

This lecture discusses four examples.

The Water Cycle

Figure 1

The earth is the only planet in the solar system where liquid water is found on the surface. Water is the only substance which, under the ranges of pressure and temperature experienced on earth, is present in solid, liquid and gaseous phase. The water cycle is therefore of fundamental importance to many processes unique to earth. In comparison, the outer planets of our solar system (Saturn, Jupiter, Uranus, Neptune and Pluto) and their moons are too cold to contain water in any form other than ice, the inner planets (Mercury and Venus) are too hot to hold water in any form other than water vapour, and Mars is presently too cold but may have had liquid water on its surface at some point in its history. In the current stage of development of the solar system Earth is the only planet that contains water in all its phases.

Like many other cycles, the water cycle links processes acting in the living and non-living world: Precipitation and oceanic evaporation link ocean and atmosphere; evaporation from land and transpiration from vegetation link the atmosphere with the biosphere.

In the context of meteorology and oceanography the effect of the biosphere is quantitatively expressed as a single process, evapo-transpiration. The water cycle then describes a basic component of the combined system ocean-atmosphere.

Associated with every cycle is a budget. Cycles represent a qualitative description of processes, budgets turn them into quantitative statements. We distinguish between static budgets, which summarize how much of a particular material is available and how it is distributed between the different compartments, and dynamic budgets, which quantify how rapidly the material is moved between compartments. Cycles define the process; budgets allow answers to questions such as; "How is the water cycle affected if a given percentage of the existing bushland in Western Australia is cleared and replaced by wheat farming ?"

The Water Budget

The distribution of water on earth (the static budget); this budget shows where the water is found:

The static budget demonstrates the importance of the ice sheets to the global water cycle: Any change in the atmospheric or oceanic conditions that releases a significant part of the water that is presently stored in the ice, will produce a major shift in the water cycle. The atmosphere seems insignificant in comparison. However, the important role of the atmosphere becomes clear when the dynamic budget is considered.

The Water Flux Budget

The branches of the water cycle on earth (the dynamic budget); this budget shows how water moves between atmosphere and hydrosphere:

The flux budget demonstrates that most of the water exchange between the compartments is between the ocean and atmosphere, so the atmosphere is an extremely dynamic element in the system despite of its small water content at any one time. The turnover of water between ocean and atmosphere over a few decades is equivalent to the total amount of water stored in the ice sheets.

The Salt Cycle

The salt cycle involves the ocean, the geosphere and to a very minor extent the atmosphere.

Minerals are leached from rocks through flowing groundwater and surface erosion. They enter the rivers and from there the ocean where they accumulate, making sea water salty. They are removed from the water and enter the sediment by chemical action.

The sediment is used to form new rock which brings the minerals back into the geosphere.

Salt gets into the atmosphere as spray from wind waves. This may be carried on to land, constituting a minute pathway from sea to land in the global salt cycle.

Because the salt cycle operates on such large time scales, establishing a static salt budget is of no relevance to oceanography.

Elements of the Salt Flux Budget

The salt cycle operates on such long scales that establishing a salt flux budget is not an important task for oceanography. The following table gives an idea of the time scales involved:

The concept of salinity is the topic of lecture 3.

The Nutrient Cycle

Figure 2

Nutrients are essential for plant and animal life. They undergo a terrestrial and an oceanic cycle.

On land nutrients are taken up from the soil by plants and return to the soil by decomposition of dead organic matter. This is a closed cycle on a relatively short time scale, determined by the process of decomposition and life spans of plants, animals and humans. In developed human societies it is only broken by the uptake of nutrients by populations of large cities, which do not return the nutrients to the land but dispose of them in sewage systems. The resulting nutrient loss in agriculture is compensated by the importation of mineral fertiliser from the reservoir of minerals in the geosphere.

This human influence introduces a link with a nutrient cycle of a much longer time scale, determined by the formation of mineral deposits. The situation is similar to the situation discussed with the carbon cycle below but does not have the same immediate consequences; the increase of nutrients available for the fast nutrient cycle on which life processes and agriculture depend is very gradual, and much of the mineral input is removed from the rapid nutrient cycle through the oceanic component.

In the ocean nutrient uptake by plants occurs in the surface layer reached by sunlight where photosynthesis takes place. Most nutrients are removed from the euphotic zone and transferred to the deeper ocean as dead organisms sink to the ocean floor, where they leave the rapid nutrient cycle. In the deeper layers organic matter is remineralized, i.e. nutrients are brought back into solution. Thus, the ocean cannot support highly productive ecosystems except where nutrients are returned to the euphotic zone from below in so called upwelling regions. The nutrient cycle is discussed in more detail in lecture 5, upwelling in lecture 6.

The Carbon Cycle

Figure 3

The carbon cycle operates naturally on two vastly different time scales. It involves the ocean, the atmosphere, the geosphere and the biosphere.

On the geological time scale carbon is released into the atmosphere and ocean through the weathering of carbonate rocks such as limestones. It returns to this vast storage reservoir as new rocks are formed through sediment deposition.

On the much shorter climate timescale carbon is exchanged between the atmosphere, the ocean and living and dead organisms.

The carbon cycle includes both timescales, but for most practical purposes the carbon budget and the carbon flux budget usually exclude the geological timescale.

This separation between the timescales has been significantly disturbed through the burning of fossil fuel. This adds carbon dioxide to the atmosphere and increases its ability to retain heat energy received from the sun (the greenhouse effect). The following tables give some current estimates for the carbon budget and the carbon flux budget.

What did we learn today ?

1. The state of the environment is determined by a balance of forces.

   Defining several cycles, such as the water cycle, the salt cycle, the nutrient cycle and the carbon cycle, is a useful way of describing the equilibrium which results from the balance of forces.

2. The concept of cycles helps to understand the world; but to manage the environment and avoid mistakes the concept has to be turned into quantitative measurement.

   Budgets and flux budgets turn the concept of cycles into quantitative statements.