'Natural' Cycles

'Natural' Cycles

Since the 19^th century observed linked natural processes have been described and rationalised in terms of 'cycles'

The hydrological cycle describes the movement of water around and through the Earth. Whilst parts of the cycle are verifiable by direct obser 18218o1418s vation, the cycle itself is conceptual or theoretical.

Global Systems

Study of the dynamics of cycles leads to the notion of natural systems which consist of processes interacting on a global scale.

Key global systems include:

Solar system

Atmosphere

Hydrosphere

Biosphere

Lithosphere

Mesosphere

Since the mid-1970's models of global evolution have sought to describe and explain the history of planet Earth in terms of the shifting dynamics of complex global systems - 'global change'

Preston Cloud and James Lovelock both developed global evolutionary models which examined not only the operation of global systems but interactions between systems.

This represents an important shift away from pure reductionism but a large part of the global change literature is less sophisticated and is still essentially reductionist.

There is widespread uncritical acceptance of models (like Cloud's) and a lot of implicit (untested) assumptions.

Geological Evidence

Science depends on evidence in the form of observations, measurements etc which if carefully collected should be repeatable or verifiable.

These data or facts may have significance or meaning which lead to a single generally accepted or 'unequivocal' interpretation - eg 'way-up criteria'

Very often, however, geological evidence may be interpreted in more than one way. The evidence is still scientifically valid but it is 'equivocal'.

Sometimes disagreement or controversy arises because of differences of opinion over whether certain evidence is equivocal or unequivocal. Disagreement also may arise over the 'relevance' of evidence.

Examples of this kind of debate often revolve around the significance of the 'absence' of a particular feature - this is sometimes referred to as 'negative' evidence.

Early Terrestrial Atmosphere

1950's  Urey, Rubey, Miller

Primordial Atmosphere - N₂, CH₄, NH₃

1960' & 1970's  early Cloud

Reducing vs Oxidising

1980's onwards

Anoxic vs Oxic

Archaean atmosphere - N₂, CO₂, H₂O, ?O₂