Class	Description
Short-term	Covers actions that take less than a tidal cycle or repeat on a time-scale of this duration or less
Medium-term	Relates to seasonal or annual changes
Long-term	Progressive changes over decades or centuries, including trends and cycles

To these are added two further classes to cover non-continuous events and single actions:

Class	Description
Intermittent	Changes that happen episodically (e.g. storms) or infrequently (e.g. accidents)
External	One off actions that, in the context of management (50-100 year time horizon), do not change with time and may, therefore, be considered constant or fixed