OFR 151.pdf - CRC LEME

1.1 Climate and climatic change
SECTION 1 (DEFINITIONS)
1.1.1 Climate
Climate is the condition of the atmosphere (weather) averaged over one to several decades on
geographic scales that can range from the local to global. It incorporates both meteorological
averages and extremes. Many geological and biological phenomena are profoundly
influenced by less common meteorological events, e.g. cyclones and severe droughts.
The principal elements of weather that make up climate are: (1) insolation (the energy input
from the sun), (2) sea surface and air temperatures, (3) precipitation and evapo-transpiration,
(4) atmospheric pressure and water content (relative humidity), (5) the origins and latitudinal
trajectories of air masses and ocean currents, (6) land and ocean heating differences
(determining the type and intensity of weather disturbances), (7) wind speed and direction
(exposure) and (8) cloud cover.
Except for sub-alpine and alpine regions where limits on plant growth include low air
temperatures and extreme events such as out-of-season glazing storms, the key factor limiting
plant growth in Australia is effective precipitation [rainfall minus (run-off plus evapotranspiration)].
Like temperature, rainfall varies diurnally and seasonally, as well as with latitude and
elevation. Over the geological time-scale, the two other factors that have changed
dramatically in Australia since the end of the Jurassic are light intensities (photoperiod)
during winter and the concentration of carbon dioxide in the atmosphere.
All climates are seasonal to the extent that precipitation, air temperature and other factors
such evaporation, photoperiod and the direction and strength of prevailing winds varies quasisystematically
throughout the year. Although seasonality strictly refers to changing day
lengths during the year (Christopherson 1997), the term is more commonly used in
palaeoecology to denote the variation in temperature and rainfall between the coolest/warmest
and wettest/driest quarters of the year.
For much of Australia, an appropriate measure of seasonality is the reliability and amount of
precipitation received in the driest month (moisture surplus or deficit) irrespective of whether
this occurs in summer (winter rainfall zone) or winter (summer rainfall zone). For highland
areas, snow and mists may be an important contribution to the total annual precipitation
whilst evaporation is reduced by the orographic cloud cover (Jackson 1999, Weathers 1999).
Mean monthly temperatures may be of less biological importance than maximum and
minimum values.
During the Cretaceous and Paleocene, regions south of palaeolatitudes 70-80 ° S experienced
partial to total darkness during winter and up to twenty-four hours sunlight during summer.
1.1.2 Climatic change
Climatic change is any change in long-term weather patterns (external boundary conditions
sensu Chappell 1983) that are sustained over periods longer than several decades.
Conversely, climatic catastrophes, which may have equal or more profound geological and
biological consequences, are short-lived perturbations of the ‘boundary conditions’ (Budyko
1999). An important caveat is that climatic change is never spatially uniform and changes in
the local or regional landscape or fauna do not necessarily vary congruently with changes in
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