Why do we use temperature anomalies instead of absolute temperatures even when studying small regions like the UK?
The last post showed that maximum annual ACORN-SAT temperatures for Australia show little change since 1910, whereas maximum temperature anomalies did indeed show some warming. Here is a plot which summarises all of this.
The top two signals are maximum temperature anomalies a) is based on the monthly highest temperature whereas b) is the monthly average daily maximum temperature. They are similar and show no change before 1990, and an increase thereafter. The bottom signal is the yearly maximum temperature for each station averaged over Australia and shows a fairly constant value of ~ 42C with little if any increase.
I wanted to look more closely into the universal use of temperature anomalies in Climate Science rather than absolute temperatures. Anomalies are always based on monthly averages relative to a set of ‘normal’ monthly values. This is intended to remove large seasonal changes, and to avoid strong time dependence caused by changes in spatial sampling. One month is the fundamental unit of time for anomalies. Daily values only contribute to the monthly average, after which they are discarded. This means that there are some implicit assumptions involved in the use of anomalies.
- The basic time interval is fixed at one month. The anomaly for one station is the delta between the average value for the current month against its 30 year average for that particular month. The 30 year average value is said to be the ‘normal’ expected value for that station and for that month.
- The global monthly temperature anomaly is the area weighted average of all stations active for that month AND which have normals defined within the selected 30-year period. The global annual temperature anomaly is simply the 12 month average of the monthly global anomaly values.
- Seasons are assumed not to vary long term as otherwise this would change the monthly normals, even if the annual average temperature for a station did not change.
- There is an underlying assumption that global warming is the same over large regions of the planet and that all stations within those regions warm in synchrony. Stations which have no readings during the 30 year interval are still often combined with nearby stations to form merged ‘hybrid’ stations, or else used to ‘homogenise’ nearby station trends. If the underlying assumption of region wide warming is not the true, then homogenisation of near neighbours via pair-wise adjustments is simply wrong. Despite this, the end result of such homogenisation adjustments will always produce an apparent region wide warming, independent of whether there is any underlying evidence for it.
Figure 2 shows a comparison between the temperature anomalies for all 1800 stations across Australia before homogenisation and that calculated from ACORN-SAT values after homogenisation.
Figure 2C shows that the homogenisation process itself has generated an extra ~0.6C increase in maximum temperatures across Australia.
Surprisingly enough it is not just observations of global warming that are always based on anomalies instead of absolute temperatures. Climate models too have difficulty in re-producing absolute temperatures correctly. One would imagine that models must balance energy between incoming solar and outgoing IR, and that this alone would constrain the global absolute average temperature. However all models disagree on exactly what that global temperature should be, and as a result they too rely on using anomalies to make useful predictions.
In the past Models have always had difficulty in achieving a net energy balance at the top of the atmosphere, and so had to impose it arbitrarily. I suspect this current disagreement in absolute temperatures is related to this underlying problem.
Comparisons of model ‘projections’ to observed global warming are only valid under the implicit assumptions which underpin the use of temperature anomalies both by models and by measurements. In fact we might as well call this dual combination as a comparison of temperature ‘abnormalities’ !
Finally I have calculated all the extreme temperatures ever recorded in Australia. These are the highest temperature in each year recorded at any ACORN-SAT station. The results are shown in Figure 4 and are quite interesting.
The three extreme temperatures that exceeded 50C occurred before 1985, and there is no evidence of any long term increase. However, there is a plateau after 2000 with all but 2 consequent years above 48C, but this is also associated with much lower inter-annual variability. This does not look right to my eye, so I suspect this could be due to homogenisation adjustments.
A full list of all temperature extremes can be seen here. These are the hottest towns in Australia and probably best to avoid in the heart of summer !