The annual range of temperature extremes in Australia is reducing as the continent warms.

The annual temperature range is the maximum recorded temperature for that year minus the minimum recorded temperature. The annual continental average is the area weighted average over all 1800 stations. This was a surprise, so I thought it could be an artefact of the changing distribution of stations. However the ACORN-SAT data, which is selected for its long term coverage, shows the same effect.

Are maximum temperatures falling or are minimum temperatures rising? The answer seems to be both, but minimum temperatures are rising slightly faster than maximum temperatures are falling.

So what is happening? Is this an artefact of some kind or can this be a real effect ?

The argument to always work with temperature anomalies when studying climate change is to remove any natural station temperature offsets due things like different elevations. Yet we know that urbanisation changes temperatures in or near cities, and that this also increases over time. I have shown how this then results in cooler temperature anomalies before the baseline normalisation period. Likewise UHI affects temperature ranges.

During hot days convection dominates cooling from the surface, whereas cold clear nights cool mostly radiatively. Therefore an enhanced greenhouse effect is more likely to increase minimum temperatures than to increase maximum temperatures. Maximum temperatures on the other hand are more affected by any increase in pollution or aerosols thereby reducing temperatures by reflecting sunlight. These are anthropogenic effects unrelated to CO2.

Are there any specific examples where this is hapenning? Well here is one: Richmond (QLD) from the homogenised ACORN-SAT

So I think this is a real effect, although I can’t yet be sure there is not a coverage bias as well.

Conclusion

The overall warming of 0.8C to 1C since 1910 seems to be associated with a fall in temperature extremes. Minimum temperatures have risen slightly faster than the rate at which maximum temperatures have fallen.

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