In 2015, the weather of Melbourne was generally cooler compared to the years gone by. What is the best term to describe this difference?

In 2015, the weather of Melbourne was generally cooler compared to the years gone by. What is the best term to describe this difference?

A.     Errors in Weather Forecast.

B.     It's a case of Natural Variability of Climate.

C.      It is the First Law of Thermodynamics.

D.     It is the Changes in Atmospheric Composition.