It seems to be a bit of a water cooler staple that this summer in Melbourne has been a cool one. That people can reach this conclusion only confirms the reality of global warming as people’s expectations of a normal summer are ratcheted upwards. This century has seen some very warm summers indeed, and didn’t the world’s elite tennis players notice the heat in January 2014. There was a one month period over January and into February 2014 where Melbourne’s mean daily maximum temperature was 6 degrees above the long term average, equivalent to a Mildura summer.
The 2013/14 summer was particularly nasty in the number of days of extreme heat. The longer term average number of days with a temperature of more than 30 degrees is exactly 30, with roughly half of those days occurring in January and February, with most of the rest in December and March and a few in November. October and April may get such a day occasionally. Summer 2013/14 broadly defined had 35 days over 30 degrees which is high but certainly not noticeably so on any statistical measure.
A “normal” summer experiences 10 days with the temperature above 35 degrees and well over half of these are in January and February. The 2013/14 summer had 14 such days which represents a higher level of variance from the longer term average than that shown by the 30+ days.
The truly frightening aspect of summer 2013/14 was the 7 days with temperatures over 40 degrees which compares with the long term average of just 1.3 days. Summer 2013/14’s experience of extreme heat was so far beyond the normal range for Melbourne that we were very fortunate indeed not to experience any seriously devastating bushfires.
This is the recent historic memory against which people say Melbourne has had a cool summer. For the record the mean maximum temperature in December 2014 was 23.9 compared to the long term average of 24.2, January 2015 recorded a mean maximum of 25.9 which equals the long run average and the February 2015 mean of 26.4 was 0.6 degrees above the longer term average of 25.8 degrees.
What people quite rightly noted this summer was the absence of devastatingly hot days. As of March 19 we have had 27 days above 30 degrees, 9 days of over 35 degrees and thankfully no 40+ days. These numbers are so close to the longer run averages that 20 years ago no-one would have noted them as a “different” summer. It is only our recent experiences with extreme heat that allows us to class a normal sort of Melbourne summer as a coolish summer.
What also seems to be occurring in our weather patterns is a contraction of winter at both ends with days over 20 degrees more likely to occur in May and in August / September than was typically the case in earlier “normal” times.
The impact of global warming is even more obvious when we look at the overnight minimum temperatures. While the daily maximum for 2013 / 14 was 1.6 degrees above the long term average and the figure thus far in 2014 /15 is 0.6 degrees above average, the corresponding figures for the overnight minimum are 2.5 degrees above the long term average in 2013/14 and 1.7 degrees so far in 2014/15.
It is expected that Melbourne will experience 7.6 days per year where the overnight low falls below 2 degrees and Melbourne may even have a day where the temperature falls below zero. In winter 2014 only one day was recorded where the temperature fell below 2 degrees. In my recollection it has been a very long time since a temperature below zero was recorded in Melbourne.
People who found summer 2014/15 a little cool for their tastes can possibly look forward to a warmer 2015/16 summer if early March projections of an El Nino event are borne out.
Is this a good time to remind everyone of the slowly boiling frog that doesn’t react to the rising heat?