“Our hot, dry future”?

THESE days, it can be hard to imagine how Melbourne ever earned a reputation as the gloomy, rain-filled capital of the south. But, growing up in the 1970s, my memories are full of muddy ovals, local creeks in flood and catching tadpoles in puddles that lasted for months on end. How things have changed.Since 1996, each successive calendar year has brought the city below-average rainfall. With 299 millimetres recorded so far this year, and with just three months to go, it seems virtually certain that this year will become the 12th in a row that has failed to get to the average of 650 millimetres. September 2008 was the driest on record in Melbourne, and the outlook for the remainder of the year suggests that below-average rainfall will continue.So why has it been so dry? The drought started in late 1996, and the subsequent El Nino years of 1997, 2002 and 2006 have each been particularly dry. Ordinarily, these events would have been interspersed with wetter years, but since 1996 the intervening periods have only approached average at best, with the deepening drought particularly evident in our reservoirs and stream-flows.

My main criticism of the article is that the BoM relies on Melbourne CBD rain data to back up their regional conclusions regarding “climate change” and drought, while the rainfall history is in fact affected by the growing urban heat island.

Melbourne Regional Office 86071 (MRO), a weather station in Melbourne’s CBD is

(a) excluded from their own High Quality (HQ) dataset and

(b) shows a negative trend of 90mm (a stunning 13% of mean annual rain) over the last 153 years when compared to the nearest HQ station, Yan Yean 35 km NNW.

So much of what they say in “Our hot, dry future”, is slanted by this amount, no wonder I am critical of much that the BoM publishes.

153 years of declining rain in Melbourne CBD

Melbourne Regional Office weather station in Melbourne’s CBD which has rain data from 1855, is a site that has undergone enormous changes in its surroundings as the city has been built and expanded over the centuries, resulting in an ever-increasing urban heat island.

Melbourne UHI transect on calm night

The above illustration is from a 1997 BoM paper.

High rise developments have increasingly affected wind and changing pollution levels over the decades could also cause variations in rain formation. Up to post WWII coal burning would have been common leading to much worse pollution than modern times, (note visibility data) and air quality data show improvements over say the last 40 years.

These are just a quick sketch of some reasons why weather data from a large and expanding urban heat island is a most unsuitable source from which to draw conclusions about, climate change, regional changes and long term rain trends.

Finally, the article contains another BoM failed prediction, saying in the second paragraph, “..the outlook for the remainder of the year suggests that below-average rainfall will continue.” Wrong BoM, the 2 month rainfall total for November-December for Melbourne Regional Office was 130.8mm compared to the long term mean of 118.7.

Jennifer Marohasy featured 5 articles on her blog examining the subject during October 2008; the first titled How Melbourne’s Climate Has Changed: A reply to Dr David Jones (Part 1) was posted on 14 October and parts 2 to 5 were later in the month.

2 comments to “Our hot, dry future”?

  • nevket240

    paul.kedrosky.com/archives/2009/04/the_end_of_chea.html

    Folks. I’m not sure if you are familiar with the chart as shown on this financial commentators website. I’m struck with the number of and duration of dry spells. Long before CO2 became the political weapon of choice.

    regards

  • WSH

    Very interesting graphic. Of course post 2000 we have seen a huge push in Australia towards more expensive water, no more dams, desal, etc. Much of which is unjustified as usable water is let waste to the sea while our water bills skyrocket.
    I am not aware of any graphic similar for Australia. But you know BoM data shows with crystal clarity from the BoM 11 year smoothed trend, that the period from circa 1920 to mid-1940′s was much drier in the MDB than our recent decade.

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