Posts Tagged ‘Rainfall’

Townsville Rainfall In Context

February 11, 2019

The rain event which caused massive floods in Townsville (and fearful stock losses in the north-west) has now ended.  There have been some who have made further political capital out of this disaster by linking it to climate change.

According to Independent Australia, a “progressive journal”,

The City of Townsville, with some 20% of its suburban zones under water today (6 February 2019), might now be a model for the world — for possible climate change impacts and handling them. 

These days, the very heavy falls have been happening more frequently — for example, in 2007, 2009 and then in 2010.

Time for a reality check.

This has indeed been a record breaking event for Townsville.  A few graphs will illustrate.  Townsville airport has had its wettest 14 day period since 1941, averaging over 100mm per day.

Fig. 1:  14 day rainfall

Tville 14d rainfall

It has also broken the record for rainfall over 31 days:

Fig. 2:  31 day rainfall

Tville 31d rainfall

And with the wet season far from over, it is very likely to break the 121 day rainfall record.

Fig. 3:  121 day rainfall

Tville 121d rainfall

Townsville’s rain is very seasonal.  Annual rainfall averages about 1127mm, and half of that falls in January and February, with another quarter in December and March, so a plot of 121 day rainfall captures the relative strength of wet seasons over the years.  There doesn’t appear to be any recent increase in wet season strength.  What is interesting is there are periods of wetter and drier years, which is more plainly seen in a plot of decadal rainfall.

 Fig. 4:  Decadal rainfall at Townsville

Tville decadal rainfall

Rainfall appears to be in a decreasing trend.

But what about the claim for greater frequency of very heavy rain events?  Heavy rain events are usually short and intense, so three day rainfall will also show relative frequency and intensity.

Fig. 5:  Three day rainfall

Tville 3d rainfall

The “Night of Noah” in 1998 is obvious, and there was another intense event in 1953.  But there is NO trend.  (The calculated trend is zero.)  Intense events are not more frequent.  Similarly, the number of days per year recording 100mm of rain shows zero trend, even though there have been eight already this year.

Fig. 6:  Count of days per year with over 100mm of rain

Tville days over 100mm

There is no climate change signal in Townsville’s rain record.

Now, to show how different locations can lead to completely different interpretations of trends in climate, I turn to two locations in wetter parts of the tropics that I have some knowledge of.  I lived for many years not far from Pleystowe and Sarina Sugar Mills near Mackay, which are about 30 km apart.  Sarina appears to have an increasing trend in rainfall:

Fig. 7:  Decadal rainfall at Sarina

Sarina decadal rainfall

While Pleystowe shows no trend.

Fig. 8:  Decadal rainfall at Pleystowe

Pleystowe decadal rainfall

Notice the similar patterns of wetter and drier periods in Townsville, Pleystowe, and Sarina.

And incidentally, the most intense and highest rainfall events in these locations occurred many years ago, in 1990-91, the 1970s, the 1950s, and 1918.  As with the recent Townsville flood, these occurred when the monsoon trough, with embedded decaying cyclones, lingered overhead for many days or even weeks.

The Townsville flood was not due to climate change, but to a frequent North Queensland phenomenon- an intense monsoon trough stuck in one place for too long.  This was an unusually intense and long lasting example, but such events are not more frequent or more intense.

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Drought and Climate Change Part 2: Rainfall deficiency

September 7, 2018

In my last post, I looked at long term rainfall trends across Southern, South Eastern, and South Western Australia, and found no cause for alarm at recent rainfall decline.  Droughts can occur at any time and cause much hardship across wide parts of the country.  Global Warming Enthusiasts are gnashing their teeth, believing man-made climate change is making droughts worse.  Greg Jericho in the Guardian wrote last Thursday 30th August, “If you are a prime minister going out to the rural areas and you’re not talking about climate change, and you’re not suggesting that droughts are more likely to occur and thus farmers need to take greater responsibility, then you are failing in your job.”

Are droughts really “more likely to occur” with climate change, and is there any evidence they are becoming more frequent, more intense, and more widespread with global warming?

The Bureau of Meteorology says:

Drought in general means acute water shortage.

The Bureau’s drought maps highlight areas considered to be suffering from a serious or severe rainfall deficiency…. for three months or more….

……

  • Serious rainfall deficiency: rainfall lies above the lowest five per cent of recorded rainfall but below the lowest ten per cent (decile range 1) for the period in question,
  • Severe rainfall deficiency: rainfall is among the lowest five per cent for the period in question.”

This map of meteorological drought (areas in the lowest ten and five percent of 12 months rainfall to 31 August) shows the extent across Australia:

Fig. 1:  Recent 12 month Rainfall Deficiency Australia

12m drought map

Parts of central and southern inland Queensland, parts of eastern South Australia, many parts of New South Wales, and small areas of Victoria are in drought.  Notice that the droughted areas are separated by areas that are not in drought.

But, but… all of NSW is in drought, isn’t it?

100% of NSW has been drought declared, and 54.7% of Queensland, and indeed some parts are in a very bad way.   But “drought declaration” is the term the media, politicians, and general public don’t understand.  They assume that because 100% of NSW is drought declared, this means all of NSW is in drought.  Not so.  Drought declaration is a political or at best administrative instrument for giving drought assistance to farmers and communities.  Some areas of Queensland that have not yet been drought declared really are in the grip of drought; some “drought declared” areas of NSW are not in drought, as this map of NSW (6 months March to August) shows:

Fig. 2: 6 month Rainfall deficiency NSW

NSW map 6m

Of course the blank areas have had below average rainfall, which may turn into full blown drought, so the NSW government is being proactive.  However, they are not at this time in meteorological drought with serious or severe rainfall deficiency.

Trends in Drought Incidence

In the bigger picture, how widespread, how intense, how long lasting, and how frequent are droughts becoming in Australia?  For this analysis I use monthly rainfall data from 1900 to July 2018 from the Bureau of Meteorology at their Climate Change page, and calculate the number of months where the rainfall total of the previous 12, 18, 24, or 36 months shows severe deficiency (in the lowest 5 percent of all months since 1900) or serious deficiency (in the lowest 10 percent).  (I am looking at droughts that last at least 12 months, not just short dry spells, and 12 months total rainfall includes rain in all seasons.)

I do this for various regions, as shown on the map below.

Fig. 3:  Australian Regions

Climate regions

I have plotted the number of consecutive months where the 12, 18, 24, and 36 month totals are in the lowest 5% and 10% of their respective values since 1900, and calculated the trend in months per century of increase or decrease. There are 96 plots, so I will only show a couple of examples, and summarise the results in Table 1 below.

Table 1:  Trends in Drought Incidence (Months per 100 Years) for various Australian Regions

Trend table

A negative trend indicates decreasing drought incidence, shaded green; a positive trend indicates increasing incidence, shaded pink.

Australia wide, and in the regions of Northern and Southern Australia and the Murray Darling Basin, and South Australia as a whole, since 1900 droughts of all lengths have become less frequent, and because these are broad regions, less widespread.  There is no evidence that climate change is making droughts more likely to occur, except for smaller areas (Victoria, Tasmania, and SW Australia) which have an increasing frequency of droughts of all lengths.

36 month dry periods are more frequent in SW Australia, SE Australia, Eastern Australia, Tasmania, Victoria, (and interestingly Queensland, but only for <10% deficiency).

Some examples will illustrate the complexity of the picture.

Fig. 4:  Number of consecutive months per calendar year of 12 months severe rain deficiency: Australia

12m 5% Aust

Fig. 5:  Periods of 36 months serious rain deficiency: Australia

36m 10% Aust

In the past droughts of all lengths and severity were more widespread across Australia.

Fig. 6:  Periods of 36 months severe rain deficiency: Southern Australia

36m 5% Sthn Aust

Similarly, multi-year periods of severe rain deficiency were much more frequent and widespread across Southern Australia before 1950.  In the last 50 years there has been only one month where the 36 month total was in the lowest 5th percentile.

Fig. 7:  Periods of 12 months severe rain deficiency: New South Wales

12m 5% NSW

Fig. 8:  Periods of 36 months severe rain deficiency: New South Wales

36m 5% NSW

Fig. 9:  Periods of 12 months serious rain deficiency: New South Wales

12m 10% MDB

Fig. 10:  Periods of 36 months serious rain deficiency: New South Wales

36m 10% NSW

Across NSW, 4 months of 2018 had 12 month totals in the serious deficiency range, but none in the severe range.  Droughts of all severity and duration have become less frequent and widespread.  The Millennium Drought lasted longer but was less severe than the Federation Drought.

The Murray-Darling Basin lies across four states including most of NSW, and is Australia’s premier food and fibre producing region.  The current drought is affecting many areas in this region.

Fig. 10:  Periods of 12 months severe rain deficiency: Murray-Darling Basin

12m 5% MDB

Fig. 11:  Periods of 12 months serious rain deficiency: Murray-Darling Basin

12m 10% MDB

Fig. 12:  Periods of 36 months serious rain deficiency: Murray-Darling Basin

36m 10% MDB

We can conclude from these plots of the Murray-Darling Basin that this drought is patchy, and while nasty, is not the most intense or long lasting even in living memory, let alone on record, and that droughts are becoming less frequent and less widespread.

Fig. 13:  Periods of 36 months severe rain deficiency: Queensland

36m 5% Qld

Fig. 14:  Periods of 36 months serious rain deficiency: Queensland

36m 10% Qld

Queensland has little trend in frequency of drought with severe deficiency over three years but less severe droughts have been more frequent- due to the droughts of the 1990s and the Millennium drought.

Fig. 15:  Periods of 36 months serious rain deficiency: Victoria

36m 10% Vic

The Millennium Drought stands out as the longest period of widespread serious rain deficiency.

Fig. 16:  Periods of 36 months serious rain deficiency: South-West Australia

36m 10% SW Oz

Here we see that all but one month of all the 36 month periods of serious rain deficiency have occurred since 1970, reflecting the marked drying trend.  This really is an example of climate changing.

Winter rainfall

Fig. 17:  Winter Rainfall Deciles across Australia, 2018

winter rain 2018

According to the Climate Council, “Climate change has contributed to a southward shift in weather systems that typically bring cool season rainfall to southern Australia.”  However the usual areas affected by this southwards shift, Tasmania, south-west Victoria, southern South Australia, and most of the south-west of Western Australia, have had an average to above average winter.  Droughted areas are to the north.   The southwards shift of weather systems caused by Climate Change cannot be claimed to have any part in this drought.

Drought is a dreadful calamity wherever and whenever it occurs.  And on top of other difficulties in Queensland is the bureaucratic approval process under Vegetation Management regulations before graziers can push mulga to feed starving stock.

This drought may get worse if a full El Nino develops.  It is unlikely to break before six months or even 18 months.  By then it will be much more severe and widespread.  However, climate change has not caused this drought.  While there is evidence for increasing drought frequency and thus likelihood of more drought in the future in Tasmania, southern Victoria, southern South Australia, and the south-west of Western Australia, across the rest of Australia there is strong evidence that droughts have become less frequent, less severe, less widespread, and shorter.  If climate change is claimed as the cause of increasing droughts in the far southern regions, then climate change must also be causing less frequent droughts across the vast bulk of Australia, where droughts are always “likely to occur”, but not “more likely”.

Not the Worst Drought in 80 Years

January 7, 2015

Last night on the 7.00 p.m. ABC TV Queensland news there was a report on the Annual Climate Statement 2014 released by the Bureau of Meteorology yesterday (January 6).  I could not believe my ears, and as soon as it was on iview ( http://iview.abc.net.au/programs/abc-news-qld/NC1530Q005S00 ) I checked- several times.

The reader, Matt Wordsworth, clearly reports that the Bureau says Queensland has experienced “the worst drought in 80 years”.  The Bureau’s Jeff Sabburg was interviewed and claimed that 37.3% of the state was covered by the lowest rainfall on record.

Now I do know that much of Queensland has been very dry for a long time, with the northwest being especially bad, having missed two wet seasons (2012-13 and 2013-14).  Before you think I am callous, uncaring, and uninformed, I should let you know I was raised on a farm, my parents, grandparents, and great-grandparents were farmers, my brother and my brother-in-law are farmers, and I take a very keen interest in the land.

However, both the ABC news item and Jeff Sabburg’s claim, if quoted correctly, are complete nonsense.  Nothing like 37% of the State has had the lowest rainfall on record, and the drought is definitely not “the worst … in 80 years”.  The facts from the Bureau’s own websites (Climate Maps and Climate Change and Variability) show otherwise.

There follows a series of graphics clearly showing the state of Queensland’s rainfall and drought record.

Fig. 1:  12 month rainfall deficiency

qld 12m drought

Obviously recent rain has improved the situation, but what about over the last two years?

Fig. 2: 24 month rainfall deficiency

qld 24m drought

A-hah, that’s showing quite a lot of serious and severe rainfall deficiency, but not a lot of “lowest on record”.  However, compare that with the 24 month period to December 2003.

Fig.3:  24 months to December 2003

qld 24m drought 2003

I have been looking at Queensland rainfall closely in the past couple of weeks.  Here are some other ways of showing rainfall for Queensland.   I have shown 2013-2014 and 1929-1935 (the supposed 80 year ago worse drought), and more recent events, for ease of comparison.

Fig. 4: 12 month running mean of rain anomalies

qld rain 12m

Fig. 5:  24 month running mean of rain anomalies

qld rain 24m

Fig. 6:  27 month running mean of rain anomalies (covering the previous two wet seasons)

qld rain 27m

There was nothing unusual about Queensland’s rain in 2014.

The next graphics are plots of the 12 month counts of 12 month running means of rainfall with below average (bottom 30%), very much below average (bottom 10%), and severe deficiency (bottom 5%) rainfall.  In other words, counts of 12 month periods (January – December, February – January, March – February etc) with rainfall in each category.

Fig. 7: Count of 12 month periods of below average rain (lowest 30% of 12 month periods)

qld drought counts 12m belavg

Fig. 8: Count of 12 month periods of very much below average rain (lowest 10% of periods)

qld drought counts 12m verybelow

Fig. 9: Count of 12 month periods of severe rain deficiency (lowest 5% of periods)

qld drought counts 12m severe

There have been no episodes of statewide severe rain deficiency since January 2004.

Fig. 10: Count of 12 month periods of 24 month below average rain (lowest 30% of 24m periods)

qld drought counts 24m belavg

Fig. 11: Count of 12 month periods of 24 month very much below average rain (lowest 10% of periods)

qld drought counts 24m verybelow

Fig. 12: Count of 12 month periods of 24 month severe rain deficiency (lowest 5% of periods)

qld drought counts 24m severe

Fig. 13: Count of 12 month periods of 27 month below average rain (lowest 30% of 27m periods)

qld drought counts 27m belavg

Fig. 14: Count of 12 month periods of 27 month very much below average rain (lowest 10% of periods)

 qld drought counts 27m verybelow

Fig. 15: Count of 12 month periods of 27 month severe rain deficiency (lowest 5% of periods)

qld drought counts 27m severe

And just for information, this map shows the 36 month rainfall deficiencies to December 1902.

Fig. 16:  36 months to December 1902- the Federation Drought

qld 1902 36m drought

Now that’s a drought!

The current drought, bad as it is, barely rates when compared with previous droughts.

Perhaps the Bureau is using different data from what is shown on their websites.  Perhaps Jeff Sabburg was quoted out of context.  Whichever way, the public has been misled- not for the first time, and unfortunately not the last.

The Bureau of Meteorology should issue an immediate clarification, and the ABC should issue an immediate apology and correction.

IPCC Dud Rainfall Predictions for the Murray-Darling Basin

April 4, 2014

The IPCC’s recently released 5th Assessment Report (AR5) dedicated Chapter 25 to impacts of climate change on Australasia. There was wide media reporting of these impacts, including that of decreasing rainfall- more droughts and floods. The relevant part of Chapter 25 outlines eight regional key risks, including:

For some impacts, severity depends on changes in climate variables that span a particularly large range, even for a given global temperature change. The most severe changes would present major challenges if realized:

……. significant reduction in agricultural production in the Murray-Darling Basin and far south-eastern and south-western Australia if scenarios of severe drying are realised; more efficient water use, allocation and trading would increase the resilience of systems in the near term but cannot prevent significant reductions in agricultural production and severe consequences for ecosystems and some rural communities at the dry end of the projected changes.

Section 25.2, Observed and Projected Climate Change, gives the details:

This pattern of projected rainfall change is reflected in annual average CMIP5 model results (Figure 25-1), but with important additional dimensions relating to seasonal changes and spread across models (seealso WGI Atlas, AI.70-71). Examples of the magnitude of projected annual change from 1990 to 2090 (percent model mean change +/- intermodel standard deviation) under RCP8.5 from CMIP5 are -20±13% in south-western Australia, -2±21% in the Murray Darling Basin, and -5±22% in southeast Queensland (Irving et al., 2012). Projected changes during winter and spring are more pronounced and/or consistent across models than the annual changes, e.g. drying in southwestern Australia (-32±11%, June to August), the Murray Darling Basin (-16±22%, June to August), and southeast Queensland (-15±26%, September to November), whereas there are increases of 15% or more in the west and south of the South Island of New Zealand (Irving et al., 2012). Downscaled CMIP3 model projections for New Zealand indicate a stronger drying pattern in the south-east of the South Island and eastern and northern regions of the North Island in winter and spring (Reisinger et al., 2010) than seen in the raw CMIP5 data; based on similar broader scale changes this pattern is expected to hold once CMIP5 data are also downscaled (Irving et al., 2012).

As the Murray-Darling Basin (MDB) is the nation’s major food bowl, contributing a very large proportion of our agricultural production, a Reality Check on these claims is in order.

The Murray-Darling Basin is the largest catchment in Australia, and is one of the Bureau of Meteorology’s climate regions:

Fig.1: MDBRegions

First, annual rainfall. The IPCC projects an annual change of -2% +/-16% from 1990 to 2090. Here are the rainfall anomalies for the MDB straight from the Bureau’s Climate Change page:

Fig.2: MDB Annual Rainfall Anomalies, 1900-2013:MDB annual anoms

Linear trends have limited use in such a manifestly non-linear dataset as rainfall, however I put one in just in case someone says rainfall is decreasing. Even with 2010 deleted the trend is still positive. Let’s now look at the 10 year means:

Fig.3: MDB Annual Decadal Means:MDB annual anoms 10yrs

Note that for the entire period before the 1950s, the 10 year mean was below the 1961-1990 mean, and in 1946 was 94mm below. While in 2009 the 10 year average was 69mm below the mean, this being the first time in six decades it had been below -60mm, for most of the 1940s it was more than 60mm below the mean. It is entirely possible that rainfall will be below average in the MDB for several more decades, and this would be completely normal.

I shall now project this historical trend through to 2090, with a 2090 rainfall of 512.35mm, 2% below that of 1990 (522.81mm).

Fig.4: MDB Annual Rain to 2090:MDB annual rain to 2090

So that’s what a decrease in rainfall looks like! Note the uncertainty range- well within historical norms, and the low figure (404.76mm) is in the below average (lowest 30% of years) rainfall category by less than 4mm.

Next, winter rainfall (-16% +/-22%, June to August). From BOM Climate Change,

Fig.5: MDB Winter Anomalies 1900-2013MDB winter anoms
There you can see the declining trend (BOM says -0.57mm per decade)- but note the size of the trend compared with the variability.

Interestingly, consider the same data for the last 54 years.

Fig.6: MDB Winter Anomalies 1960-2013MDB winter anoms 1960-2013
But of course, the authors have detected the drying trend since the 1990s!

Now, decadal means:

Fig.7: MDB winter decadal means:MDB winter anoms 10yrs

Note the 10 year mean about -10mm in past decade, but -15mm in the 1970s and -19mm in the 1940s. Note also that the 10 year average was below zero for the better part of two decades, twice, in the past. Below average winter rain for the next few years would be completely normal, if the past is anything to go by.

Here is a chart showing the number of dry winters per 10 year period in the MDB.

Fig.8: 10 year count of below average winters.MDB winter anoms  under30%10yrs

Below average winters were more frequent in the past.

Projecting the winter anomalies into the future, with a decrease of -16±22%, June to August, we get:

Fig.9: MDB Winter Rain to 2090:MDB winter rain to 2090

109.74mm is almost exactly the 1961-1990 mean (111.1mm). The low end of the uncertainties, 85.6mm, is in the below average range but well outside the severe deficiency or even serious deficiency level. Yet this will cause “significant reductions in agricultural production and severe consequences for ecosystems and some rural communities”?

Note: these projections are based on continued warming by up to 2 degrees. Consider that we have already seen warming in the MDB of about +0.8 C since 1910 (according to BOM analysis based on ACORN-SAT).

It appears that the IPCC can’t be wrong, whether rainfall is higher, lower, or stays the same. They’re having two bob each way.

In discussing agricultural production, I would have been less underwhelmed if rainfall in other seasons had been considered. If winter rain is down (marginally), but annual rain is up, when did it fall?
Briefly, autumn, like winter, is almost flat (-0.59mm per decade), spring is up by 1.61mm per decade, but summer rain has increased 3.86mm per decade. If heavy rain falls before the wheat harvest is off, the crop is seriously downgraded, so late spring- early summer rainfall increasing would be of concern.

Fig.10: MDB Summer Rain AnomaliesMDB summer anoms

Fig.11: MDB Decadal Summer RainMDB summer anoms 10yrs

Note that summer rain increase is all since 1950. For 60 years farmers have been contending with this. It’s nothing new. Farmers adapt farming methods to changing conditions and with new technology. Moreover, the recent decadal peak is about the same as the 1960s and 1990s. Note also that the low decadal mean of the Millennium Drought is nowhere near the levels of past dry periods.

The warming to now has ‘resulted’ in increased annual rain, made up mostly of stronger summer rains since 1950, and marginally less winter and autumn rain which is less variable than in the early decades of last century.  The IPCC’s projections are thus the result of climate models and not historic observation, are subject to large uncertainty, and not greatly different from patterns of the past 114 years.

The AR5 prediction of dire consequences for the Murray-Darling Basin, based on rainfall projections that are essentially no different from historical observations, is nonsense. It is beyond parody, beyond ridicule. It treats the citizens and farmers of Australia with contempt.