With record rainfall in large parts of Australia in recent months there has been a bit of a tendency to think that normal service has been returned. But record rainfall is by definition exceptional. This is how the last three months look on the BOM maps:
The research shows that when the IOD is in its negative phase, with cool Indian Ocean water west of Australia and warm Timor Sea water to the north, winds are generated that pick up moisture from the ocean and then sweep down towards southern Australia to deliver higher rainfall.
The study also shows that the IOD has a much more significant effect on the rainfall patterns in south-east Australia than the El Niño-Southern Oscillation (ENSO) in the Pacific Ocean.
The La Niña increases the chances of higher than average rainfall in most of the rest of eastern Australia as well as a more active cyclone season. On average.
The exceptionality of the current season can easily be seen by comparing it to this decile mean of 12 La Niña events:
The following image shows what happens in negative IOD years:
That page also carries examples of the rainfall effect of combined IOD and La Niña years, showing how truly exceptional the current season is.
If you overlay these maps with a map of the Murray Darling Basin you can see that both would have a considerable influence on flows:
It is particularly misguided to think that current rainfall in the Murray-Darling Basin represents ‘normal’.
The MDBA deals with the impact of climate change in Chapter 4.2 of the Technical background paper which carries this graph of modelled river levels at Wentworth assuming no diversions:
I would suggest that it’s not possible to establish what the ‘normal’ pattern is. Nor can we say that the pattern of flows has reduced decisively and won’t recover. But it would be heroic indeed to suggest that the pattern from now until 2030 will return roughly to the average of the entire historical record for 1985-2009, yet to cut to the chase the MDBA has used a reduction of surface flows of only 3% in their planning and has deemed that likely changes in groundwater and sea level rise near the mouth will be too negligible to be considered.
In my view, admittedly limited, their stance can be justified by the science if their focus is the next decade, but it seems to me that the science is not particularly helpful. In other words we really don’t know what the heck is going to happen during the climatically short period we are planning for.
To back up a bit, climate models suggest that there is going to be an expansion of tropical weather systems leading to mid-latitude drying, as shown in this image from IPCC AR4 WG1:
I understand that climate modellers work in grids of about 300 km square, so the predictions are coarse-grained and uncertain over a short time period like 2010-2030. But recent information indicates that we’ve seen in both the northern and southern hemispheres the westerlies retreating towards the poles.
So we have these winds that wrap around and around the mid-latitudes, and basically those belts of winds have been moving like shrinking doughnuts towards the poles of both hemispheres. And in the Southern Hemisphere it’s been even more dramatic. In the north it’s been about four degrees of latitude, in the south it’s been six degrees of latitude.
That’s 670 km. I’d suggest that ‘normal’ has gone forever.
There are something like 23 river valleys in the MD Basin and according to Chiew et al there is a clear run-off gradient from less than 10 mm in the west to more than 200 mm per annum in the SE. This is illustrated as follows:
It’s no surprise, then, to learn that the Upper Murray catchment alone with 1.4% of the area accounts for 17.3% of runoff.
And, indeed, the South East is where the biggest reduction has been:
The image on the right shows the decrease averaged over the 18 MDB regions.
In looking at the 2009 Steffen Report on Australian climate I gave this summary:
Progress has been made in sorting out what is happening with the climate in Australia. The shorter story is that there is a clear climate change signal in the drying pattern in south-west Australia in recent decades and in the lower edge of the Murray-Darling Basin. There is some evidence of a climate change signal in the drying trend in Victoria and the lower half of South Australia. In Northern NSW and Queensland it is too early to say.
Steffen notes that increased heat alone will reduce run-off by 15%.
The most important factor in terms of MDP planning is that they intend to do it all over again by 2020. Hence the focus is effectively instant in terms of climate change.
The MDB Technical report notes the extreme variability of run-off in the past. In the wettest 15-year sequence (1050-1964) flows at Wentworth were 42% higher than the long-term average. In 1995-2009 flows were 32% lower. Climate models indicate a variability of +7% to -37% by 2030. A ‘median’ climate scenario of -10% is derived, which they scale back to -3% for the current planning period.
You can’t fault this on scientific terms but variability and uncertainty seem to be key concepts, especially since water allocations are low in dry years and high in wet years. Since, according to Chiew et al, run-off is only 27.3 mm of precipitation of 457 mm (long-term average) the whole business of irrigation seems extremely marginal. Some usage can be opportunistic, but people, trees, vines, farm animals and many of the environmentally sensitive areas require continuous access to water. Moreover the lower part of the basin, say from Mildura down, contributes very little run-off and hence relies on rain falling further up.
Clearly priorities are going to have to be set across all users, in each valley and across the basin as to what kind of river and what uses are sustainable. The proposed reallocation to the environment is astonishing when represented graphically and numerically (see Figure 4.11 and table 4.4, pp 111-112).
The Lower Lakes present a peculiar problem in terms of climate change.
The best that science can offer is that current levels of CO2 imply an eventual sea level rise of 25 metres, plus or minus five. Everyone concedes that to reach this level will require centuries and probably upwards of a millennium. But most scientists and planning authorities that take sea level rise seriously are reckoning on 1.0 to 1.2 metres by 2100, with risk on the upside. The long-term viability of the Lower Lakes as a freshwater system becomes a question. If you want to return the Lower Lakes tidal system to a ‘natural’ state a prime condition is that you have to cease all water diversions upstream.
But that’s assuming no sea level rise, past or future. Without a barrage, saltwater incursions would reach Murray Bridge at 1 metre rise. You can check levels up to 14 metres on this flood map. At 14 m you get significant incursions up as far as Loxton.
Skeptical science has recently done a post looking at The Eemiam interglacial 116,000 to 130,000 years ago when the temperatures reached 1.9C higher than pre-industrial. That equates with the 2C guard rail currently adopted for a ‘safe’ climate. Sea levels were 6.6 to 9.4 metres higher (midpoint 8 metres) than now. This is what you get:
So removing the barrages is not an exercise in restoration, rather a commitment to continuously increasing saltwater incursion. Yet if we are not going to adopt aggressive mitigation it may be better to relinquish the lower lakes and put the barrages above them so that Adelaide can continue to draw water.
One way or another as the century progresses we are going to have to make some stark choices about what kind of river we want. Already particular river communities are suggesting (for example the Lachlan) that their water goes practically nowhere downstream. The technical report stresses the interconnectedness of the whole basin, but with drying this will become less and less true. No community of interest across the whole basin can be assumed.
The earlier post is here.
The House Standing Committee on Rural Australia has a Inquiry into the impact of the Murray-Darling Basin Plan in Regional Australia
Here’s a perspective from rural Australia.
Here’s a somewhat jaundiced appraisal of the worth of the Wentworth Group.