Australia: Global warming and the ‘Big Dry’— What prospects for the Murray-Darling river system?
By Renfrey Clarke
July 20, 2009 -- From desert-fringe villages and drowning atolls, global warming is predicted before long to set climate refugees on the move. But arguably, the first climate refugees to reach Australia’s major cities are arriving already. And the places from which they have come are not exotic — rural towns like Mildura, Renmark and Griffith in Australia’s south-east.
In settlements throughout the Murray-Darling, residents are quietly deciding the irrigation-based economy has no future. For many orchardists and viticulturalists, allocations of water in recent years have been too low to keep plantings alive.
When barely a trickle is coming down the rivers, farmers are concluding it’s best to sell the next-to-meaningless water rights, accept a government exit package, bulldoze the trees and vines, and walk away.
In the southernmost regions that provide most of the Murray-Darling’s flow, the last “normal” year of rainfall and of runoff into the rivers was 1996. The subsequent “Big Dry”, during its first decade, had rough parallels in the Federation drought of 1895-1902 and the Second World War drought of 1937-1945.
But the present drought has continued and intensified. Its hold on Victoria is suggested by the rainfall records for Melbourne. The January-June perIndian Ocean Dipole in 2009 was the driest first half-year ever recorded in Melbourne, with just 126 millimetres of rain. Prior to 1997, the city’s average annual rainfall was 660ml; since then, it has dropped by 21% to 520ml.
At the best of times, the Murray-Darling Basin loses vast quantities of water to evaporation. In the past, each 1% shortfall in rainfall brought a 3%-4% reduction in streamflow. But with the dryness of the landscape now unprecedented, past relationships between rainfall and streamflow have broken down; average rainfall no longer brings average runoff.
From 2006, the flow deficit has worsened sharply, much more than the decline in rainfall would suggest.
Total inflows to the basin’s rivers in June 2008, the Melbourne Herald-Sun reported, were less than one-seventh of the average figure. For the three years ending in March 2009, an April 7 Murray-Darling Basin Authority press release says, Murray system inflows were only 46% of the previous three-year minimum, recorded in 1943-46.
Murray inflows in the first three months of 2009 were the lowest in 117 years of record-keeping.
As well as reflecting historically low rainfall, the declining streamflows also result from greater evaporation due to higher temperatures. CSIRO climate scientist Wenju Cai has calculated that a 1ºC rise in temperature in the Murray-Darling Basin results in a 15% reduction in river flows. Temperatures in the basin in 2007, were the warmest ever at 1.1ºC above average, the Australian said in June 2008.
None of this seems to have made any fundamental impression on federal minister for climate change and water Penny Wong. Addressing a conference on water management in Adelaide on May 18, Wong acknowledged that less water was available as a result of climate change, and that Australians would need to become “much smarter” about the way they used it.
But in 10 years, she predicted, “the basin’s rivers and wetlands will be in a condition that resembles their natural state, sustaining biodiversity for which this nation is internationally renowned”, the May 19 Australian said.
Others are less hopeful. Also in May, professor Mike Young of the Wentworth Group of Concerned Scientists observed to the Australian Broadcasting Corporation’s science website that the Murray-Darling Basin might be entering a “step-wise” reduction of rainfall, as in the south-west of Western Australia since the mid-1970s. Between 1975 and 2002, inflows to Perth’s water supply dams averaged just under half of the figure for 1911-1974, and the average for the decade from 1997 to 2006 was lower still.
Is the drought in the Murray-Darling Basin semi-permanent, and can it be attributed to global warming?
Computer modelling shows that by 2006, the Big Dry was already a once in more than 300 years event. As the dry years succeed one another, the chances that the drought is a random occurrence dwindle to the infinitesimal.
In October 2007, Cai said in a media release: “There is no longer any doubt that climate change caused by increases in greenhouse gases is influencing seasonal shifts in rainfall patterns.”
Of four key mechanisms known to affect rainfall in the Murray-Darling Basin, at least three and possibly all four are trending in directions that suggest further drying. In each case, scientists have been able to point to clear links with global warming.
A survey of these mechanisms — at least as they pertain to the southern part of the basin — is provided by Bertrand Timbal of the Bureau of Meteorology in a recent paper called, The continuing decline in South-East Australian rainfall: update to May 2009.
Most of the decline, Timbal explains, is due to a 25% reduction since 2006 in autumn rainfall across the region. Eleven of 13 autumns since 1996 have been drier than the long-term average.
Of the climate mechanisms in play,
Timbal argues, the only one that adequately explains the decline in autumn
rainfall is a marked strengthening of the subtropical high pressure ridge —
that is, of the “highs” that drift across Australian weather maps. This
strengthening is tied to global warming through an expansion of the so-called
Hadley circulation, which governs the flow of air between the tropics and
For climate purposes, the tropics can be understood as the zone near the equator where moist air brought by the trade winds rises, cools and releases its moisture as rain. Warmer air can hold more water vapour, and as global temperatures have risen, one result is that the tropical zone has expanded — by about 350 kilometres on each side of the equator over the past 50 years.
Driven by the Hadley circulation, the air that has risen over the tropics moves towards the poles at high altitudes. Now dry, and growing denser as it cools, it descends eventually over the mid-latitudes as the familiar high-pressure cells. As global warming has proceeded, these cells have become larger and more intense. To an increasing degree, they block Great Southern Ocean storms and force them south of the Australian continent. The outcome has been a band of reduced rainfall across southern Australia from Perth to the eastern seaboard.
Of other climate mechanisms, the Southern Annular Mode — which also affects the flow of moist south-westerly winds over southern Australia — is claimed by some scientists to be a further important “driver” of the long drought. Timbal, however, discounts the Southern Annular Mode as a major influence because there has been no clear trend in the number of its “dry” phases during the winter months when rainfall and runoff in the southern part of the Murray-Darling Basin are at their peak.
By contrast, the El Nino Southern Oscillation, which has a profound impact on weather in the Pacific and beyond, is generally recognised as trending long-term in a direction that will reduce rainfall in Australia, especially over Queensland and northern New South Wales.
Indian Ocean Dipole
The fourth climate engine, the Indian Ocean Dipole, has now been shown by research at the University of NSW to be closely correlated with droughts in the southern Murray-Darling Basin.
The two “poles” of the Indian Ocean Dipole are represented by areas of warm and cool water on opposite sides of the Indian Ocean. Usually, the warm pool is in the east, to the south of Sumatra, with cooler water and higher air pressure off Africa. Air flowing from the high pressure cell grows wetter as it moves over the warmer water to the east. This warm, humid air often penetrates over Australia as belts of tropical moisture. This moisture coming overland from the Indian Ocean gives south-eastern Australia much of its rainfall.
In the negative Indian Ocean Dipole phase, the above pattern is enhanced, and rainfall across the southern part of the Murray-Darling Basin tends to be well above average. But in the positive phase, the water off Sumatra is relatively cool, and the flow of moist air from the Indian Ocean is much less.
All of south-eastern Australia’s extended droughts, the UNSW research has shown, have been accompanied by a marked dearth of negative Indian Ocean Dipole episodes. This has been especially true of the current Big Dry. “What we have found is that there has not been a single wet event, not a single negative event in the Indian Ocean Dipole since 1992”, UNSW researcher Caroline Ummenhofer told Reuters on February 5.
“That means all you are left with in south-east Australia is dry events.”
Positive Indian Ocean Dipole episodes occurred in each of the three years of severe drought from 2006 to 2008. In a study published in June, Cai said that positive Indian Ocean Dipole episodes had increased from about four per 30 years early in the 20th century to about 10 since the late 1970s. Negative events decreased from about 10 to two over the same respective periods.
In a 2008 study based on the chemistry of ancient corals, scientists led by Nerilie Abram of the Australian National University concluded that the Indian Ocean Dipole had operated for at least 6500 years, and that the big driver was the East Asian monsoon. Years with vigorous monsoons see increased wind strength over the eastern Indian Ocean, and high evaporation. Abram and her colleagues established that for millennia, such years have also tended to see strong ocean cooling and positive Indian Ocean Dipole events.
With global warming, the strength of the East Asian monsoon is on a marked upward trend. This suggests that positive Indian Ocean Dipole events — and droughts in south-eastern Australia — will become still more frequent.
Water for the future?
If the drying of the Murray-Darling Basin is driven by global warming and can be expected to get worse, how far and how fast is the water likely to ebb? CSIRO scientists attempted to answer this question in an October 2008 report, Water Availability in the Murray-Darling Basin. Their modelling concluded that the median value for likely outcomes was an 11% reduction in surface water availability across the basin — 9% per cent in the north, and 13% in the south — by 2030.
This would not amount to a return to Wong’s “natural state”, but it would be a dramatic improvement on the actual situation since 1996. Unfortunately, the CSIRO’s relatively encouraging predictions seem unlikely to pan out.
The CSIRO based its report on scenarios and modelling from the 2007 Fourth Assessment Report of the United Nations Intergovernmental Panel on Climate Change. Though formidable as a work of science, the IPCC’s report was slow to appear because of exhaustive checking and consultation. No sooner was the report published than it was criticised for being out of date.
On the whole, the findings from the most recent per Indian Ocean Dipole point to climate outcomes considerably more dire than those predicted by the IPCC in 2007. Much of the reason is that world greenhouse emissions have increased far more quickly than the IPCC anticipated.
Arguably, the best estimates now available for future global warming are those provided by the Integrated Global Systems Model developed at the Massachusetts Institute of Technology. MIT’s figures incorporate scientific advances of the past few years, fresh data on how emissions have actually risen, and modelling of the future world economy.
The results of an exhaustive run of MIT’s model were published in May in the American Meteorological Society’s Journal of Climate. For a business-as-usual “no policy” emissions scenario, MIT said, the results “indicate a median probability of surface warming of 5.2 degrees Celsius by 2100, with a 90 per cent probability range of 3.5 to 7.4 degrees”. This figure of 5.2° contrasts with the IPCC’s 2007 “best estimate” of 4.0° for its high-emissions scenario.
What effects might a rise of 5ºC over pre-industrial levels have on the Murray-Darling Basin? Here we can turn to the relevant sections of Mark Lynas’s 2007 book Six Degrees. Summarising climate modelling and paleoclimatic evidence, Lynas projects that “almost the whole of Australia” would become part of a huge mid-latitude desert belt.
If we extrapolate today’s data for temperature and water runoff, the Murray and its tributaries would become no more than intermittent streams, dry by early summer. Substantial irrigation would be out of the question. Not that many people would want to live in the basin, since summer temperatures there would commonly reach above 50ºC.
In time, the Murray-Darling system as such would cease to exist; distinct rivers would discharge their rare peak flows into a broad, shallow sea. Five degrees of global warming would be more than enough, over perhaps a thousand years, to melt the Earth’s icecaps. The new mouth of the Murray would be somewhere near Swan Hill in Victoria.
Human society — even profit-driven capitalism — may yet stop short of the “business as usual” emissions that underlie this scenario. But the point remains that no effort to save the rivers that merely seeks to reapportion water, without also campaigning for drastic cuts in greenhouse emissions, will make any difference in the long term.
Meanwhile, Australians will have to recognise that with present climate change, plus additional warming already built into the system, the historical Murray-Darling has almost certainly been lost. The only reasonable basis for policy is to accept Young’s suggestion that a “step-wise” decline in rainfall and runoff has occurred.
The new level around which river flows might stabilise in coming decades has yet to become clear. But on the basis of the decline in streamflows in the Perth hinterland — the main cause of which also appears to be the strengthened Hadley circulation — governments would be foolish to count on even half of the 20th century flows in the Murray-Darling Basin being regularly available.
The focus for water use must now be on saving viable examples of the river ecosystems. Water will have to be apportioned carefully on the basis of where it can do most good. Many floodplains and already-damaged wetlands will have to be allowed to dry out, so that others can survive to illustrate the richness that preceded large-scale water diversions.
Should any irrigation continue? If nature merits preserving, human communities, too, have at least some claim on the rivers. But the quantities of water diverted can only be a fraction of those typical in the last century.
An integrated basin-wide plan needs to be developed, with strong input from irrigators and other basin residents, to ensure that the “stepping down” of irrigation proceeds humanely, with the interests of smallholders and communities protected before those of big agribusiness. Where irrigation continues, state-of-the-art water efficiency must be mandatory.
To sustain river towns, new industries that do not require significant amounts of water should be developed with government funding. With their flat landscapes, clear skies and established infrastructure, the main irrigation areas could be the sites for massive solar thermal energy installations.
So far, the governments that run the Murray-Darling Basin have shown no inclination to take the recent science and its lessons on board. Indeed, the way these governments resist warnings suggests they have accepted future eco-catastrophe as a price for keeping big investors happy and popularity polls buoyant.
But the dilemmas that confront the basin have sources that threaten to put advanced civilisation out of business — starting within a few decades in hot, drought-prone countries like Australia, and extending ultimately across the planet.
There is no excuse for complacency in the face of dangers like this. Where governments will not act, informed and mobilised populations must.