From desert-fringe villages and drowning atolls, global warming is predicted 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 settlements throughout the Murray-Darling, residents are quietly deciding the irrigation-based economy has no future.
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 drought has continued and intensified. Its hold on Victoria is suggested by the rainfall records for Melbourne.
January to June 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 660mm; since then, it has dropped by 21% to 520mm.
Murray River 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 an average 1ºC rise in temperature in the Murray-Darling Basin would result 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.
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 drought was already a one in more than 300 years event.
As the dry years succeed one another, the chances that the drought is a random occurrence dwindle even further.
In October 2007, Cai said: "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 are changing in ways that suggest further drying is likely. In each case, scientists have been able to point to clear links with global warming.
A survey of these mechanisms — at least as they affect the southern part of the basin — was provided by Bertrand Timbal of the Bureau of Meteorology in a recent paper.
Most of the decline, Timbal explained, is due to a 25% drop 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 the only one that explains the decline in autumn rainfall is a big strengthening of the subtropical high pressure ridge — that is, of the "highs" that drift across Australian weather maps.
The stronger "highs" are tied to global warming through an expansion of the so-called Hadley circulation, which governs the flow of air between the tropics and mid-latitude regions.
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. 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.
Because of global warming, these cells have become larger and more intense. More and more, they block storms from the Southern Ocean and force them south of the Australian continent.
The outcome has been reduced rainfall across southern Australia.
Also, the El Nino Southern Oscillation, which has a big impact on weather in the Pacific and beyond, is predicted to further reduce rainfall in Australia, especially over Queensland and northern New South Wales.
A third climate engine, known as the Indian Ocean Dipole, has now been shown by research at the University of New South Wales to be closely connected 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 water is in the east. The cooler water and higher air pressure tends be in the west, near Africa.
Air 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 and gives south-eastern Australia much of its rainfall.
In the "negative" Indian Ocean Dipole phase, the eastern waters are even warmer, and rainfall tends to be well above average.
But in the "positive" phase, the water in the eastern Indian Ocean is relatively cool, and the flow of moist air across Australia is much less.
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 its big driver was the East Asian monsoon.
Stronger monsoons lead to higher wind strength over the eastern Indian Ocean, and more 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 even 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?
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).
New results from the MIT's model were published in May. For a business-as-usual emissions scenario the results "indicate a median probability of surface warming of 5.2°C by 2100, with a 90% probability range of 3.5°C to 7.4°C".
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 the work of climate scientists he predicts at 5°C of warming "almost the whole of Australia" would become part of a huge desert belt.
The rivers of the Murray-Darling Basin would become irregular streams, dry by early summer. Large-scale 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 higher than 50ºC.
In time, the Murray-Darling system as such would cease to exist. 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.
The point is that no effort to save the rivers that merely seeks to reallocate 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 focus for water use must now be on saving viable examples of the river ecosystems. Water will have to be allocated carefully on the basis of where it can do most good.
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 residents, to ensure that the "stepping down" of irrigation proceeds humanely.
The interests of smallholders and communities must be protected before those of big agribusiness.
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 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.
[Renfrey Clarke is a climate activist based in South Australia and a member of the Socialist Alliance.]