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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
At the best of times, the
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
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
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
Is the drought in the
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
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
Of other climate mechanisms, the
Southern Annular Mode — which also affects the flow of moist south-westerly
winds over southern
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
The fourth climate engine, the Indian
Ocean Dipole, has now been shown by research at the
The two “poles” of the Indian Ocean
Dipole are represented by areas of warm and cool water on opposite sides of the
In the negative Indian Ocean Dipole
phase, the above pattern is enhanced, and rainfall across the southern part of
All of south-eastern
“That means all you are left with
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 over the same respective periods.
In a 2008 study based on the
chemistry of ancient corals, scientists led by Nerilie Abram of the
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
Water for the future?
If the drying of the
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
If we extrapolate today’s data for
temperature and water runoff, the
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
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
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
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
There is no excuse for complacency in the face of dangers like this. Where governments will not act, informed and mobilised populations must.