(Repeats March 23 item with no changes. The opinions expressed here are those of the author, a columnist for Reuters)

* Graphic: Chinese aluminium volatility - http://tmsnrt.rs/2mvGXl9

By Andy Home

LONDON, March 23 (Reuters) - The political heat is rising in the aluminium market, with a trio of industry bodies calling on the G20 to address global market imbalances resulting from China's burgeoning output.

"China's state-sponsored support is contributing to an unsustainable structural overcapacity that will impact growth and contribute to heightened instability until it is addressed."

So wrote the heads of the U.S. Aluminum Association, its cross-Atlantic peer European Aluminium and the Aluminium Association of Canada in a March 15 open letter to the Group of 20 leading economies.

What they want is the sort of global forum created to discuss steel overcapacity at last year's G20 summit.

The charges against China's leviathan aluminium sector are largely the same as those against its equally huge steel industry.

"Both the massive increase in production and the excess capacity have had a downward effect on the prices, generating significant economic and employment losses for our respective producers and economies," the three associations said.

As with steel, trade tensions are rising with the United States taking the lead. It has launched a broad complaint about Chinese aluminium subsidies with the World Trade Organization, while the Aluminum Association has filed a petition seeking anti-dumping duties on aluminium foil.

All of which is somewhat ironic.

The price of aluminium traded on the London Metal Exchange is up 14 percent this year and, at $1,930 a tonne, close to two-year highs.

This, of course, is also all about China, as the market tries to price in the potential for significant anti-pollution production curbs next winter.

"Heightened instability", it appears, works both ways.

Graphic on monthly Chinese aluminium production figures:

http://tmsnrt.rs/2mvGXl9

UNSTABLE STATISTICS

China has lifted its share of global primary aluminium production from about 10 percent in 2000 to more than 50 percent, with the International Aluminium Institute's (IAI) latest figures showing that China produced 54.4 percent of the global total in February.

Give or take a percentage point or two, because even the country's production numbers are unstable.

Annualised output dropped by 1.7 million tonnes in February relative to January, if you believe the figures supplied to the IAI by China's Nonferrous Metals Industry Association (CNIA).

It seems highly unlikely that such a huge amount of capacity simply went offline last month.

The largest monthly swing in production in the rest of the world over the past 10 years was a drop of 584,000 tonnes in January 2012, a period of falling prices and multiple capacity closures.

Seasoned aluminium hands have grown accustomed to this sort of volatility in Chinese production figures, particularly around the end of both calendar and lunar years, as well as the occasional revisions to historic production figures.

CNIA shocked the market at the start of 2016 when it added more than 4 million tonnes to the production ledger over the 2011-14 period.

It has just sprung another little surprise; this time deducting 154,000 tonnes from 2015.

The official monthly figures, in other words, should carry a strong health warning.

Production seems to be trending higher. Taking the cumulative net change over the past four reported months (November 2016 to February 2017), national run rates have increased by about 925,000 tonnes annualised.

That would seem to tally with anecdotal reports of new capacity coming online and restarts to take advantage of the improving price environment.

FUTURE AND PRESENT INSTABILITY

The market is still digesting the implications of Beijing's anti-pollution measures in regions around the Chinese capital over the winter heating months from mid-November to mid-March.

A requirement that local aluminium smelters cut capacity by 30 percent puts at risk about 1.3 million tonnes of production, with potential further hits from mandated closures of plants producing other smelter inputs such as carbon anode and pet coke.

Affected Chinese producers might be expected to run their smelters as fast they can to compensate.

But Beijing's environmental crackdown is far more extensive than the winter pollution measures and it is already affecting production rates.

Environmental inspection teams were in Henan province last month and at least one operator, Linfeng Aluminum and Power, closed some capacity as a result, according to specialist Chinese aluminium consultancy AZ China.

Capacity closures across the metals production sector are now increasingly happening pre-emptively ahead of inspections.

Moreover, environmental inspectors are returning at regular intervals to check on progress. Shandong province will receive its fourth environmental check next month, AZ China says.

Beijing is committed to sending inspection teams to every province over the course of this year, including key aluminium production hubs such as Xinjiang.

Chinese policymakers' search for "blue skies" has injected a new level of uncertainty in the world's largest supplier of aluminium.

TIME TO TALK

And that means a previously unknown degree of uncertainty in a global aluminium supply chain that has historically not been prone to the sort of disruptions that plague other industrial metals, such as copper.

All this at a time when aluminium usage is growing fast as the metal takes an ever-growing materials share in key sectors such as transport.

Its success has in large part been predicated on a global tendency to overproduce, particularly in China, and the resulting relative low level of pricing volatility.

It's ironic that just as international pressure is rising on China to rein in its aluminium smelters, the country is doing precisely that in the form of an environmental clampdown.

But the three aluminium bodies are certainly right when they refer to heightened instability.

China's dominance of global production has left the aluminium market highly vulnerable to shifts in the country's policy.

That needs to be addressed and the G20's steel forum is as good a template as any.

If nothing else, it could start with getting a better statistical picture of just how much China is producing.

(Editing by David Goodman)