Anyway central banks are calling the shots, though not by choice, and mostly by sabotaging each other's efforts to make their exchange rates more competitive.

In a world verging on deflation there's something to be said for a devalued currency if you can get away with it. But it's a zero sum game where nobody can be the winner.

Even if the G20 leaders agree on fighting tax avoidance by companies such as Apple and Google, removing impediments to free trade, regulating big banks and tackling global warming, it's their independent central banks that will determine growth, interest rates and currencies.

The best we can hope for from G20 is that the job-creating stimulus from more infrastructure spending, culminating in a modest rise in global growth after five years, also lifts confidence, something the global economy needs dearly.

The US is in the throes of a cheap energy revolution as shale mining drives down oil and gas prices, the US dollar strengthens, which pulls down the prices of commodities because it's their home currency, and zero official interest rates reduce the cost of capital.

With growth picking up the US Federal Reserve has even dropped its extra money printing known as quantitative easing or QE which proved such a boon for global markets.

But everywhere else is slowing.

That includes China, the second-biggest economy. Unfortunately it's in the middle of a property downswing that could get out of hand.

Europe is looking weak again, prompting the European Central Bank to step up its own version of QE.

Japan has started the mother of all QEs, so far to no avail except for an unfortunate side effect in holding our dollar up as commodity prices fall.

Mind you, some of those extra yen will almost certainly find their way into our sharemarket and AAA-rated Australian Government bonds.

Such is the offshore demand for our bonds it borders on greed, a missed opportunity if ever there were one since this has slashed their yields. It's a very cheap way for the government to borrow and get cracking on some spending on infrastructure.

Anyway bonds are the financial system's benchmark, so their lower yields pull down other interest rates.

Russia is being shirtfronted – sorry – into a recession as commodity prices slump. Some of the biggest emerging markets aren't faring much better either.

The one thing all the central banks have in common, with the possible exception of ours, is a well-grounded fear of deflation. The inflation rate in Britain,  for example, is struggling to keep above 1 per cent.

In the US the Fed is keeping official rates low for longer because it's trying to push inflation up to 2 per cent.

You might wonder why it's not grateful for small mercies. But inflation any lower or, worse, falling and consumers would naturally delay their spending, expecting everything to become cheaper.

It had better succeed because it seems the US is the last one standing.

 @moneypotts