Try this riddle. A lily in a pond is doubling in size every day. It takes 31 days to grow to cover half the pond. How many more days will it be before it covers the pond completely?

The answer, of course, is just one more day.

The object of the riddle is to point out the insidious nature of exponential growth. A sustained growth process – like a cancer – is almost imperceptible when it starts. But if you ignore it, it will ‘suddenly’ overwhelm the organism.

The lily, for example, goes from covering 0.75 per cent of the pond on its 25th day, to 1.5 per cent on its 26th – and the average gardener wouldn’t even notice it. Yet a few days later, it grows from 12.5 per cent to 25 per cent – and clearly it’s a problem. But by the time it has reached that size, your pond may well be doomed.

This is the story behind Australia’s private debt. It has been growing more than 4% faster than our GDP for 53 years. Back in the 1960s, that meant very little – the ratio of debt to GDP would increase by no more than 1 per cent a year, and it was at comparatively trivial levels anyway. It grew from 27.3% of GDP in 1966 to 28.2% in 1967.

Forty years later, that ratio is increasing ten times as fast. It is 147.1% now. If the rate of growth doesn’t slow down, it will crack 150 per cent of GDP by March 2007, and it will exceed 160 per cent of GDP by the end of 2007. We simply can’t keep borrowing at that rate. We have to not merely stop the rise in debt, but reverse it.

Unfortunately, long before we manage to do so, the economy will be in a recession.

The reasons are simple: paying down excessive debt causes borrowers to stop spending – whether that means households that cancel order for the latest LCD TV, or firms that put off that planned expansion of capacity. Income plummets, but debt continues to rise, simply because of the effect of compound interest. The debt to GDP ratio starts to fall only when a substantial slab of income is devoted to paying debt, but that in turn means a serious recession.

We have suffered two such debt-driven downturns since the end of WWII: the long 1973-1983 recession, when unemployment blew out from a mere 1.8% to over 10%; and the 1990 ‘recession we had to have’, when unemployment exploded from 5.6% to 10.6% in just over 3 years.

Both recessions were preceded by booms in which private debt rose to previously unheralded levels. During both, the ratio ‘headed south’, against its long term trend. But the momentum of debt meant that the turnaround in the ratio followed the start of the recession itself. It continued rising, for nine months after unemployment began to rise in 1973, and for a whole two years in 1990.

So when will this recession begin? On current data, the domestic economy may already be in one – though the China boom has more than compensated for the domestic downturn.

What can be done to avoid it? Unfortunately, almost nothing. We have two sources of spending power: what we earn, and what we borrow. During a boom, borrowing adds to our spending power – and it has added massively in the last decade, as debt has blown out from 88% of GPP at the end of 1997 to 147% now. But during a slump, once we get on top of the momentum of debt, repayment of debt subtracts from our spending power. This feeds back on income itself, reducing its growth still further, because investment ends, consumer spending drops, and unemployment rises even more.

Thanks to Sean Leahy

There is an ominous similarity here between debt and global warming. Rising carbon dioxide levels cause rising temperatures, that in turn cause events like this month’s bushfire infernos – and they in turn add to carbon dioxide levels. Even John Howard seems to understand that now.

When the debt bubble too begins to burn, government will be captive to it rather than in control. The RBA will have to reduce interest rates – as its counterpart did in Japan when its Bubble Economy imploded in 1990. And the government surplus will evaporate, as tax receipts fall and unemployment benefits rise.

The government of the day, whether Liberal or Labor, will have no say in that – and nor should it. We’ve been conditioned to regard government deficits as always and everywhere bad. But during a downturn, increased government spending provides a cash flow that wouldn’t otherwise be there. The consumer spending that government deficits allow turns up in corporate balance sheets, enabling them to pay down their debt. Without a deficit, the process of reducing debt would be that much more protracted.

That was certainly the story of the 1990 recession; the coming one, though, will dance to the beat of a slightly different drum. Business was on the debt binge during the 1980s boom; this time, households have led the charge into bankruptcy, as they borrowed up big to speculate on real estate. They are the ones who really have to repair the balance sheets this time round, and they don’t have some of the easy options that businesses have. They can’t simply go bankrupt and disappear – like Laurie Connell or Alan Bond. Nor can they sack the kids to cut down on their pocket money bills. So this recession will take longer to turn around.

It also mustn’t lead to yet another debt-financed boom. We’ve thrown as much debt money onto the economic fire as we can possibly afford. Just as we have to cut carbon dioxide concentrations in the atmosphere to avoid catastrophic global warming, we have to reduce debt levels now. We can’t let them resume their cyclical but exponential trend upwards once this crisis is over.