Prosperity cannot be paid forever by maxing out our green credit

The most thought-provoking comment I've seen on the budget came from Senator Christine Milne of the Greens. ''Every Australian knows,'' she said, ''that if you have two credit cards, it is very bad management to pay off your debt on one of them by racking it up on the other.'' The budget ''pulled down the national economic debt, but it continued the process of racking up our ecological debt''.

Sadly, it's true. The budget formally records Kevin Rudd's failure of leadership with his cowardly and illogical decision to shelve his emissions trading scheme.

It shows he took steps to avoid being accused of using the abandonment of the scheme to hasten the budget's return to surplus by using the net cash saving involved - $653 million - to increase spending on renewable energy.

The reversal did make it possible for the Government to meet its commitment to limit the real growth in its spending to 2 per cent a year.

And it did mean it was abandoning a ''great big new tax on everything'' in favour of a great big new tax on the mining companies, with the proceeds to be used to buy votes with a range of tax cuts and concessions - surely a net political gain.

Even so, if the government wants to insist it was motivated more by lack of political courage than by budgetary expediency, I accept its protestation.

No, that's not the point. It's that the budget continues our practice of worrying intensely about what we're doing to the economy while ignoring what we're doing to the environment. We just took a decision to take our chances on global warming - to do nothing to prepare for global action on climate change and nothing to set an example others might follow - but nowhere does that show up as a cost or liability.

It's not in the budget, nor in gross domestic product. It's invisible. We carefully measure and hugely publicise any increase in government debt or setback in economic growth, but what our actions and inactions are doing to the environment is largely out of sight.

When we run down our non-renewable resources (as we're hoping to do at a much faster rate with the return of the resources boom), nowhere does this show up as a cost or reduction of our assets. When we continue to deplete renewable resources at a rate much faster than they can renew themselves, nowhere does this show up as any kind of negative.

When we continue pumping our waste back into the environment - including greenhouse gases, but also other air and water pollution, garbage and human waste - at a faster rate than it can absorb, nowhere is this recorded as a cost.

GDP, our great de facto measure of progress, counts the short-term benefits from all this exploitation, but ignores its long-term costs. So Milne is right: we have been paying off our economic credit card by racking up debt on our environmental credit card.

But as the still-unfolding global financial crisis reminds us, you can get away with racking up debt only for so long. And with the environment the day of reckoning has already started to dawn. Lift your head from the economic statistics and you see rising average temperatures, the clearing of native forests, the destruction of habitat, the decline in fish stocks, the damage we've done to the Murray-Darling and other river systems and the degrading of our soil.

So far we've managed to keep the economy separate from the environment, but we won't get away with that much longer. Why not? Because, in the words of a former US senator, ''the economy is a wholly owned subsidiary of the environment''.

The economy exists within the natural environment and is dependent on it. Logically, you could have the natural world without an economy - that is, without human activity - but you couldn't have an economy without a natural world.

We can go for a period running our economy at the expense of the environment - plundering its natural resources on one hand, pumping out our waste on the other - but eventually we start to get feedback. The despoiled and depleted ecosystem begins to malfunction, with serious consequences for the continued functioning of our economy.

We get a lot more extreme (and thus expensive) weather events, a rising sea level forces us to move back from the coast, we start running out of native forests and some mineral resources and fossil fuels (making energy and fertiliser a lot dearer), we see the destruction of international tourist attractions such as the Great Barrier Reef,

we have to move agriculture north to where the rain is, but the elimination of fish stocks and degradation of soil makes food production a lot harder and more expensive the world over.

How did we get into the mindset that allowed us to take the environment for granted? Well, mainly it's because economic activity is simply more visible than the environment. And because, until relatively recently, we could plunder the natural world with impunity.

But also because we're wedded to a way of thinking about (and measuring) the economy that, because it has changed little in the past 150 years, simply ignores the environment. Because at the time global economic activity was so small relative to the huge natural world, it made sense for the early economists to treat the environment as a ''free good'' - something so plentiful it comes without cost.

But with the human population having more than trebled since 1927 and the global standard of living also having risen considerably, it's no longer sensible to treat the environment as an ''externality''.

We need a new economic model - and a new way of measuring progress - that recognises the centrality of the environment to our wellbeing and keeps recording and reminding us when we charge things up on our environmental credit card, as Rudd has just done.