W hile the word "encourage" features about 50 times in the draft text of the global agreement on green development that will be agreed in Rio this week, the word "must" makes just three appearances.

That says it all. Billed as a "once in a generation" opportunity to put the world on a sustainable path, the Rio +20 Summit was a damp squib before it even started.

Jim Leape, head of WWF, describes the limp text proposed by hosts Brazil as "pathetic" and sounding more like a seminar paper. Gro Harlem Bruntland too is dismayed by its weakness. She was the Norwegian minister behind the first Rio Earth Summit in 1992, which generated the epoch-making agreements on biodiversity and climate change.

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In that year I wrote an article for this newspaper headlined "Green agenda in pivotal year for the environment", based on the predictions of six environmentalists, including Kevin Dunion, then the new director of Friends of the Earth Scotland. It is suffused with vigour and hope. John Elkington of SustainAbility reported: "For 20 years environmentalists have been campaigning to get green issues on to the public agenda. They have succeeded. In the next two decades, I think we'll see them working with industry on implementation and delivery. It's time for action."

What happened? Despite the Kyoto agreement, greenhouse gas emissions have risen by 40% as Western hyperconsumption ballooned and manufacturing was outsourced to (largely coal-powered) China. Levels of CO2 now exceed 400 parts per million, only 50ppm less than the level many scientists believe will trigger catastrophic irreversible climate change. Global population has grown by 22% but consumption has grown far faster, as the populations of rich countries grow ever fatter by polluting the planet and stripping its resources.

Yet in the face of this incipient environmental Armageddon what does the world do? The Vatican attempts to veto family planning. Despite a 100,000-strong Twitter-storm on the subject, Canada and Venezuela block a commitment to phase out fossil fuel subsidies, which could reduce emissions by 6% at a stroke. The developing countries, led by China, red-pen large chunks of the text, fearing that the rich world is attempting to control their growth by the back door. The US attempts to weaken the principle of shared responsibility for tackling climate change. New protection for open oceans is delayed for another three years. The resultant document fails to offer clear timetables or anything more than vague aspirations. Urgency has been met with torpor.

It's wrong to blame the UN, which cannot be more than the sum of its parts. This agreement is merely the profoundly depressing common denominator of what the world can agree on. Most of us must know deep down that we will have to return to a simpler, less profligate lifestyle. Yet our politicians cannot wean themselves from the language of perpetual growth because they fear the wrath of the voters in a political system that is overwhelmingly focused on the short-term. Meanwhile multinational corporations may like to talk the language of sustainability but their executives and shareholders rely on persuading us to buy stuff we don't need with money we don't have. So, faced with the choice between a little pain now or a lot more for our grandchildren, greed wins every time. The same applies to sharing more with the world's billion hungry people, or making modest sacrifices to protect biodiversity.

The core challenge remains the same as it was in 1992: to reduce the rich world's disastrously huge and growing environmental footprint, while supporting the developing world to boost sustainably. George Osborne is wrong: this isn't about saving the planet by putting our country out of business. It's more subtle than that. It's about committing to doing things differently. So, faced with the need to take a few big steps quickly, we can't even manage to take small, tentative ones. How will future generations judge us?