The trouble with the Rio+20 UN conference, which began yesterday and ends tomorrow with more than 100 world leaders in attendance in the Brazilian city, is that it is a summit in search of a purpose.

Nobody really wanted it. There was no demand for it from the world community. It was merely an opportunity, 20 years on from the original Earth Summit in Rio de Janeiro in June 1992, to mark the anniversary with a similar-sized international get-together which might, just might, lead to some positive steps to help the global environment.

But as the anguished cries of virtually all commentators on the draft final text are making clear, it is unlikely to achieve much of substance – if, indeed, it achieves anything. Even the best potential outcome, an agreement to draw up sustainable development goals along the lines of the celebrated Millennium Development Goals, which the British Government has been resolutely supporting for a year, looks shrouded in doubt.

For in trying to replicate the success of the original Earth Summit, today's delegates have lost sight of what it was that made that summit successful. It was not its colossal, 750-page agenda for saving the planet ("Agenda 21"), which all nations signed up to and promptly forgot about, that made the summit historic. It was the use that was made of the occasion. Its crowning achievement was that the deadly serious UN treaty on climate change (and also the treaty on wildlife) was negotiated and signed in record time.

UN treaties can take an age to negotiate – the Law of the Sea took a decade, and the Americans still didn't sign. But the climate treaty was done and dusted in a mere 18 months, because it had to be signed in Rio.

No sign of any such treaties in Rio this time. No sign of much of anything. Anniversary diplomacy promises more than it delivers.