There was no law against genocide in the early 1940s; it became an internationally recognised crime only after the worst genocide of modern history had actually happened. Similarly, there is no law against "ecocide" now. That will only come when the damage to the environment has become so extreme that large numbers of people are dying from it even in rich countries.
They are already dying from the effects of environmental destruction in poor countries, but that makes no difference because they are powerless. By the time it starts to hurt large numbers of people in powerful countries, 20 or 30 years from now, most of the politicians who conspired to smother any substantial progress at the Rio+20 Earth Summit will be safely beyond the reach of any law. But eventually there will be a law.
Rio+20 was advertised as a "once-in-a-generation" opportunity to build on the achievements of the original Earth Summit, held in the same city 20 years ago.
That extraordinary event produced a legally binding treaty on biodiversity, an agreement on combating climate change that led to the Kyoto accord, the first initiative for protecting the world's remaining forests, and much more besides.
This time, few leaders of the main powers even bothered to attend. They would have come only to sign a summit statement, "The Future We Want", that had already been nibbled to death by special interests, national and corporate.
"[The] final document... contributes almost nothing to our struggle to survive as a species," said Nicaraguan representative Miguel d'Escoto Brockmann. "We now face a future of increasing natural disasters." A plan to stop the destruction of the world's oceans was blocked by the US, Canada and Russia. The final text simply says that countries should do more to prevent over-fishing and ocean acidification, without specifying what.
A call to end subsidies for fossil fuels was removed from the final text, as was language emphasising the reproductive rights of women. And of course there were no new commitments to fight climate change.
The 49-page final declaration of Rio+20 contained the verb "reaffirm" 59 times. In effect, some 50,000 people from 192 countries travelled to Rio de Janeiro to "reaffirm" what was agreed there 20 years ago.
Rarely has such a large elephant laboured so long to give birth to such a small mouse. The declared goal of the conference, which was to reconcile economic development and environmental protection by giving priority to the goal of a "green" (that is sustainable) economy, simply vanished in a cloud of vague generalities.
The final text does say that "fundamental changes in the way societies consume and produce are indispensable for achieving global sustainable development", but it does not say what those fundamental changes should be.
"This is an outcome that makes nobody happy. My job was to make everyone equally unhappy," said Sha Zukang, secretary-general of the conference, but that is not strictly true. Governments seeking to avoid commitments are happier than activists who wanted some positive results from the conference, and the hundreds of large corporations that were represented at Rio are happiest of all. How did it end up like this? Global greenhouse gas emissions have grown by 48 per cent in the past 20 years, we have lost another 3 million square kilometres of forest, and the world's population has grown by 1.6 billion - yet there is less sense of urgency than there was in 1992.
You can't just blame the economy: Rio+20 would probably have ended just as badly if there had been no financial crash in 2008.
Twenty years ago the issues of climate change, biodiversity, preservation of oceans and forests, and sustainable development were relatively fresh challenges. Moreover, the world had just emerged from a long Cold War, and there was plenty of energy and hope around. Now everybody understands how tough the challenges are, and how far apart are the interests of the rich and the poor countries.
We now have a 20-year history of defeats on this agenda, and there is a lot of defeatism around. Politicians are always reluctant to be linked to lost causes, and the struggles against poverty and environmental destruction now seem to fall into that category. Thus we sleepwalk towards terrible disasters - but that doesn't absolve our leaders of responsibility. We didn't hire them to follow; we hired them to lead.
At the recent World Congress on Justice, Law and Governance for Environmental Sustainability, a group of "radical" lawyers proposed that "ecocide" should be made a crime. They were only radical in the sense that a group of lawyers agitating for a law against genocide would have been seen as radical in 1935.
One day, after many great tragedies have occurred, there will be a law against ecocide. But almost all the real culprits will be gone by then.
Gwynne Dyer is a London-based international journalist.