Twenty years ago, Nature proclaimed the first Earth summit in Rio de Janeiro to be a success (see Nature 357, 523–524; 1992). The article offered a sober defence of a political process that had suffered intense criticism, but it also provided an impassioned assessment that pulled no punches. It challenged Rio's detractors as naive optimists operating in a bubble chamber of “utopian rhetoric”. And it hit out at many of the political elite, including the summit chairman Maurice Strong, for feeding the delusion with dangerously grandiose proclamations, seemingly suggesting that one good summit on sustainable development could solve all of the world's problems. But Nature took heart in humanity's formal acknowledgement of the monumental challenges ahead, and applauded the creation of an evidently incremental process to address those issues.

As the world heads into the second United Nations Conference on Sustainable Development in Rio later this month, the collective failure to fulfil those initial pledges is all too evident. Countries have increased the rhetoric and their political commitments, but there is little to show for 20 years of work, apart from an impressive bureaucratic machine that has been set to indefinite idle. On urgent environmental issues, the world has perfected the art of incremental negotiation and redefined circular motion. Meanwhile, as documented elsewhere in this issue, pressure on the planet continues to build, greenhouse-gas emissions are still rising and species are still disappearing (see page 19).

In short, development continues apace, as it must and should in order to lift the world's poorest out of poverty, but it is hardly sustainable. The goal of stabilizing greenhouse-gas emissions seems just as daunting today as it did two decades ago, and people continue to devour the world's remaining wild habitat at an alarming pace.

Time to reassess

The world has perfected the art of incremental negotiation and redefined circular motion.

So what is the purpose of the Rio+20 meeting? It cannot be a celebration. Nor should it be a platform for major new treaties and commitments — the world is awash with both, and to no avail. Instead, the second Earth summit is a chance to take honest stock of the situation and present ways to break political deadlock and hasten progress on the ground, in the air and in the oceans.

Diplomats, politicians, scientists and environmentalists alike must acknowledge where environmental politics has failed — although it is just as important to recognize where there has been progress. Greenhouse-gas emissions may be climbing at a breakneck speed, but governments around the globe have at last begun to take global warming seriously and to prepare their citizens for a changing climate. And although the global picture for biodiversity loss is gloomy, Brazil itself shows what can be achieved, despite an ongoing row about forest policy (see page 13). Deforestation in the Brazilian Amazon is down by a whopping 78% from its recent high in 2004. If Brazil can maintain that progress — and Norway has put a US$1-billion reward on the table as encouragement — it would be the biggest environmental success story in decades, and would set an example to other countries that want to protect their tropical forests.

Global effort remains important to address global problems but, as deforestation in Brazil shows, much of the current progress on the environment occurs at the national and, often, sub-national level. Fed up with the slow pace of international negotiations, state and national governments are moving forwards on their own, experimenting with ideas and policies that may one day spread around the world. This activity is helping to bridge the increasingly arbitrary gap between industrialized and rapidly emerging economies. If countries make any progress on their pledge to sign a new global-warming treaty by 2015, it will be thanks in no small part to the fact that many involved are developing their own independent climate policies.

The international process remains important because it has focused resources and intellectual energy on global warming, biodiversity and sustainable development, and scientists can rightly claim a place at the heart of this political process. Climate diplomats have tied all of their major decisions over the past two decades to reports produced by the Intergovernmental Panel on Climate Change. Governments have invested in Earth and environmental sciences. And scientists have improved their understanding of climate and ecological processes while providing policy-makers with new tools and indicators to assess the threats ahead. Many scientists are frustrated by the lack of political progress, and rightly so, but there is little else to do but keep calm and carry on.

Nevertheless, it is hard to avoid a certain sense of gloom, if not doom. Despite progress on some issues — ozone loss, for example — the disconnect between science and politics seems to be growing, not shrinking. The accumulating evidence screams that the consequences of inaction could be dire. As each day passes, the problems become more expensive to solve and the number of available options decreases. New clean-energy technologies could make all the difference to climate, but many governments in the industrialized world are investing less money in clean energy now than they were just a few years ago.

In 1992, Nature warned against thinking that a single summit could eradicate poverty and redistribute wealth while setting specific limits on greenhouse gases. The expectations for Rio+20 are so low that almost any agreement or affirmation would qualify as a success. The fact is that politicians know what needs to be done, and countries committed to doing it 20 years ago; what is missing is political leadership and solutions that are cheap, scalable and politically viable. For the second time, the world has a chance to craft a workable agenda, but the elusive key to success lies in finding a way to overturn the widespread reluctance to make the necessary investments in time, money and intellect to get the job done.