The agreement for modest commitments to sustainable development after 2015, reached at last week's Earth conference in Rio de Janeiro, has been roundly condemned as inadequate, or even an outright failure. The document is full of legalese and vague assertions, and it postpones the making of potentially significant decisions and assigning accountability to an uncertain future. When it comes to sustainable development, the agreement neither secures new resources nor defines the parameters for success. But did anybody, really, expect more?

Rio was never intended as a venue for the signing of major new environmental treaties, so it should come as no surprise that governments did not do so. But the actual purpose of the conference was never made clear. In the run-up to the meeting, Nature said that the event presented an opportunity to take stock, acknowledge past failures and identify opportunities to break political deadlock. It may have succeeded with the first two, but has certainly fallen flat with the all-important third.

The world needs hope, a sustainable goal, and a clear path to reach it.

The world needs hope, a sustainable goal and a clear path to reach it. It needs a strong message to invigorate the global conversation and to motivate current and future generations of scientists, thinkers and leaders. Right now, politicians need to lay the groundwork and implement productive policies on the ground. They also need to find practical and politically viable ways to scale things up. With a clear aim in view and a workable means to achieve it, political leaders would find it much harder to turn their backs on the problems the world faces.

Perhaps Rio really was a failure, but not for the reasons levelled at the conference over the past week. Put simply, the agreement failed to capture the imagination. The wording does not lay out a clear and compelling vision — a brave new message that will spark debate among friends, family and colleagues. Sadly, it doesn't even capture the spirit and energy that enlivened the conference itself over the past two weeks.

For the more than 45,000 people who attended the conference, the picture was very different. Countless individuals, businesses and non-profit organizations unveiled their latest ideas to eager audiences. Governments announced initiatives to reduce emissions, protect forests, expand access to energy and generally make the world a cleaner and greener place for all. Visitors from around the world were introduced to global problems — and, in the case of Rio's favelas, smart solutions — clarifying the connection between poverty and the need for sustainable development. Millions of Brazilians, at least, were bombarded with news coverage detailing the full suite of issues under discussion. This is not enough, but it is not nothing either. By focusing too much on the final text and what it contains, critics are ignoring what was there.

Although the agreement that came out of Rio last week did not define sustainable-development goals, it did create a process to do so. These goals will need to be informed by science if they are to be meaningful. And policy-makers will need to find a way to assess whether they are making progress on the social, economic and environmental issues they face. Governments must cope with a dizzying array of interrelated challenges, from freshwater shortages and carbon emissions to issues of food, poverty, biodiversity and demographics. They have limited resources with which to tackle a bewildering portfolio of complex problems, and they need to understand where the environmental boundaries lie when they try to solve them.

At its heart, this is a scientific challenge, and one that scientists and funding agencies have already set out to tackle through the Future Earth initiative, a framework of research into sustainable development paid for by the National Science Foundation. It represents an opportunity to analyse and assess competing uses for various resources across the global landscape. To help translate this useful work into relevant information, scientists must find a way to integrate their knowledge of natural systems with economics and other social sciences, to better assess the solutions that people will be willing to accept and to encourage new technologies and ideas to ripple through society. Speaking in Rio, Hans Joachim Schellnhuber, a physicist by training and director of the Potsdam Institute for Climate Impact Research in Germany, said that the next decade could be “the decade of the social sciences”. He may well be right. Just as fear does not sell, it should also be clear by now that simply presenting the bare facts on their own, no matter how starkly, will not be enough.