The benefits of some environmentally friendly policies will not be apparent until decades after they have been enacted. That is one of the messages of a report from the United Nations Environment Programme, which, even by the standards of global environment assessments, is sobering reading.

Global Environmental Outlook 3 (GEO-3), a study of the links between environmental, social and development issues, contains a range of dire but familiar predictions about the impact of factors such as climate change and industrial development. But the report, released last week in the run-up to August's World Summit on Sustainable Development in Johannesburg, was unusually pessimistic about the prospects for reversing the damage.

The new predictions are contained in one of four possible futures outlined in the report. The authors considered scenarios in which global politics were dominated by concerns over either markets, environmental and social policies, security or sustainability. These were based on attempts to quantify the effect of the different approaches on population levels, economics, technology and governance.

Some of the scenarios produced a familiar picture. In a world dominated by a market mentality, for example, land and forest degradation becomes a critical issue, particularly in Latin America and the Caribbean.

But the sustainability scenario's predictions shocked some of the authors. “The delays between changing human behaviour and environmental recovery came as the biggest surprise to the regional experts,” says Jan Bakkes of the National Institute of Public Health and the Environment in Bilthoven, the Netherlands, one of the report's authors.

The report found that even if environmentally friendly approaches were adopted now, carbon dioxide concentrations would continue to rise until 2050. Water shortages would continue and coastal pollution would increase slightly. Bakkes blames difficulty in altering energy and transport infrastructures.

Originally used during the 1950s to simulate future conflicts, scenarios were revived in an improved form by the Intergovernmental Panel on Climate Change (IPCC) in the early 1990s. “By adding scenarios to assessments you come up with a credible story about how the world might evolve and can translate that into quantifiable information,” says Bert Metz, also at the Bilthoven institute and co-chair of the IPCC working group on strategies for tackling climate change.

More than 1,000 scientists contributed to GEO-3, which divides the world into no less than 17 different regions. By contrast, the IPCC has used just four regions in previous assessments, although the panel's new chair, energy economist Rajendra Pachauri, has pledged to improve regional detail in future studies (see Nature 417, 106; 200210.1038/417106b).

→ http://www.unep.org/GEO/geo3