''However, these thresholds are in many cases not yet fully understood, nor are the environmental, social and economic consequences of crossing them,'' the report admits. In which case, they're not allowed for in the projections. Over the past four decades, human endeavour has unleashed unprecedented economic growth in the pursuit of higher living standards. While the world's population has increased by more than 3 billion people since 1970, the size of the world economy has more than tripled. Although this growth has pulled millions out of poverty, it has been unevenly distributed and has incurred significant cost to the environment. Natural assets continue to be depleted, with the services those assets deliver already compromised by environmental pollution. The United Nations is projecting further population growth of 2 billion by 2050. Cities are likely to absorb this growth. By 2050, nearly 70 per cent of the world population is projected to be living in urban areas. ''This will magnify challenges such as air pollution, transport congestion, and the management of waste and water in slums, with serious consequences for human health,'' it says.

The report asks whether the planet's resource base could support ever-increasing demands for energy, food, water and other natural resources, and at the same time absorb our waste streams. Or will the growth process undermine itself? With all the understatement of a government report we're told that providing for all these extra people and improving the living standards of all will ''challenge our ability to manage and restore those natural assets on which all life depends''. ''Failure to do so will have serious consequences, especially for the poor, and ultimately undermine the growth and human development of future generations.'' Oh. That all? Without policy action, the world economy in 2050 is projected to be four times bigger than it is today, using about 80 per cent more energy. At the global level the energy mix would be little different from what it is today, with fossil fuels accounting for about 85 per cent, renewables 10 per cent and nuclear 5 per cent. The emerging economies of Brazil, Russia, India, Indonesia, China and South Africa (the BRIICS) would become major users of fossil fuels. To feed a growing population with changing dietary preferences, agricultural land is projected to expand, leading to a substantial increase in competition for land.

Global emissions of greenhouse gases are projected to increase by half, with most of that coming from energy use. The atmospheric concentration of greenhouse gases could reach almost 685 parts per million, with the global average temperature increasing by 3 to 6 degrees by the end of the century. ''A temperature increase of more than 2 degrees would alter precipitation patterns, increase glacier and permafrost melt, drive sea-level rise, worsen the intensity and frequency of extreme weather events such as heat waves, floods and hurricanes, and become the greatest driver of biodiversity loss,'' the report says. Loss of biodiversity would continue, especially in Asia, Europe and southern Africa. Native forests would shrink in area by 13 per cent. Commercial forestry would reduce diversity, as would the growing of crops for fuel. More than 40 per cent of the world's population would be living in water-stressed areas. Environmental flows would be contested, putting ecosystems at risk, and groundwater depletion may become the greatest threat to agriculture and urban water supplies. About 1.4 billion people are projected to still be without basic sanitation. Urban air pollution would become the top environmental cause of premature death. With growing transport and industrial air emissions, the number of premature deaths linked to airborne particulate matter would more than double to 3.6 million a year, mainly in China and India.