Climate of change: the world has the resources to respond to climate change, maintains Alan Porteous. But we need to get on with it.(LEAD STORY)

HUMANITY is faced with a menacing question. What happens if we hit nature's thresholds, the so-called tipping points of the global systems that sustain us? And will we be left with the resources to adapt?

The evidence for global climate change is becoming ever more visible. Nine out of ten of the world's glaciers are in retreat. Arctic sea ice is melting at a rate of about 9 per cent per decade. Ten of the warmest years since the beginning of the 20th century have occurred since 1995; there have been a cluster of ' 100-year' droughts; and hurricanes are becoming more intense.

Disturbing images of polar bears swimming desperately between remnants of sea ice illustrate the impact of climate upheaval on wildlife and ecological systems. Global warming also poses threats to food production.

Early in 2007 the Intergovernmental Panel on Climate Change (IPCC) will present its Fourth Assessment Report on the state of the world's climate. This will embody the most comprehensive collective understanding yet of the scientific, technical and socio-economic implications of climate change.

For sheer volume and detail, the report may not be an easy read. The process of writing has been laborious, involving hundreds of lead authors and thousands of contributors and reviewers. There have been vigorous debates between strongly held views, and word-by-word arguments in the editing process. Consensus has been achieved across a spectrum of languages and political environments.

In preparing the report, the IPCC has had to contend with a fair amount of scepticism and disagreement arising from its Third Assessment Report, published in 2001. This proposed that the global temperature would increase by between 1.4[degrees]C and 5.8[degrees]C by the end of the 21st century. This wide range made it difficult for policy makers, conservationists and engineers to develop precise strategies.

At the time, there was controversy over satellite data from the 1990s which appeared to show that the upper atmosphere was cooling, contradicting the rising temperatures shown by surface measurements. It has since been discovered that these data were misinterpreted. Once this was recognised and fixed, the upper atmosphere data backed up the trend at the surface.