British chemist Robert Watson had strict instructions when he sat down to write the opening chapter of the first U.N. scientific overview of climate change in 1990: "Keep it short."

On Monday, the sixth set of reports by the same U.N. panel hit a record length of more than 10,000 pages, with the release of a third section of almost 3,000 pages on solutions to climate heating, showing it is "now or never" to head off the worst.

By contrast, in 1990, the initial report series by the Intergovernmental Panel on Climate Change (IPCC) totaled a relatively modest 1,000 pages.