He said the "world's eyes" were on the panel, adding: "Since 1990 the IPCC has provided regular, unbiased assessment of the mounting impact of a warming planet.
"You are the world's authorities on climate change, recognised with a Nobel Peace Prize.
"We need to build resilience and seize the opportunities of a low-carbon future. The heat is on. Now we must act."
Reaction to the report has been almost unanimous in accepting its findings and calling for a reduction in carbon emissions.
Achim Steiner, executive director of the United Nations Environment Programme, described the report as a "dramatic reminder of both the significance, the pace, and also our ability to increasingly understand what is happening to our planet".
He said that while scientists may not know everything, they know enough about "the risks of not acting".
Greenpeace chief scientist Doug Parr said: "This report is about what kind of future we want. We either compound climate change by burning fossil fuels, then suffer the consequences, or we take a different path.
"Dangerous levels of warming of two degrees or more can be avoided, but this requires countries like the UK accepting the folly of grasping for yet more oil and gas in the Arctic, the tar sands and the Home Counties, and instead taking rapid action on renewables."
Alison Doig, Christian Aid's senior climate change adviser, said: "Scientific evidence confirms man-made climate change is a reality and that, without urgent action globally, the impacts will get worse."
She added: "The time for debate is over, it is time to act."