While people quibble, Arctic seabeds are belching massive quantities of methane

James Hansen of NASA is the most eminent contemporary climate scientist. He says that 20,000 years ago the Earth was 5°C colder and the sea level 110 metres lower than today. As temperatures began to elevate 14,000 years ago, sea levels rose at a very rapid rate, about 1 metre every 20 to 25 years.

Seven thousand years ago global temperature and sea levels stabilised. This is the period when what we sometimes speak of as human civilisation developed. According to Hansen, this stability is now clearly under threat. He argues it is certain that unless dramatic action is now taken – essentially the end of all coal-burning – various "tipping points" will soon be passed that will eventually make inevitable the melting of the Greenland and West Antarctic ice sheets and the carbon-intensive Siberian and Arctic permafrost, and the radical extension of what the relevant scientists are already calling the sixth mass extinction of species in the history of the Earth.

Hansen argues that .."If humanity burns most of the fossil fuels, doubling or tripling the pre-industrial carbon dioxide level, Earth will surely head toward the ice-free condition, with sea-level 75 meters higher than today. It is difficult to say how long it will take for the melting to be complete, but once ice sheet disintegration gets well under way, it will be impossible to stop."

Given what is at stake – the future of the Earth – for non-scientists to dismiss Hansen's warning seems folly or arrogance of an astonishing kind.