Each morning over their coffee some of the most powerful people in the world turn to the financial pages of their newspapers to check on the health of their investments by looking at how the Dow Jones and the FTSE 100 are performing. But there is another graph, also updated every day that is far more significant for charting the long-term wellbeing of all of humankind.

Available to view at the website of the US-based National Snow and Ice Data Center is a graph that uses satellites and computer models to chart the shrinking of the floating cap of Arctic sea ice on the top of the world. Nowhere else is as sensitive to the rise in global temperatures as the polar regions, and this simple graph shows quite how rapidly the total area of summer sea ice in the Arctic is diminishing. Between September 2006 and 2007 a staggering 1.5 million square kilometres was lost. Anyone who logs on to look at the graph in coming days will learn if the 2011 melt outstrips even 2007.

Forty years ago today (September 15) Greenpeace was born when a small band of environmentalists sailed their small fishing boat towards a US nuclear test site, seeking to stop the most tangible threat to their humanity. Now we're sailing to the frontline for my generation.

As I lie on my bunk typing this aboard the renovated sealing vessel that is Greenpeace's icebreaker Arctic Sunrise at 81 degrees north, I am just a few hundred miles from the North Pole. Here I'm witnessing at first hand what is surely the most visible indicator anywhere on Earth of the impact of our continued burning of fossil fuels. Standing on deck, surrounded by nothing but ice and water as far as the eye can see, it seems almost impossible to imagine that by the time my son is collecting his A-level results and perhaps thinking about going to University, the ice will be gone in the summer months. But that is entirely possible. If the Earth is photographed from space around that time, where now there is white, there will be blue. The ice will have been replaced completely by open water.

Last week Cambridge University's Professor Peter Wadhams, the world's foremost expert on the Arctic sea ice, emailed me via satellite with his latest assessment. "This month will certainly feature the lowest sea ice volume ever seen in the Arctic. The trends in sea ice extent and thickness continue inexorably downwards, as the Arctic responds to increasing greenhouse gases in the atmosphere."

Wadhams has sent two of his team with us for a month to investigate the thickness and volume of the sea ice at ten different sites. In between visits from polar bears, these scientists are using drills, core-ing, aerial imagery, snow depth measurements and GPS readings to calibrate and validate other data from models and satellites. For the first time they are also pioneering the use of 3D laser scanners to get even more accurate measurements. This research will help bring understanding of the rate at which warming is affecting this region.

But the impact of the melt outside my porthole will also be felt far beyond the Arctic Circle. The sea ice is vital, not just for the polar bears, but for all of us. We all rely on it to act as the world's air conditioning system and keep the global climate cooler than it otherwise would be. Eighty per cent of the sunlight that hits the ice here is reflected straight back out to space, so without it global temperatures will rise and rise as more solar radiation is absorbed by the Earth.

The great Arctic melt should be a wake up call, but in a bitter twist of irony it is instead being seen as a last chance for the global oil giants to grab the last drops of the fuels that caused the ice to shrink in the first place. Working on the basis that a barrel of crude oil produces around 300kg of carbon dioxide emissions after refining and combustion, the Arctic's recoverable reserves could yet become responsible for another 27 billion tonnes of CO2 emissions. That's about the same as the world's annual carbon footprint.

The night before I began this journey, I read my son 'Arctic Song.' Like the stories of steam ships on the Nile, and anacondas in the Amazon, this fragile wilderness occupied a special place in the books of my own childhood. Right now, as the ice floes crisply break around our hull, and we are likely the furthest north of any ship anywhere in the world, with our only company the snowy-white ivory gulls circling our ship, I can tell you the Arctic of all our imaginations exists. But for how much longer, that will depend.

Joss Garman works for the climate campaign of Greenpeace UK and is a founder of Plane Stupid