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d ~ 159'513'600 km: day 62

Kate Humble and the 23 Degrees team are in Kulusuk on the east coast of Greenland. Their mission is to travel out to the very edge of the Arctic sea ice. If we're lucky their arrival will coincide with a critical moment in the annual cycle of the sea ice.

They will be taken onto the ice by Inuit hunters from the village. The beginning of March is an important time for the seal hunters. The long winter nights are over, the days are getting longer, and the increasing sunlight attracts seals onto the sea ice to moult their winter coat. The sea ice allows the hunters to travel quickly over great distances to find the seals.

What's remarkable about the sea ice is how late in the year it melts. Records of the extent of the sea ice over the last 50 years show that on average it reaches its maximum in early March, more than two months after the winter solstice when the northern hemisphere received the least amount of sunlight. So the team are hoping to be on the ice at the tipping point - the moment when the sea ice is neither expanding nor contracting. They're going to discover why there's such a big lag between increasing solar radiation and the melting of the sea ice

The reason for this lag is the relationship between air temperature and the thermal properties of water. The ocean has huge thermal inertia - it takes a long time to cool down and a long time to heat up. Air on the other hand responds quickly to solar energy. So at this time of year the air is starting to warm up as solar radiation increases. But critically, it's not yet warmed enough to get above freezing. The sea on the other hand is still cooling down; the air temperature is still below freezing, so the ocean continues to lose heat. This process continues into March until the air temperature rises above freezing. Now the sea can start to warm up, the sea ice advance is halted, and gradually the process goes into reverse.

Early surveys of the ice for 2011 suggest that the extent of sea ice this year has been the smallest in living memory. There has been a particularly warm winter in Greenland so the seas haven't cooled down as much as normal. Scientists believe it is part of a trend linked to Global Warming. There have been significant annual variations, but the trend seems to be that the extent of the sea ice has been getting smaller for around ten years. It's also been getting thinner, which is not good news for the hunters of Kulusuk, because without the sea ice, hunting is a lot more difficult.

In a few weeks, the planet's tilt will bring even more solar energy to the Arctic, and the sea ice will finally begin to melt and recede. The ebb and flow of the sea ice here in Greenland reveals the extraordinary complexity of our planet's response to the energy it gets from the sun. It's a fascinating relationship, and one that determines the fate of all life on earth.

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