It could be the greatest change to the planet's environment many of us will ever see. Within a few decades the vast sea ice that spreads over the North Pole could disappear for weeks or even months in the Arctic summer. The last time this happened, scientists tell us, was long before humans set foot on the earth. The Arctic sea ice is retreating as climate change advances. The change being felt in this fragile world is caused, in part, by us. And it's happening so fast, it's defying scientific models.