A group of British explorers just back from a 60-day trip to the North Pole said Monday they had encountered unusual conditions, including ice sheets that drifted far faster than they had expected.

The three-member team walked across the frozen Arctic Ocean to study the impact of increased carbon dioxide absorption by the sea, which could make the water more acidic and put crucial food chains under pressure.

Expedition leader Ann Daniels said the ice drifted so much that they eventually covered 500 nautical miles (576 miles) rather than the 268 nautical miles initially envisaged.