Figure 3: Transiting the Arctic.

Monthly average number of vessels along the two main Arctic routes: (A–C) the Northeast (NE) and D) Northwest (NW) Passages. For the Northeast Passage, the average number of ships per month shows a seasonal pattern that is highest between June-November with a maximum in September ((A): cargo and tank, (B) cargo, and (C) tank). Shipping by cargo and tanker vessels increased by 20% in 2014 compared to the 2012 maximum. For completeness, we also display the values obtained from the report of the Northern Sea Route¹⁴ (empty red circles). (D) The Northwest Passage only shows one or two cargo or tank vessels transiting each year between 2012 and 2014. The number of ships from the “other” category account for around 50% of the traffic in the longitudes corresponding to the Northeast Passage and around 80% of those corresponding to the Northwest Passage (Supplementary Figures 2 and 3).