Rapid retreat of Arctic sea ice extends the area of open ocean for new trans‐Arctic shipping routes. However, the projected routes may be too optimistic in terms of savings in shipping costs from shortened trans‐Arctic distances as they do not consider the increased sea fog frequency (SFF) over areas of the retreating sea ice. We show that delays due to sea fog can be 1–4 days, about 23%–27% along the Northwest Passage and 4%–11% along the Northern Sea Route than previous estimated. We design a route based on the projected sea‐ice extent and SFF. The new route can reduce the sailing time by 0.3–1 day by detouring the routes with lighter impacts of sea fog. More importantly the new route will lower the risk of catastrophic accidents compared to the shortest route and saves the additional costs due to unscheduled port calls.

Sea fog often penetrates adjacent coastal areas, a process called sea fog penetration (SFP). SFP can cause traffic accidents and other economic losses. Qingdao, an international port city with a dense population, suffers from SFP originating over the Yellow Sea in the boreal spring (March–May); the process, however, is not well-studied. Based on hourly observations from buoys and automatic weather stations distributed in Qingdao and its adjacent islands, we composite SFP events to reveal their spatiotemporal features and to investigate the mechanisms involved. Results show that these SFP events often penetrate inland areas from southeast to northwest and last 5–8 h at night. We further use reanalysis data to reveal that during the daytime before SFP, strong moisture advection at 925–975 hPa brings sufficient water vapor from the Yellow Sea to Qingdao; the water vapor then transfers downward to the surface via background descending motion and turbulent mixing. The daytime anomalous moistening, together with the following diurnal cooling at night, saturates the surface atmosphere and, hence, facilitates SFP. The strength of SFP depends on the strength of daytime anomalous moistening. Considering that moistening leads SFP by about a day, we use this relationship to predict the intensity of SFP. The accuracy of predicting SFP events could reach 50–80%, which highlights the predictability of intensity of SFP in Qingdao.

Rapid retreat of Arctic sea ice extent in response to global climate warming expends the area of open ocean for new trans-Arctic shipping routes. Ship companies will benefit from the shortened sailing distance across the Arctic, but threats of intensified sea fog induced by the retreated sea ice, especially the potential risk of accidents due to low visibility, are not well considered when designing the trans-Arctic shipping routes. Here, we show that the sailing time along the previous suggested routes will increase nearly 10–30% caused by the impacts of sea fog. We further design a new route detouring to the low-fog-frequency area, based on the projected sea ice extent and the fog frequency in 21st century. The new route is 5–20% longer than the original one, but can save as much as 10% of total sailing time, and most importantly, will lower the risk of catastrophic accidents. Our estimates are similar in both RCP4.5 and 8.5 of CMIP5 simulations.