The rapid Arctic warming is causing sharp declines in sea ice extent (SIE) and volume, and this will tend to continue (Kwok, 2018;Liu et al., 2013;Stroeve & Notz, 2018), indeed, the first and second lowest SIE were detected by space-borne measurements in 2012 and 2020 (NSIDC, 2020;Witze, 2020). In particular, it is projected that there will be likely at least one ice-free summer before 2050 in the very recent IPCC AR6 report (IPCC, 2021). The retreat of sea ice makes Arctic shipping possible. Major savings in navigational distances can be achieved compared with traditional routes. Specifically, travel distance can be reduced by approximately 40% via the Northern Sea Route (NSR) compared to the Suez Canal (Rotterdam to Yokohama) (Liu & Kronbak, 2010;Schøyen & Bråthen, 2011); as a result, sailing time (ST), fuel consumption and shipping emissions can be significantly reduced (Browse et al., 2013;Melia et al., 2016;Schøyen & Bråthen, 2011). The high economic benefit of trans-Arctic shipping routes has spurred great interest from policymakers and communities (Brigham, 2011;Stephenson et al., 2011). In practice, the number of ships navigating in the Arctic has increased since 2010, as has the navigational distance (Eguíluz et al., 2016;Gunnarsson, 2021;PAME, 2020). Here we use the Chinese research ships and commercial ships as an example to give an instanced picture of the operational voyages in the Arctic. All the sailing paths of research ships and merchant ships (2013)(2014)(2015)(2016)(2017)(2018)(2019)(2020) are shown in Figure S1 in Supporting Information S1. In total, 42 merchant voyages operated via the NSR from 2013 to 2020. The research ship, Xuelong (red solid line), has successfully sailed through the Transpolar Sea Route (TSR) with a real-time sea ice forecast service from the Arctic Ice Ocean Prediction System in the summer of 2017 (Mu et al., 2019). The

Arctic navigation has become operational in recent decades with the decline in summer sea ice. To assess the navigability of trans-Arctic passages, combined model and satellite sea ice thickness (CMST) data covering both freezing seasons and melting seasons are integrated with the Arctic Transportation Accessibility Model (ATAM). The trans-Arctic navigation window and transit time are thereby obtained daily from modeled sea ice fields constrained by satellite observations. Our results indicate that the poorest navigability conditions for the maritime Arctic occurred in 2013 and 2014, particularly in the Northwest Passage (NWP) with sea ice blockage. The NWP has generally exhibited less favorable navigation conditions and shorter navigable windows than the Northern Sea Route (NSR). For instance, in 2013, Open Water (OW) vessels that can only safely resist ice with a thickness under 15 cm had navigation windows of 47 days along the NSR (45% shorter than the 2011–2016 mean) and only 13 days along the NWP (80% shorter than the 2011–2016 mean). The longest navigation windows were in 2011 and 2015, with lengths of 103 and 107 days, respectively. The minimum transit time occurred in 2012, when more northward routes were accessible, especially in the Laptev Sea and East Siberian Sea with the sea ice edge retreated. The longest navigation windows for Polar Class 6 (PC6) vessels with a resistance to ice thickness up to 120 cm reached more than 200 days. PC6 vessels cost less transit time and exhibit less fluctuation in their navigation windows compared with OW vessels because of their ice-breaking capability. Finally, we found that restricted navigation along the NSR in 2013 and 2014 was related to the shorter periods of navigable days in the East Siberian Sea and Vilkitskogo Strait, with local blockages of thick ice having a disproportionate impact on the total transit. Shorter than usual navigable windows in the Canadian Arctic Archipelago and Beaufort Sea shortened the windows for entire routes of the NWP in 2013 and 2014.