Climate models project continued Arctic sea ice reductions with nearly ice-free summer conditions by the mid-21st century. However, how such reductions will realistically enable marine access is not well understood, especially considering a range of climatic scenarios and ship types. We present 21st century projections of technical shipping accessibility for circumpolar and national scales, the international high seas, and three potential navigation routes. Projections of marine access are based on monthly and daily CCSM4 sea ice concentration and thickness simulations for 2011-2030, 2046-2065, and 2080-2099 under 4.5, 6.0, and 8.5 W/m 2 radiative forcing scenarios. Results suggest substantial areas of the Arctic will become newly accessible to Polar Class 3, Polar Class 6, and open-water vessels, rising from~54 %, 36 %, and 23 %, respectively of the circumpolar International Maritime Organization Guidelines Boundary area in the late 20th century to~95 %, 78 %, and 49 %, respectively by the late 21st century. Of the five Arctic Ocean coastal states, Russia experiences the greatest percentage access increases to its exclusive economic zone, followed by Greenland/Denmark, Norway, Canada and the U.S. Along the Northern Sea Route, July-October navigation season length averages~120, 113, and 103 days for PC3, PC6, and OW vessels, respectively by late-century, with shorter seasons but substantial increases along the Northwest Passage and Trans-Polar Route. While Arctic navigation depends on other factors besides sea ice including economics, infrastructure, bathymetry, and weather, these projections are useful for strategic planning by governments, regulatory agencies, and the global maritime industry to assess spatial and temporal ranges of potential Arctic marine operations in the coming decades.

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ABSTRACT. The last major calving event along the Ross Ice Shelf (RIS, Antarctica) front occurred a decade ago, following a substantia l increase in the rate of ice-front advance in the few years preceding the event. This "B-9" event, on the eastern part of the front between Edward VII Peninsula and Roosevelt Island , removed ~ 5100 km 2 of ice, about 100 years of advance in that sector, but reduced the ice-shelf area by only 1%. Since 1987 the entire ice front has continued to advance, more than regaining the area lost during the B-9 event. The western front is now well north of any position recorded during the last 150 years, and it has not exp eri enced major calving for at least 90 years. Ice-front heights generally decrease from east to west, but local variability is high. Elevations are relatively low from 17l ° to 177° W, the location of "warm" Modified Circumpolar Deep Water circulation beneath the outer ice shelf. Modern heights considerably exceed historical heights between 179 0 Wand 178 0 E and are lower west of 174 0 E, probably due to r ecent dynamic changes such as rifting a nd the western advance. The general advance of the RIS front and the period of several decades to more than a ce ntury that elapses between major calving events is consistent with a relatively stabl e ice front. This contrasts with several sm aller ice shelves along the Antarctic Penin sul a and McMurdo Ice Shelf in the Ross Sea which have retreated substantially during the past few decades.

Abstract. There is an emerging need for regional applications of sea ice projections to provide more accuracy and greater detail to scientists, national, state and local planners, and other stakeholders. The present study offers a prototype for a comprehensive, interdisciplinary study to bridge observational data, climate model simulations, and user needs. The study's first component is an observationally based evaluation of Arctic sea ice trends during 1980-2008, with an emphasis on seasonal and regional differences relative to the overall pan-Arctic trend. Regional sea ice loss has varied, with a significantly larger decline of winter maximum (January-March) extent in the Atlantic region than in other sectors. A lead-lag regression analysis of Atlantic sea ice extent and ocean temperatures indicates that reduced sea ice extent is associated with increased Atlantic Ocean temperatures. Correlations between the two variables are greater when ocean temperatures lag rather than lead sea ice. The performance of 13 global climate models is evaluated using three metrics to compare sea ice simulations with the observed record. We rank models over the pan-Arctic domain and regional quadrants and synthesize model performance across several different studies. The best performing models project reduced ice cover across key access routes in the Arctic through 2100, with a lengthening of seasons for marine operations by 1-3 months. This assessment suggests that the Northwest and Northeast Passages hold potential for enhanced marine access to the Arctic in the future, including shipping and resource development opportunities.

Global economics, not declining sea ice, is driving ships to the Arctic Ocean. Only international regulation will protect the region, says Lawson Brigham.