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Arctic-wide observations of sea ice thickness are essential for estimating trends in sea ice volume, and for assessing the fidelity of the numerical models that form the basis of future climate projections 1-3 . Unfortunately, past observations of arctic sea ice thickness have been spatially incomplete and temporally sporadic [4][5][6][7] . Despite the ability of global climate models to relate dynamic and thermodynamic processes of the Arctic region 8 , many still underestimate the rate at which sea ice extent has declined 9, 10 , which reduces confidence in their capacity to simulate past and future trends in sea ice volume. The Pan-Arctic Ice-Ocean Modelling and Assimilation System (PIOMAS) -a coupled ocean and sea ice model that, unlike other numerical models, assimilates sea ice data by including measurements of near-real-time sea ice concentration and drift 11 provides an alternative approach to estimating regional trends in volume. However, although PIOMAS has shown good agreement with 2 contemporaneous observations of sea ice volume in the high-Arctic region derived from satellite observations 12 , the system has not yet been evaluated across its whole domain. Here, we quantify five years of inter-annual variations in the volume of sea ice across the entire northern hemisphere using observations acquired by the European Space Agency s ESA CryoSat-2 mission 13 . Measuring sea ice volume using CryoSat-2We use 88 million individual CryoSat-2 altimeter measurements to estimate changes in northern hemisphere Figure 2) changes in sea ice volume during the sea ice growth period (October to April) of each year. To estimate uncertainties in sea ice thickness and volume, we account for uncertainties in the sea ice density, snow loading, sea ice area, sea ice concentration, and for spatial variations in the measurement of sea ice freeboard -by far the smallest error source we consider (see Methods and Supplementary Table 1).To assess the accuracy of the CryoSat-2 observations, we compared them to 772,090, 430, and 80 million independent estimates of thickness and draft derived from springtime airborne laser and electromagnetic sensor campaigns 20, 21 and year-round upward looking sonar observations, respectively (see Supplementary Information). CryoSat-2 estimates of ice thickness agree with these independent measurements to within 2 mm, on average a difference that is much smaller than the certainty of either dataset (10 to 40 cm). Recent trends in Arctic sea ice volumeBetween 2010 and 2014, there have been marked variations in the quantity of sea ice in key sectors of the Arctic (Figure 1). During this period, the average northern hemisphere springtime (March/April) sea ice thickness was 3 2.09 ± 0.28 m. In autumn (October/November), after the summer melting season, the average thickness reduced to 1.41 ± 0.19 m. While the thickest sea ice is in most years concentrated around the coast north of Greenland andEllesmere Island, it often extends into the central Arctic -a region that has been, until now, beyond the ...

Abstract. Timely observations of sea ice thickness help us to understand the Arctic climate, and have the potential to support seasonal forecasts and operational activities in the polar regions. Although it is possible to calculate Arctic sea ice thickness using measurements acquired by CryoSat-2, the latency of the final release data set is typically 1 month due to the time required to determine precise satellite orbits. We use a new fast-delivery CryoSat-2 data set based on preliminary orbits to compute Arctic sea ice thickness in near real time (NRT), and analyse this data for one sea ice growth season from October 2014 to April 2015. We show that this NRT sea-ice-thickness product is of comparable accuracy to that produced using the final release CryoSat-2 data, with a mean thickness difference of 0.9 cm, demonstrating that the satellite orbit is not a critical factor in determining sea ice freeboard. In addition, the CryoSat-2 fast-delivery product also provides measurements of Arctic sea ice thickness within 3 days of acquisition by the satellite, and a measurement is delivered, on average, within 14, 7 and 6 km of each location in the Arctic every 2, 14 and 28 days respectively. The CryoSat-2 NRT sea-ice-thickness data set provides an additional constraint for short-term and seasonal predictions of changes in the Arctic ice cover and could support industries such as tourism and transport through assimilation in operational models.