Abstract. Changes in sea-ice thickness are one of the most visible signs of climate change. However, to gain a comprehensive understanding of mechanisms involved, long time series are needed. Importantly, the development of more accurate predictions of sea ice in the Arctic requires good observational products. To assist this, a new sea-ice thickness product by ESA Climate Change Initiative (CCI) is here compared to the ocean reanalysis ORAS5 by ECMWF for the first time. The CCI product is based on two satellite altimetry missions, CryoSat-2 and ENVISAT, which are combined to the longest continuous satellite altimetry time series of Arctic-wide sea-ice thickness, 2002–2017 and continuing.

Time series of sea-ice volume for the CCI coverage reveal years of extremely low volume as well as recovery during the winter season. The 15-year trends in sea-ice volume are clearly negative over the time series and despite large variability between years statistically significant. The 15-year ORAS5 trends have larger interannual variability than the CCI trends and are therefore not statistically significant despite of a good match in terms of year-to-year variability. The observed negative trends result from changes in both atmospheric and oceanic forcing.

The CCI product performs well in the validation of the ORAS5 reanalysis: overall root-mean-square difference (RMSD) between sea-ice thickness from CCI and ORAS5 is below 1 m. However, seasonal and interannual RMSD variations during the time series are large, from 0.5 m to 1.3 m. The differences are a sum of reanalysis biases, such as incorrect physics or forcing, as well as uncertainties in satellite altimetry, such as the snow climatology used in the thickness retrieval.