A Spurious Jump in the Satellite Record: Has Antarctic Sea Ice Expansion Been Overestimated?

Monday, 15 December 2014: 8:45 AM
Ian Eisenman, University of California San Diego, Scripps Institution of Oceanography, La Jolla, CA, United States, Walter Meier, NASA Goddard Space Flight Center, Greenbelt, MD, United States and Joel R Norris, University of California San Diego, La Jolla, CA, United States
According to the IPCC AR5, the Antarctic sea ice cover expanded during 1979-2012 at a statistically significant rate with a magnitude 1/3 as large as the rapid sea ice retreat in the Arctic. This was a substantial change from the IPCC AR4, which reported the 1979-2005 trend to be considerably smaller and statistically indistinguishable from zero. Both estimates were based on satellite retrievals that use NASA's "Bootstrap algorithm" to estimate the sea ice concentration from passive microwave radiometer measurements. The increase in trend between the two IPCC reports has generally been attributed to an acceleration in the ice advance. Here, we show instead that most of the increase occurred due to a change in the way the satellite observations were processed. The Bootstrap algorithm underwent an update in 2007 to make the dataset more consistent with a 2002-2006 record from another satellite, and the entire dataset was reprocessed with the updated algorithm. We find that this reprocessing of the Bootstrap dataset caused a previously undocumented change in the inter-calibration across a 1991 sensor transition, which caused a four-fold increase in the 1979-2006 trend. The question remains whether this undocumented change corrected an error or introduced one. Our analysis is not able to definitively answer this, but the results show that either the older dataset or the newer dataset contains a substantial and previously undocumented inter-calibration error. This suggests that numerous studies that have relied on these observations should be reexamined to determine the sensitivity to this change in inter-calibration. Implications for the uncertainty in satellite sea ice observations will also be discussed. Furthermore, a number of recent studies have investigated physical mechanisms for the observed expansion of the Antarctic sea ice cover. The results of this analysis raise the possibility that much of this apparent expansion may be a spurious artifact of an error in the processing of the satellite observations.