Evolution of the trend in observed Antarctic sea ice extent

It was widely reported in the early to mid-2000s that Antarctic sea ice was expanding at a small and statistically insignificant rate of approximately 2-5 × 10³ km² per year. More recently, the rate of expansion has been reported to be considerably larger and statistically significant at approximately 10-15 × 10³ km² per year. Here, the influence of the end date on the ice extent trend is investigated. We show that the change in the reported trend during the past decade did not occur due to an acceleration in the ice advance; rather, it occurred due to the previously undocumented effect of a change in the "Bootstrap" retrieval algorithm that is widely used to estimate sea ice cover from passive satellite microwave measurements. Using the current version of the algorithm, the expansion of the Antarctic sea ice cover has been statistically significant above the 99% regression confidence level since 1999. Comparison with results from the "NASA Team" satellite retrieval algorithm, however, demonstrates that the regression confidence interval substantially underestimates the uncertainty in the trend in both hemispheres.