This MSA record (1841-1995) is from a Law Dome ice core called "DSS" in East Antarctica. It was calibrated against satellite sea ice records and used to reconstruct sea ice extent prior to the satellite era. The following is taken from the abstract of the paper (Curran et al., 2003).
The instrumental record of Antarctic sea ice in recent decades does not reveal
a clear signature of warming ... despite observational evidence from coastal Antarctica. This work shows a significant correlation (P less than 0.002) between methanesulphonic acid (MSA) concentrations from a Law Dome ice core and 22 years of satellite-derived sea ice extent (SIE) for the 80 degrees E to 140 degrees E sector. Applying this instrumental calibration to longer term MSA data (1841 to 1995 A.D.) suggests that there has been a 20% decline in SIE since about 1950. The decline is not uniform, showing large cyclical variations, with periods of about 11 years, that confuse trend detection over the relatively short satellite era.
This work was completed as part of ASAC project 757 (ASAC_757).