Reference
Bornemann, A., Norris, R.D., Friedrich, O., Beckmann, B., Schouten, S., Damste, J.S.S., Vogel, J., Hofmann, P. and Wagner, T. 2008. Isotopic evidence for glaciation during the Cretaceous supergreenhouse. Science 319: 189-192.
What was done
The authors used δ18O data, which are responsive to both temperature and salinity, that were derived from both surface- and bottom-dwelling foraminifera samples taken from a 40-m-thick core of laminated organic-carbon-rich marlstone retrieved from the western equatorial Atlantic Ocean at Ocean Drilling Program Site 1259, together with a salinity-independent temperature proxy, based on an index of organic compounds unique to the membranes of certain microbes, to infer variations in both sea surface temperature and global ice sheet volume during portions of the Turonian period of 93.5 to 89.3 million years ago.
What was learned
Bornemann et al. say their data "show synchronous shifts ~91.2 million years ago for both the surface and deep ocean that are consistent with an approximately 200,000-year period of glaciation, with ice sheets of about half the size of the modern Antarctic ice cap," although they actually calculate an ice-sheet volume range "equivalent to 44 to 105% that of the modern Antarctic ice sheet." As a result, they conclude that "even the prevailing supergreenhouse climate [with exhibited sea surface temperatures 5 to 9°C warmer than at present] was not a barrier to the formation of large ice sheets, calling into question the common assumption that the poles were always ice-free during past periods of intense global warming."
What it means
The nine-member team of German, Dutch, UK and US researchers speculates that the development of the Turonian ice sheets could have been caused by a warming-induced "increase in the activity of the hydrological cycle, which must have initiated more humid conditions and enhanced precipitation in the high latitudes." Consequently, since there is no reason not to believe that a similar phenomenon would be operative in our day, there is reason to believe that Antarctic ice may be considerably more immune to warming-induced destruction than climate alarmists claim it is. And this observation suggests that sea levels may also be much more sluggish in their response to rising temperatures than what they contend.