Ice
shelves that have already disintegrated on the eastern side of the Antarctic
Peninsula may have been predisposed to collapsing because they were thinning
for the past 300 years.
An analysis of fossils suggests the ice shelves in the region have been thinning since around 1700, leaving those such as Larsen B vulnerable to their recent break-ups as human-caused climate change took hold.
What scientists know about past losses of the Antarctic ice sheet is limited because satellite records only extend back to the early 1990s. To go further back in time, James Smith at the British Antarctic Survey and colleagues looked at fossils of single-celled algae in a mud sample taken from near the South Orkney Islands, off the tip of the peninsula.
The team used the ratio of two oxygen isotopes in the fossils to estimate past changes in the climate and glacier discharge – the melting of the ice into the ocean.
The isotopes revealed the rate of glacier discharge was stable for most of the past 6000 years, and started to speed up around the start of the 16th century but not beyond natural variations. A stronger acceleration started around 1700, followed by an even more rapid one at the start of the 20th century.
The greater losses from 1700 correlate with a strengthening of a climate phenomenon known as the Southern Annular Mode, bringing stronger westerly winds, warmer air temperatures and possibly channelling warmer water into the area, melting the ice from above and below.
“The ice in this region may have been losing mass for several hundred years. [But] the rate of change during the past 100 or so years has been far greater than any point during the last 6000 years in this record,” says Smith.
Eric Rignot at the University of California, Irvine, says the study should be treated with caution, and puts too much emphasis on the recent collapses being down to changes around 1700 rather than the dramatic warming seen in the past 50 years. “The results need to be confirmed by independent data, and the reasoning that the collapse of the ice shelves was caused by enhanced melting over hundreds of years is highly speculative,” he says.
Andrew Shepherd at the University of Leeds in the UK also sounds a note of caution and says it is hard to pinpoint events as precisely as we’d like. “Although there is a clear rise in oxygen isotopes related to iceberg production during the 20th century, it’s hard to be certain before then. It’s clear though that rapid ice melting has really only occurred during our lifetimes, and that’s no coincidence in my book.
A guest editorial