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.
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