The climate system is an angry beast, and we are poking it with sticks. The late Wally Broeker geophysicist at Columbia
An international team of top climate scientists has found that it only takes a decade for the Antarctic ice sheet to pass the dreaded tipping point. In the millennium past, natural warming periods repeatedly impacted the bitter cold continent by calving multiple and enormous icebergs that crumbled along the continent’s edges. The breaking of the great ice sheets marine extensions was not the only consequence of natural climate changes; the interior ice plunged into the southern oceans.
There, they searched for evidence of icebergs that broke off the Antarctic continent, floated in the surrounding ocean and melted down in the major gateway to lower latitudes called “Iceberg Alley.” In the process, the icebergs released encapsulated debris that accumulated on the ocean floor. The team took sediment cores from the deep ocean in 3.5 km water depth from the area, dated the natural climate archive and counted the ice-rafted debris.
The scientists identified eight phased with high amounts of debris which they interpret as retreat phases of the Antarctic Ice Sheet after the Last Glacial Maximum about 19,000 to 9,000 years ago, when climate warmed and Antarctica shed masses of icebergs repeatedly into the ocean. The result of the new data-model study: each such phase destabilized the ice sheet within a decade and contributed to global sea-level rise for centuries to a millennium. The subsequent re-stabilization was equally rapidly within a decade.
The research team found three other independent pieces of evidence for such post-glacial tipping points: Model experiments showing the melting of the entire Antarctic ice sheet, a West Antarctic ice core documenting ice-sheet elevation draw-down and drill cores revealing a step-wise ice-sheet retreat across the Ross Sea shelf.
The study, published overnight in Nature Communications and co-authored by Dr Zoë Thomas and Professor Chris Turney from UNSW Sydney, used geological data from Antarctica combined with computer models and statistical analyses to understand how recent changes compared to those from the past going back thousands of years.
“Our study reveals that during times in the past when the ice sheet retreated, the periods of rapid mass loss ‘switched on’ very abruptly, within only a decade or two,” says Dr Thomas.
“Interestingly, after the ice sheet continued to retreat for several hundred years, it ‘switched off’ again, also only taking a couple of decades.”
Dr Thomas says the Antarctic Ice Sheet went through many of these on/off episodes, each time contributing to global sea level rise as the world warmed at the end of the last ice age, about 20,000 years ago.
The researchers’ findings confirm computer modelling that had indicated that the diminishing ice sheet had passed a critical tipping point leading to irreversible loss of parts of the ice sheet below sea level.
“We have already observed over the last two decades that the Antarctic Ice Sheet has suddenly started losing ice which has contributed to rising sea levels around the world,” says Prof. Turney.
“But the satellite data showing this speed-up only go back about 40 years, so we needed longer records to put this change in context.”
Exhibit A: Satellite imagery of a rapidly crumbling Pine Island Glacier.
We are not seeing the natural warming that the sediment cores revealed in the Bonn study. Our predicament is that the warming event we are in at the moment has only taken a century or two to unfold. Nothing is going to happen overnight. Within decades though, that is the worrying code red alert that this study reveals.
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