Place your bets: the collapse of the Antarctic ice sheet Climate change and the melting of the polar ice caps are always making headlines, but how fast is it happening? Tamsin Edwards doesn’t know – exactly. And that was one of the most important points in her 2016 Charles Lyell Award Lecture. Alan Barker listened and learned. Ice is the only commonly occurring solid that floats in its own liquid. As a result, lakes and oceans freeze top-down, insulating the life beneath from the intense cold above. But ice also protects us from heat by reflecting sunlight back into space. The ice caps at the planet’s poles regularly wax and wane with the seasons; and, importantly, the minimum ice cover in September in the Arctic is steadily reducing due to climate change. By the middle of the century, that minimum may reduce to nothing and reduce the protective effect of the polar ice. Dry dock ice berg. Credit: antarcticash [http://www.coolantarctica.com/gallery/antarctic_assorted/Dry_Dock_Iceberg.php] Tamsin, lecturer in environmental sciences at the Open University, has a particular interest in the Antarctic ice sheet. Predicting its collapse is tricky: Antarctica, unlike the Arctic, is a vast land continent. Antarctic ice rests on solid land, flows as glaciers along solid beds, and floats on the surrounding sea as icebergs. Earlier predictions had been contradictory. According to the Intergovermental Panel on Climate Change report in 2013, instability in the Antarctic ice sheet might cause up to “several tenths of a metre” of sea level rise by the end of the century. (You can see a summary of the report, delivered as a set of 19 haiku, here.) Just a year later, satellites had revealed big changes in the coastline of the Amundsen Sea in West Antarctica, an area the size of Texas. Computer models predicted that vast glaciers were collapsing, and would continue to do so, maybe for centuries. In the press, headlines screamed about irreversible decline, and – not for the first time – the public wondered whether scientists knew what they were talking about. Tamsin’s research, published in ‘Nature’, focused on the glaciers. Understanding their rate of flow is critical to estimating the reduction in ice cover; and that rate of flow is influenced in part by the position of the grounding line – the point at which the base of the glacier leaves the continent’s bedrock and starts to float in the sea. If the grounding line moves inland – which it could do if warm water reaches the edge of the ice or a floating ice shelf collapses – then the amount of ice entering the sea increases and sea levels rise. Tamsin and her team asked: where is the bedrock most vulnerable to shifts in the grounding line, when might change occur, and how fast might it be? They created a range of predictions using three thousand computer-generated models. They scored each model on its ability to simulate recent changes, using exactly the same maths that spam filters use. (It’s called Bayesian statistics.) Spam score = 5? Reject from the inbox of the future. Not spam? Let it through. The grounding line will shift inland more slowly if the underlying bedrock is lumpy (imagine ice being dragged across a series of speed bumps). A tiny increase in lumpiness can make a big difference (think of the princess and the pea). We have no accurate map of those lumps and bumps. But, by looking at that uncertainty, Tamsin can make predictions that are more accurate, even if they’re not more definite. Tamsin’s team predicted there was only a 1 in 20 chance Antarctic instability would produce more than 30 cm sea level rise by the end of the century. That’s substantial, but much less than some of the earlier, simpler predictions. Antarctic ice presents fascinating and complex challenges – challenges that can affect climate change and our response to it. In Tamsin’s words, “the story definitely isn’t over yet.” If there’s one idea she wanted us to take from her lecture, it’s that uncertainty is not a bad thing. Uncertainty is the engine of science, and the public has a right – and an obligation – to understand that science can rarely give definite answers. Alan Barker, British Science Festival Swansea, September 2016. Alan Barker is a writer and training consultant specialising in communication skills. He is Managing Director of Kairos Training Limited. Banner photo: Early morning sun halo in Pine Island Glacier, Antarctica. Credit: Jan De Rydt.