The Northwest Passage from the Atlantic Ocean to the Pacific Ocean was once an explorer’s dream. Eventually, big icebreakers made their way through the ice. Soon it could become a routine summer cruise: by 2050 the North Pole will experience at least one summer without pack ice.
Unlike the Antarctic (south), the Arctic contains no continent of its own. The ice covering the North Pole is pack ice, i.e. sea ice, the thickness and area of which vary greatly depending on the place and season: approximately 15 million km2 (500 times the size of Belgium), an average of four to five meters thick at its winter maximum and half that thickness in summer. But summer melt is accelerating: at the end of last summer, in September 2019, pack ice covered only 4.3 million km2, the third-lowest value since the first satellite observations, in 1979. ‘Pack ice has a seasonal rhythm,’ explains François Massonnet, FNRS research associate at the UCLouvain Earth and Life Institute. ‘It melts in summer, reforms in winter, and so on. But year after year, we find that less and less forms in winter and more and more melts in summer.’ Several factors explain this phenomenon. Pack ice is formed by freezing sea water. Winter temperatures are increasing, however, which results in reduced thickness of winter ice. In addition, summer melting is intensifying more frequently owing to hot air masses migrating from the south. ‘So in recent years we’ve experienced a rather unfortunate combination of circumstances. Because if half of the decrease in Arctic ice can be attributed to human activities (global warming due to greenhouse gas emissions), the other half is caused by the climate’s natural variability, especially the warm southerly winds.’
Models to the rescue!
Member of SIMIP (Sea-Ice Model Intercomparison Project), which brings together 21 of the world’s research institutions, Prof. Massonnet is a man of models: ‘Our task in SIMIP is to compare climate models. It started in the 1990s and the results we’re publishing today are those of the most recent comparison phase, which involved around 40 different models.’ Climate models have systematic errors that call into question the reliability of forecasts. In addition, there are various types of models, some regional, others focused on specific environments, others global. So it’s crucial to compare them and the data with which they are supplied. The proof: this sixth round of comparison sprung a surprise, and it concerns Arctic pack ice.
An ice-free ocean
What does the comparison of the 40 climate models say about pack ice? The results, published in Geophysical Research Letters, created a stir within the scientific community. ‘When I started my research on the subject ten years ago,’ Prof. Massonnet recalls, ‘there was always hope that summer pack ice at the North Pole wouldn’t disappear, provided that greenhouse gas emission reductions were sufficient. Our latest comparison of models wiped out that hope.’ In other words, even if average global warming doesn’t exceed 2° C – and that’s no given – there will be ice-free summers at the North Pole: the explorer’s dream of an unimpeded passage will become reality. At what frequency? ‘If the increase in average temperature doesn’t exceed 2° C, one summer in ten will have no pack ice. If the temperature stays below a 1.5° C increase, it’ll only be one summer in a hundred. But what the models also tell us is that even if we stay below 2° C, we’ll experience at least one summer without polar ice by 2050.’ These new trends could be identified because the means of calculation have greatly improved over ten years, but also because climate models have evolved thanks to a better understanding of the climate’s natural variability.
Video profile François Massonnet :