Geological records can help improve sea-level forecasting
Global sea levels rose around 20 cm in the last 120 years(opens in new window), faster than at any time in the last 3 000 years(opens in new window). Historically largely driven by thermal expansion(opens in new window) and glacier melt, in recent decades melt from the Greenland and Antarctic ice sheets has accelerated this process. Even with emissions reductions, this upward trend is expected to continue for centuries. “It is virtually certain that global mean sea levels will continue rising, but the rate, height and time at which it might stabilise are uncertain,” says Fiona Hibbert, Marie Skłodowska-Curie fellow from the EU-supported ExTaSea project. Using a combination of geological data, statistics and modelling, Hibbert from the University of York(opens in new window) analysed the rate and magnitudes of past sea-level changes to better understand the overall system and its natural variability.
Credible worst-case scenarios
Under a high-emissions scenario, the Intergovernmental Panel on Climate Change projects(opens in new window) sea levels could rise 0.77 metres on average globally by 2100; also concluding that, although unlikely, levels could potentially rise over 15 metres by 2300. While the Greenland and Antarctic ice sheets only contributed around 13.5 and 7.4 millimetres, respectively, from 1992 to 2020, their complete melt would contribute around 7 and 57-metre sea-level rises, respectively. “Melting all remaining ice caps and glaciers would only contribute around 0.32 metres, so melt from the polar ice sheets is a serious concern, especially as recent observations show total ice-sheet losses tracking the upper range of current sea-level projections,” explains Hibbert. Predictions have been mainly based on relatively short-term data sets including 20 years of satellite data and around 120 years from tide gauges, but as Hibbert explains: “The geological record includes real-world episodes of large-scale and rapid ice-sheet loss, offering more comprehensive data.” Going back 200 000 years, ExTaSea analysed the geological records for six intervals, covering three different climate states: two warm interglacial periods with conditions like now; two ‘cold’ glacial intervals; and two deglacial intervals (the transition from glacial to interglacial with some of the largest warming and ice-sheet loss in Earth’s recent past). To produce sea-level reconstructions, statistical analysis of the geological records – over 5 900 data points – was combined with modelling which accounted for variations caused by gravitational, rotational and solid Earth deformation processes. “For the two most recent warm interglacial intervals, most recently around 125 000 years ago, sea levels were 5 to 10 metres above the present day levels, despite similar or only slightly warmer global temperatures,” remarks Hibbert. Hibbert also found that sea levels can rise very rapidly during deglacial intervals, approximately 3.6 metres a century during the last one, ten times the current rate. “This suggests processes undiscovered by instrumentation and latent potential for very rapid ice loss and consequently sea-level rise. Put simply, the geological record contains a warning,” adds Hibbert.
The value of credible ‘worst-case’ scenarios
Rising sea levels could cause significant coastal flooding with approximately 50 million people in Europe living within low-elevation coastal zones(opens in new window) and around 10 % of the world’s population(opens in new window). Without effective planning, it has been calculated that damage costs could increase in the EU and United Kingdom from EUR 1.4 billion today to almost EUR 240 billion by 2100. “ExTaSea’s research into low-probability but high-impact sea-level rise driven by polar ice-sheet loss, while important for protecting coastal populations and ecosystems, is vital for critical infrastructure,” concludes Hibbert. Hibbert’s next steps will be to tailor these worst-case scenarios to specific locations or situations. This research was undertaken with the support of the Marie Skłodowska-Curie Actions programme(opens in new window).