Less than one metre or up to two? Predicting sea level rise in the 21st century
Leaked versions of the UN’s coming climate report suggest the forecast for sea level rise will be higher than previously thought. According to a new article in the journal Nature, the report could predict “close one metre” of sea level rise by 2100. But even that might be a lowball, according to some climate models which suggest a rise of up to two metres can’t be ruled out.
We explain how the latest sea level rise forecasts are made, and why some scientists feel the threat could be greater than the IPCC’s report looks set to suggest.
An upward revision
While the physics of how human-caused warming can drive up sea levels have been understood for a long time, the science of predicting how fast sea levels will rise is less certain.
The last IPCC report, published in 2007, predicted sea levels would rise by between 18 and 59 centimeters by the last decade of this century. But according to leaked versions of the forthcoming report seen by the some media outlets, the IPCC has estimate has revised that estimate upward.
By the last two decades of the century, sea levels are likely to rise by between 26 and 81 centimeters, according to the reports – pushing the likely range of sea level rise this centiry up substantially.
So what’s changed?
This time around, scientists are trying to anticipate how much extra water will be added to the sea as ice sheets in Antarctica and Greenland melt using new ice sheet models. Scientists knew back in 2007 that their predictions were too low because they didn’t have the knowledge or the computer models to properly factor the ice sheets in – as the Nature report states:
“Models used to date do not include… the full effects of changes in ice sheet flow, because a basis in published literature is lacking.”
More or less than a metre?
Nowadays, computer models do a better job of recreating past sea level changes and predicting what will happen in the future.
But there are two different types of model that can get the job done, and disagreement about which is best, as the Nature article explains.
‘Process’ models, which the IPCC uses in its climate reports, work out how numerous factor affecting sea level rise might change and sum up their contributions. These models suggests sea levels could rise by up to one metre by 2100.
‘Semi-empirical’ models, on the other hand, make predictions about future sea level rise based on the way it fluctuated in relation to past temperatures. And they suggest higher levels of change. These models suggest sea levels could rise by up to two metres by the end of the century.
Image - Sea Levels Process & Empirical (note)
Both types of model have their pros and cons.
Semi-empirical models are – as you’d expect – good at recreating past sea level rise, since they’re built on past trends. They also allow scientists to predict sea levels without having to know about every process involved.
But there’s no telling how long the relationship between temperatures and sea levels, which these models are based on, will hold true. At the moment, a large proportion of sea level rise comes from the expansion of water as it warms, making the temperature-sea level link a pretty useful predictive tool. In coming years, however, melting ice is expected to make a much bigger contribution – and that could make semi-empirical models less useful.
Process-based models aren’t without their failings either. Back in 2007, when researchers added up all the individual processes that contribute to rising seas – such as the expansion of warming water, extra water from melting glaciers – they could only account for only 60 per cent of the sea level rise witnessed in recent decades. As a scientist quoted in the Nature piece puts it, “[T]he whole was bigger than the sum of its parts.”
Advances in scientific understanding mean these models have since improved, and scientists now understand the processes involved better. These changes, plus the recent advances in simulating ice sheet changes mean this type of model can now account for all sea level rise witnessed in recent years.
Cautious forecasts
When it comes to making projections, deciding which type of model is more reliable has big consequences.
At the moment, the IPCC reports use the improved process models. With semi-empirical models predicting up to two metres of sea level rise by 2100, however, some scientists may criticise the report for being too conservative in its estimates.
It’s worth remembering that none of this is final – the IPCC’s projections on sea level rise could change when scientists meet next week to go over the report with a fine-toothed comb.
But even if they remain cautious compared to other studies, the IPCC’s sea level estimates look set to be considerably higher than they were just a few years ago.