Scientists from the University of Copenhagen have created a mathematical model disputing commonly expected rates of sea level rise, arguing that most projections underestimate the speed at which the levels are increasing.

Their study, published Tuesday in Ocean Science, finds that sea levels will likely rise at a much higher rate than anticipated — by an additional 25 centimeters or so per century, according to Aslak Grinsted, an associate professor at the university’s Niels Bohr Institute research section and the paper’s lead author.

As temperatures increase, sea levels go up due to ice melting and the volume of seawater expanding under heated conditions. The last 150 years have seen sea levels climb exponentially. 

Today, scientific bodies like the United Nations’ Intergovernmental Panel on Climate Change study data within this time frame to understand where sea levels are headed, including the speed at which they are increasing. However, Grinsted proposes that such institutions may be omitting important trends from their models.

Current projections are not sensitive enough because they focus heavily on traditional, static climate data, instead of focusing on sea level data that is in flux, according to Grinsted. As it stands, he said, models to depict sea levels rising often rely on conglomerates of individual factors, such as radiation trapped in the atmosphere and greenhouse gas emissions, that haven’t been assessed comprehensively enough.

“These models might be completely realistic by themselves, these individual components, but when you add them together, they don't add up to the correct total for the historical period,” Grinsted said. “The problem is that since they haven't done any [sensitivity] analysis for the past data, we can't really accept the way they are doing it now.”

The concept of transient sensitivity, which refers to measuring the outcome of a variable based on changes to its system over time, is often used to examine the greenhouse effect with regard to larger-scale climate models. These new findings are the first to suggest its use in determining the rate of sea level increase specifically.

The paper emphasizes that sea level forecasts for a given point in time should reflect the average temperature of the century preceding that point, rather than only the specific temperature expected.

For their study, the authors tested a linear model that includes what they called a more precise representation of temperature by utilizing that average, which they say better accounts for variations in sea levels over time.

Grinsted and his team coined the resulting slope “Transient Sea Level Sensitivity,” or TSLS. It is a novel way to evaluate sea level projections for precision through mathematical modeling.

To test its validity, the team’s new model was compared with past sea levels. Once its accuracy was confirmed, TSLS was used to test future projections of sea levels. Here the researchers found a discrepancy, as the rate of sea level rise consistently fell below projections that fit their model.

“The sea levels are rising faster than the models predict, even for a scenario that is warmer,” Grinsted said. “That simply does not make sense from a physical point of view. That is what leads us to conclude that the mismatch means the models must be biased.”

Because TSLS is isolated from external variables like fossil fuel emissions and can focus exclusively on sea level rates, Grinsted said, the model could help in forecasting more accurate sea level measurements without having to simulate particular conditions.

The use of transient sensitivity in measuring each distinct factor of climate change could make for a more realistic view of the situation at hand, Grinsted believes. He said that the need to reduce emissions is already strong, but relying on overly conservative data will create greater challenges with every passing century.

“We need to make sure that not only do the individual processes look realistic and fit the data, but also that the total adds up to be correct,” he said.

The article “The transient sensitivity of sea level rise” was published Feb. 2 in Ocean Science. The authors of the study were Aslak Grinsted, University of Copenhagen and Jens Hesselbjerg Christensen, Bjerknes Centre for Climate Research. The lead author was Aslak Grinsted.