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Ocean levels are set to rise faster than expected

Researchers at the University of Copenhagen have developed a new method for predicting how fast the sea level will respond to global warming. The study reveals that previous projections have been too conservative, and that ocean levels will rise faster than what has been predicted.

Professor Aslak Grinsted explained that ocean levels have been rising throughout the last 150 years, which is called the industrial period.

“We expect, of course, that there is a connection between rising temperature and the rate indicating the momentum of the rise. Observations are telling us that the rate has been accelerating over the past 150 years,” said Professor Grinsted. 

“This means we can create a picture of how the connection between temperature and sea level rise has been, historically. But 150 years is not very long, actually, because of the great inertia in the warming of the oceans and inland ice sheets, so several hundreds of years can pass before we see the full consequences of warming in the atmosphere.” 

“This is why we compare the observations with the results from the detailed computer models we use to depict a future scenario. Among others, the climate panel of the United Nations Intergovernmental Panel on Climate Change (IPCC), has gathered these projections, made from a collection of many smaller models. These, in turn, have been validated, obviously, as well as can be done.”

The IPCC predictions of future ocean levels are based on a complex network of models for ice sheets, glaciers and rising sea temperatures.  However, when there is only a limited amount of data for the models, the projections are not reliable. For example, there was no data available to account for melting ice in Antarctica before satellite observations in the 1990s. 

“We have better historical data for the sea level rise in total, which, in principle, allows for a test of the combined puzzle of models. However, it has not been part of the routine to make sea level hindcasts at IPCC, so presently we are not able to tell if these models are capable of reproducing the historical sea level,” explained Professor Grinsted.

“At the Niels Bohr Institute, we have used this situation as our starting point, and so we observe how sensitive the models are in reacting to warming. We expect that if we compare observational data from the fairly short period of time from 1850 onwards with the sensitivity of the models, it should allow us to assess whether the models are realistic or not.”

The researchers hope their new method of validating future scenarios by looking into the past can improve how sea level rise will be analyzed in the future. 

“Apparently, the models we are basing our predictions of sea level rise on presently are not sensitive enough. To put it plainly, they don’t hit the mark when we compare them to the rate of sea level rise we see when comparing future scenarios with observations going back in time,” said Professor Grinsted.

According to Professor Jens Hesselbjerg Christensen, it is crucial for our faith in model-based climate predictions that they are able to reproduce the climate as realistically as possible.

“We hope this new comparison metric will be adopted to as large extent as is possible and can become a tool we can apply in comparing different models,” said Professor Christensen. “A good example is that we don’t expect the sensitivity to be the same all the way back to the last millennium or several millions of years back in time, but the added understanding of how the sensitivity might change over time is something we can add to the comparisons between models and observations.” 

“Besides, we’d like to see the method applied to the individual processes contributing to sea level rise. This might make the understanding of the sensitivity even more detailed, we believe.”

The study is published in the journal Ocean Science.

By Chrissy Sexton, Staff Writer

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