Here’s what the latest climate models say about what will happen to Greenland and Antarctic ice sheets
More warming doesn’t necessarily mean that meltwater from Antarctica will contribute to even more sea level rise in 2100, but there’s still a great deal of uncertainty about the future.
The latest segment of the Sixth Assessment Report from the UN Intergovernmental Panel on Climate Change (IPCC) is based on a new generation of climate models. These models have become better and even more detailed, which you can read about in more detail here.
An international group of researchers has investigated what a number of the new models say about ice melting in Antarctica and Greenland, compared to earlier model predictions. When this study was started, results from four of the models were ready.
The study has been published in the journal Geophysical Research Letters.
Must be run separately
“The effective climate sensitivity in the new models is higher,” says Tore Hattermann, one of the researchers behind the new study, to sciencenorway.no.
“This means that with the same increase in atmospheric CO2, the temperature increases more strongly. This is a general feature of the updated climate models,” he said.
Hattermann is a researcher at the Norwegian Polar Institute and an associate professor at UiT The Arctic University of Norway. His work is focused on climate modelling and the oceanography of the Southern Ocean.
The researchers have used the results from the climate models to create ice models.
“Today’s climate models don’t contain dynamic ice models. They can’t say anything about how the ice cover in Greenland and Antarctica will change. To find out, we had to run ice models separately,” says Hattermann.
More melting in Greenland
Stronger warming has different effects on Greenland and Antarctica, the study shows.
Under the new models, Greenland loses more ice than in previous calculations.
“It’s actually quite easy to understand,” says Heiko Goelzer, another researcher who also participated in the study, to sciencenorway.no.
Goelzer works at NORCE and the Bjerknes Centre for Climate Research, where he focuses on modelling inland ice.
Higher climate sensitivity and warmer air means that the models show more melting from the surface of Greenland’s ice sheet, says Goelzer.
“The meltwater then flows into the sea and raises the sea level,” he said.
How much will the sea rise?
In the Working Group 1 contribution to the Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report, researchers provide an estimate of how much the sea level could rise by 2100.
- Scenario SSP1-1.9 (Very low greenhouse gas emissions; CO2 emissions reduced to net zero around 2050, followed by net negative emissions): 0.28 metres to 0.55 metres.
- SSP1-2.6 (Low greenhouse gas emissions; CO2 emissions reduced to net zero after 2050, followed by net negative emissions): 0.32 metres to 0.62 metres
- SSP2-4.5 (Medium high greenhouse gas emissions; CO2 emissions remain at current levels until around 2050): 0.44 metres to 0.76 metres
- SSP5-8.5 (Very high greenhouse gas emissions; CO2 emissions tripled in 2100): 0.63 metres to 1.01 metres
Sources: AR6 Climate Change 2021: The Physical Science Basis and the Norwegian Environment Agency.
Different in Antarctica
The story is different in Antarctica.
Even if air temperatures increase in Antarctica, the ice sheet will not start to melt, because temperatures over large parts of the continent are well below 0 degrees C.
"You start to get melting due to climate influences from the atmosphere only if there are very high temperature increases of 5 degrees or more," says Goelzer.
Higher temperatures in the atmosphere will more likely increase snowfall in Antarctica, Goelzer explains.
“Warmer air retains more moisture. This warm air is transported to Antarctica, and falls as snow. This increases the mass of the ice,” he said.
Since the water in the snow comes from moisture evaporated from the sea, this will remove water from the sea and counteract sea level rise.
Melting from the underside
At the same time, the ice shelves hugging Antarctica’s coasts will also be affected by sea temperatures.
“The warming in the sea means that the ice sheets will melt from below, which leads the ice to lose mass,” Goelzer says.
The two effects act as a counterweight to each other. Which process becomes dominant? Here, the models show slightly different results, says Goelzer.
“Some of our experiments show that this balance shifts towards the positive, and the Antarctic ice mass increases. In others it shifts towards the negative, and the continent loses a lot,” he said.
A more intense cycle
Ice loss in Antarctica has been increasing since the 1990s.
“Antarctica is currently losing a lot. If that trend continues, then we will definitely have ongoing contributions to sea level rise from Antarctica. But as the climate gets warmer, this could have a compensatory effect,” Goelzer says.
Hattermann says there’s not much difference between the new and old models in how much ice loss can be expected in Antarctica. This is due to the compensatory effect.
“Both snowfall and melting are increasing. There’s always a cycle where snow falls and where some melts, and that cycle becomes more intense,” he said.
Nevertheless, he doesn’t find these results particularly reassuring.
“If the cycle intensifies, it means change. This change is happening on a short time scale, so we basically don’t really know what’s going to happen. As soon as we start to change a complicated system, unforeseen things happen. I don’t see this as a sign that we can relax in any way.”
Slight difference in latest IPCC report
The new climate report estimates that sea level rise from Antarctica will be 11 centimetres in 2100 under a low-emission scenario. For the scenario with very high greenhouse gas emissions, the panel estimates that the sea level will rise 12 centimetres. These figures are the best estimates of possible increases between 3 to 34 centimetres.
“There are fairly similar estimates for the low- and high-emission scenarios, which is also reported in our study,” Goelzer said. “This is because of the compensatory effects of atmospheric and oceanic warming”.
Hattermann points out that there is still a lot of uncertainty around the figures, which may make it appear that the response to increased warming will be small.
“The truth is probably that our models are still not good enough to capture the response, so they provide a relatively stable mean value, but very large spread and a low degree of certainty,” he said
He says that we have to look at the Antarctic system over a longer time scale, beyond 2100, to really see the impact of climate change that is happening right now.
Sea level rise after 2100
Researchers are constantly working to gain a better understanding of how Antarctica will behave in the future.
“The work is being done from a range of different approaches,” Hattermann said. “We at the Polar Institute do a lot of observations, such as under the ice sheets in Antarctica, to see how sea temperatures are developing over time. One of the big challenges with climate science is that you need a long time series to see changes. We have to separate the weather from the climate.”
Goelzer and Hattermann agree it’s important to look at what will happen with sea after 2100.
Sea level rise will not stop in 2100, Goelzer said.
“It’s also important to look 200 to 300 years into the future. At least for scenarios with high warming, it’s expected that the ice cover will continue to decrease, probably in an accelerated way. There are backlinks in the system that aren’t so strong over a scale of a hundred years, but that could become very important later,” he said.
In the latest climate report summary, the IPCC writes that sea levels over the longer term "are committed to rising for centuries to millennia due to continued deep ocean warming and ice melting and will remain elevated for thousands of years."
Over the next 2000 years, sea levels will rise by 2 to 3 metres if warming is limited to 1.5 degrees. It will rise by 2 to 6 metres if the Earth’s average temperature climbs 2 degrees, and 19 to 22 meters if warming reaches a ceiling of 5 degrees. This estimate is described as having “low confidence”, meaning a low degree of certainty.
Translated by Nancy Bazilchuk
Read the Norwegian version of this article on forskning.no
Antony J. Payne, et al.: “Future Sea Level Change Under Coupled Model Intercomparison Project Phase 5 and Phase 6 Scenarios From the Greenland and Antarctic Ice Sheets”, Geophysical Research Letters, May 4, 2021.