A major new analysis on the impact melting polar ice sheets could have on sea level rise has given rise to some worrisome conclusions.
Researchers found that sea levels increased some 20 feet during three warming periods of 1.8 to 3.6°F (1 to 2°C) that took place at different interglacial periods over the past three million years. The study’s findings mean that the planet could be in for major sea level rise even if warming is kept to 2°C — a limit that the world is set to exceed without major action on climate change.
Published in the journal Science, the review compiled more than 30 years of research from scientists around the world to show that changes in the planet’s climate and sea levels are closely linked. It found that even a small amount of warming can lead to significant sea level rise.
Andrea Dutton, a geochemist at the University of Florida, led the study. She told ThinkProgress that her team looked at periods of time that took place 125,000, 400,000, and three million years ago in order to get a range of possibilities, as no one will be a perfect analog to the warming period the Earth is experiencing now.
“What’s important to note is that the ice sheets appear to be out of equilibrium with the climate based on what’s happened in the past,” she said.
Last year was the warmest on record — a record that 2015 is on pace to break. The International Energy Agency recently warned that temperatures could jump by as much as 7.7°F (4.3ºC) by 2100 — more than double the amount that caused sea level to rise 20 feet in previous eras. Global average temperatures have already risen almost 1.8°F (1ºC) since the 1880s.
Modeling sea level rise is notoriously challenging, and Dutton said one of the biggest questions for policymakers is how fast sea levels will rise. While glacier melt and thermal expansion of the oceans could account for about three feet of rise, Dutton said anything else will come from the polar ice sheets in Greenland and Antarctica.
“Estimates say we could get multiple meters of sea level rise within thousands of years, centuries, or even decades,” said Dutton. “That’s a lot of uncertainty.”
It’s not just about timescale either. It’s about trying to determine if the rise will be gradual or if it will jump abruptly. While Dutton said “it won’t happen overnight” the real point is considering long-term commitments to mitigating greenhouse gases and preventing numerous climate-related catastrophes, including sea level rise.
“People always talk about the year 2100 when they talk about sea level rise,” she said. “It’s not going to stop then; it will keep rising after. It’s important to realize the decisions we make today will influence that trajectory.”
Sea level rise of 10 or 20 feet could impact hundreds of millions of people living in coastal areas around the world. Many major urban centers — New York City, Hong Kong, Tokyo, Bangkok — would be overcome by the elevated seas. The authors of the study point out that most of Florida has an elevation of 50 feet or less, and Miami averages just six feet about sea level. Dhaka, the capital of Bangladesh, has some 15 million residents all inhabiting the low-lying coastal delta.
Climate Central put together a graphic illustrating the countries with the most people living on land that would be impacted by a 20-foot rise in global average seal levels. In Vietnam, some 36 percent of the population, or 32 million people, would be impacted; In Egypt, 15 percent, or 12 million people, would suffer the direct consequences; In Brazil, 6 percent, or 11 million people, would have to relocate. Overall, Climate Central determined the land on which more than 375 million people currently live would be usurped by water.
When sea levels rise, there are other corollary impacts, including storm surge, erosion and inundation, according to Anders Carlson, an Oregon State University glacial geologist and paleoclimatologist, and co-author of the Science study.
Carlson also said that we are starting to see these changes already.
“It takes time for the warming to whittle down the ice sheets,” he said. “But it doesn’t take forever. There is evidence that we are likely seeing that transformation begin to take place now.”
Carlson told ThinkProgress that “we are nearing one degree Celsius warming,” and that the “worst case scenario is what we are already on.”
Recent studies have shown that both the Greenland ice sheet and West Antarctic Ice Sheet have seen massive increases in ice loss in just the past five years. The rates of losses far exceed even those imagined a few years ago. Furthermore, there is evidence that the West Antarctic Ice Sheet has begun an irreversible process of collapse, in part because it is melting from underneath.
For now the Greenland and Antarctic ice sheets are contributing less than a millimeter a year to sea level rise, but they have the potential to add 20 feet and 200 feet to ocean levels, respectively, if greenhouse gas emissions continue to rise unabated. The higher greenhouse gas emissions get, the more Arctic sea ice melts, which allows the oceans to gather more heat. Dwindling sea ice in the Arctic Ocean creates large areas of relatively dark ocean surface that reduce the albedo, or reflectivity, of the polar region. More open water causes the Earth to absorb more of the sun’s solar energy rather than reflect it back into the atmosphere.
As the temperature warms in the Arctic regions, more permafrost also melts, thus adding to the greenhouses gases already in the atmosphere and eventually turning the Arctic from a carbon sink to a carbon source. It has been estimated that defrosting permafrost could add up to 1.5°F to total global warming by 2100. Permafrost covers nearly a quarter of the land surface of the northern hemisphere.
If and when significant sea level rise happens, it won’t be uniform across the globe. It will rise at different rates in different places due to changes in gravitational fields as the polar ice melts, according to Sutton.
“It’s not like turning on the tap,” she said. “There would be a gravitational effect depending on whether the changes came from the north or south. Lots of times you hear about the global average, but it’s also important to try and understand what’s going to happen in your own backyard.”