Sea levels worldwide have been rising at a much faster rate than has been predicted by climate change models. The reasons for this are clear according to University of Colorado geologist Bill Hay; there are many large feedback loops that are just beginning to kick in, that will contribute significantly to future sea level rise.

The most recent official report from the IPCC was released back in 2007, and only predicted a global sea level rise of 0.2 to 0.5 meters by 2100. But current sea-level rise measurements are already meeting or exceeding the high end of that range, and have been suggesting a rise of at least one meter by 2100, possibly much more.

“What’s missing from the models used to forecast sea-level rise are critical feedbacks that speed everything up,” says Hay.

The primary feedbacks that he is referring to are ones to do with the melting Arctic sea ice, the melting Greenland ice cap, and also soil moisture and groundwater extraction.

“There is an Arctic sea ice connection,” says Hay. While melting sea ice itself doesn’t raise sea level, it causes considerable changes in the amount of solar energy being absorbed by the water. This overall increase in the warming of the Arctic in turn causes ice losses in nearby Greenland and northern Canada. The overall effect is that when sea ice melts, “there is an oceanographic effect of releasing more fresh water from the Arctic, which is then replaced by inflows of brinier, warmer water from the south.”

“So it’s a big heat pump that brings heat to the Arctic,” says Hay. “That’s not in any of the models.” The warm influx creates “ice-free waters, which absorb sunlight rather than reflect it back into space like sea ice does. The more open water there is, the more heat is trapped in the Arctic waters, and the warmer things can get.”

Also worth considering is that there are massive stores of ice in Greenland and Antarctica. Sea levels rose by over 10 meters when all of that ice melted during the last interglacial. And new research has been strongly suggesting that that considerable rise in sea levels happened over just a few centuries.

“You can lose most of the Greenland ice cap in a few hundred years, not thousands, just under natural conditions,” says Hay. “There’s no telling how fast it can go with this spike of carbon dioxide we are adding to the atmosphere.”

This was made very clear just this last summer when Greenland was observed undergoing an unprecedented, record-setting melt.

Hay notes, “Ten years ago we didn’t know much about water under the Antarctic ice cap.” But it is there, and it allows the ice to move — in some places even uphill due to the weight of the ice above it.

“It’s being squeezed like toothpaste out of a tube,” explains Hay. “The one thing that’s holding all that ice back from emptying into the sea is the grounded ice shelves acting like plugs on bottles at the ends of the coastal glaciers.”

“Nobody has any idea how fast that ice will flow into the oceans once the ice shelves are gone.”

Another factor that has remained under researched is the effect that groundwater extraction has. All over the globe massive amounts of water are being mined from underground for agricultural, industrial, and urban uses. All of that water ends up contributing to sea level rise, as visualized here (a recent visualization of this effect in the U.S. posted by NASA’s Earth Observatory):

These are all known positive feedbacks that could considerably speed up the changes that are occurring in the climate, and contribute to a faster than expected sea-level rise.

“You would expect negative feedbacks to creep in at some point,” says Hay. “But in climate change, every feedback seems to go positive.”

The reason for this seems to be that the climate has only certain states that are stable. Everything in between these states is chaotic and fast changing, until it finally settles into a new state. “Under human prodding, the system wants to go into a new climate state.”

Source: Geological Society of America

Image Credits: Map by Emanuel Soeding, Christian-Albrechts University, using NOAA data