Sea level will rise 20 feet--but when?

The most recent reviews of climate change and sea levels lead to important conclusions—sea levels will rise more than we’ve understood, certainly dramatically, even catastrophically…

The threat that emerges from the new research is so immense, so scary, that you’re not seeing it covered in most serious media. Partly, perhaps that's because it seems to outlandish, but also it is because there is still so much uncertainty in the timing.

The likelihood, according to a new study, is of sea levels 20 feet higher than they are now. It has happened before. Indeed, for those of us who have walked the limestone of the ancient coral reefs of Ewa, it's no surprise that sea levels have been far higher than they are now.

The report says those high ocean levels are pretty much baked-in by current greenhouse gas levels and anticipated warming. It’s just not clear when. Could be decades, but it could be centuries. A lot of smart people are trying to figure out which.

“We're debating about timescales that are orders of magnitude different--decades, centuries, a thousand years,” said Andrea Dutton, the new paper’s lead author and a University of Florida carbonate geochemist.

First, some background.

The International Panel on Climate Change in 2007 figured sea levels could be up .2 to .6 meters--almost two feet--by 2100. That would be problematic for low places in the Islands, but perhaps not catastrophic in most places. 

But then the potential impacts of the melting Greenland and Antarctica ice sheets were figured in, and the numbers increased to 1-2 meters, or 3 to six feet by 2100.

During storm periods and super tides, that puts a lot of the coastal areas underwater—badly underwater—including in the Hawaiian Islands. 

Other researchers confirm the suggestion of sea level rise of at least 3 feet. 

This report, published this week in the journal Climate Research by researchers from Denmark, China, Holland and England, comes up with sea level estimates for every major coastal city in Northern Europe. Depending on where you are, and how the land itself moves, the number could be close to 3 feet by 2100, they say.

But they add an ominous warning: “There is a considerable risk that relative sea level rise will exceed recent high-end scenarios.”

And they also warn not to put too much focus on the 2100 level, because planning for that won’t be planning enough: “Sea level rise will continue for centuries beyond 2100, and sea level rise over the 22nd century is projected to exceed that of the 21st century. This long-term aspect should be considered in adaptation plans.”

But another new report, published last week in the journal Science, has thrown all that onto the back burner. 

Based on detailed study of coastal changes in ancient sea level as a result of the warming-caused melting of the globe’s great ice sheets, the authors say there is an excellent chance that even very small amounts of warming—just 1 to 2 degrees—could raise sea levels six meters. 

That’s up to 20 feet above current levels.  That is catastrophic by any standard for coastal communities.

Dutton and her colleagues calculated that the amount of water lost from the immense ice sheets of Greenland and Antarctica is expected in time to be the major contributor to sea level rise. To try to assess their impact,  they studied previous interglacial periods when temperatures were higher than current levels. 

Their finding: "During recent interglacial periods, small increases in global mean temperature and just a few degrees of polar warming relative to the preindustrial period resulted in" as much as 20 feet or more of sea level rise."

And they assess that these levels are probable, because similar temperatures have caused similar sea heights repeatedly in the past. 

RaisingIslands contacted lead author Andrea Dutton at the University of Florida. She said the sea level rise is probably coming, but the researchers still can’t tell how quickly it will arrive. That’s what they’re working on next.

“As we state in the paper, perhaps the most societally relevant information we can provide from the record of past sea-level rise is the rate at which sea level rose as the polar ice sheets retreated. At this stage, the data we have on rates is still highly uncertain and hence is an important target for future research. 

“The physics in the models is not good enough to make projections of rapid ice sheet retreat and/or collapse…There is no consensus on how long it will take. 

"In general, multi-meter sea level rise is thought to take at least centuries, though a model published earlier this year suggested it could be possible over several decades.  So we're debating about timescales that are orders of magnitude different--decades, centuries, a thousand years.  Hence the answer is that it is too uncertain to say,” Dutton said in an email.

What does that mean for people living in coastal areas, and those responsible for planning? 

“From a planning perspective, it would seem to make sense to want to plan for a rate of sea-level rise that is higher than most projections since major storm events will cause extreme sea levels that are even higher than the background rate of rising sea levels,” Dutton said in her email.

Because the impacts of sea levels more than two stories high are so severe the development of better data of global mean sea level (GMSL) is critical, Dutton and her co-authors wrote.

“Improving our understanding of rates of GMSL rise due to polar ice-mass loss is perhaps the most societally relevant information the paleorecord can provide, yet robust estimates of rates of GMSL rise associated with polar ice-sheet retreat and/or collapse remain a weakness in existing sea-level reconstructions,” they wrote.

How does a community plan for something like this? In Hawai`i, consider that at the predicted levels, most of our airports are underwater, most of our harbors are gone, most of our resort areas are awash, many of our water wells turn salty. Sewer lines, power plants, coastal roads are destroyed, and many communities on almost every island are isolated.

Most of the Northwestern Hawaiian Islands and most of the atolls and other low-lying islands throughout the Pacific would be gone. The impacts on the turtles, seals and millions of nesting seabirds are beyond imagining.

© Jan TenBruggencate 2015