The Hawaiian Islands, seen from space, are the most vulnerable to uneven global sea level rise due to melting glaciers and ice sheets.
Melting ice in Greenland, Antarctica and elsewhere will push up seas unevenly around the world, according to a new study that finds some of the highest waters will inundate Honolulu, Hawaii.
At the poles, sea levels will actually fall because of the way sea, land and ice interact. For example, the sheer mass of water held in ice in Greenland and Antarctica generates a gravitational field that pulls in the surrounding water. As ice there melts, the gravitational pull weakens and the water is redistributed.
In addition, the melting ice on Antarctica and Greenland will lighten the load on the land beneath it, allowing the land to rebound up and the seafloor to drop a corresponding amount.
"Meaning that as the seafloor deepens, there is another component of sea-level fall, ironically, around these piles of ice," Charles Fletcher, a geologist at the University of Hawaii at Manoa, explained to NBC News.
Fletcher was not involved in the new study but is familiar with its findings, particularly the vulnerabilities of Hawaii — and, more broadly, tropical islands in the Pacific Ocean — to global sea-level rise.
Modelling glacier wastage
The research was co-led by Giorgio Spada, a professor of Earth physics at the University of Urbino in Italy. It is "the first study to examine a regional pattern of sea-level changes using sophisticated model predictions of the wastage of glaciers and ice sheets over the next century," he told NBC News via email.
He and colleagues used the model to investigate sea level under two future scenarios of sea-level rise: a mid-range, likely-to-happen one and one closer to the upper limit of what’s plausible. Under both, maximum sea-level rise is expected at Honolulu, the researchers reported Feb. 13 in the journal Geophysical Research Letters.
In the mid-range scenario, worst-affected equatorial oceans could see as much as two feet of sea-level rise, when the fact that water expands as it warms is taken into account. Under the high end, the rate of sea-level change in Honolulu will exceed 0.3 inches a year during the second half of this century.
Sea-level rise will also be greater than average in Western Australia and throughout the atolls and islands in the tropical Pacific Ocean. In Europe, sea levels will rise, but will likely be lower than the global average given the continent’s proximity to Greenland and the dropping sea levels there.
But the picture is far from complete, cautioned Fletcher.
"What this paper and other models have not been able to do because, ultimately, it is too chaotic, is to predict what the winds will be doing," he explained.
For example, enhanced trade winds associated with a phenomenon called the Pacific Decadal Oscillation are pushing the Pacific Ocean surface westward. As a result, annual sea-level rise in Hawaii is currently about half the global average rate of about 0.1 inch a year, while in the western Pacific the rate is more than 0.3 inch a year.
This may, or may not, change with the next phase of the oscillation, noted Fletcher, whose own research recently highlighted how groundwater beneath land rises with sea levels and could breach the surface, causing severe flooding in places such as Honolulu.
"Overall, the (new) paper is one more twig in the bundle of concerns that low-lying coastal cities, and especially Pacific islands, are highly vulnerable to this problem of sea-level rise," he said, adding that "these Pacific islands have contributed almost nothing to the problem of global warming."
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John Roach is a contributing writer for NBC News. To learn more about him, check out his website.