Seas Are Rising at Fastest Rate in Last 28 Centuries
The oceans are rising faster than at any point in the last 28 centuries, and human emissions of greenhouse gases are primarily responsible, scientists reported Monday.
They added that the flooding that is starting to make life miserable in many coastal towns — like Miami Beach; Norfolk, Va.; and Charleston, S.C. — was largely a consequence of those emissions, and that it is likely to grow worse in coming years.
The scientists confirmed previous estimates, but with a larger data set, that if global emissions continue at a high rate over the next few decades, the ocean could rise as much as three or four feet by 2100, as ocean water expands and the great ice sheets in Greenland and Antarctica begin to collapse.
Experts say the situation will grow far worse in the 22nd century and beyond, likely requiring the abandonment of many of the world’s coastal cities.
“I think we can definitely be confident that sea-level rise is going to continue to accelerate if there’s further warming, which inevitably there will be,” said Stefan Rahmstorf, a professor of ocean physics at the Potsdam Institute for Climate Impact Research in Germany and co-author of a paper released Monday by the Proceedings of the National Academy of Sciences.
“我认为，我们绝对可以相信，如果出现进一步升温，海平面上升的速率将会继续加快，而进一步升温在所难免，”德国波茨坦气候影响研究所(Potsdam Institute for Climate Impact Research)的海洋物理学教授斯特凡·拉姆斯托夫(Stefan Rahmstorf)说。他是《美国国家科学院院刊》(The Proceedings of the National Academy of Sciences)本周刊登的一篇相关论文的合著者。
“Ice simply melts faster when the temperatures get higher,” Dr. Rahmstorf added. “That’s just basic physics.”
In a report issued at the same time as the scientific paper, a climate research and communications organization in Princeton, N.J., Climate Central, used the new findings to calculate that roughly three-quarters of the tidal floods now occurring in towns along the American East Coast would not be happening in the absence of sea-level rise caused by human emissions.
The lead author of that report, Benjamin H. Strauss, said the same was likely to be true on a global scale, in any coastal community that has seen an increase of saltwater flooding in recent decades.
该报告的第一作者本杰明·H·施特劳斯(Benjamin H. Strauss)表示，在全球范围内，情况很可能也是一样的，很多海岸社区近几十年经历了潮汐洪水的增多。
Local factors do come into play, though: Communities on land that is sinking, as in the Chesapeake Bay region of the United States, are being hit especially hard by the rising sea level.
Tidal floods are occurring more frequently, and are becoming a strain in many towns by killing lawns and trees, polluting supplies of fresh water, blocking streets in the middle of sunny afternoons and sometimes stranding entire island communities for hours by covering the roads to the mainland.
“I think we need a new way to think about most coastal flooding,” Dr. Strauss said in an interview. “It’s not the tide. It’s not the wind. It’s us. That’s true for most of the coastal floods we now experience.”
The new research was led by Robert E. Kopp, an earth scientist at Rutgers University who has won respect from his colleagues by bringing elaborate statistical techniques to bear on longstanding problems, like understanding the history of global sea level.
这项新的研究由罗格斯大学(Rutgers University)的地球科学家罗伯特·E·科普(Robert E. Kopp)牵头进行。他将精密的统计学技术应用于一些长期存在的问题，比如怎样了解全球海平面的历史，赢得了同行的敬重。
Scientists already knew that the sea level rose drastically at the end of the last ice age, by almost 400 feet, causing shorelines to retreat by up to 100 miles in places. They also knew that the sea level had basically stabilized, like the rest of the climate, over the past several thousand years, the period when human civilization arose and spread across the earth.
There were small variations of climate and sea level over that period, and several recent papers have tried to clarify these. The new paper confirms a central finding of the earlier research, that the sharp increase of sea level in the 20th century was unprecedented over thousands of years, but does so with a larger data set that may add to the confidence scientists place in the results.
The paper confirms that the ocean is exquisitely sensitive to small variations in the earth’s temperature — a portentous finding, given that human emissions are inducing a large temperature rise.
The researchers found that when the average global temperature fell by a third of a degree Fahrenheit in the Middle Ages, for instance, ice started to build up on land, and the volume of ocean water contracted, causing the average surface of the ocean to fall by about three inches over 400 years. When the climate warmed slightly, that trend reversed.
“Physics tells us that sea-level change and temperature change should go hand in hand,” Dr. Kopp said. “This new geological record confirms it.”
In the 19th century, as the Industrial Revolution took hold, the oceans began to rise, and have gone up by about eight inches since 1880. That may sound small, but the increase has caused extensive erosion worldwide, and governments are spending billions of dollars to try to shore up beaches and other coastal defenses.
Largely because of human emissions, global temperatures have jumped by about 1.8 degrees Fahrenheit since the 19th century. Land ice has started to melt all over the planet, and seawater is expanding as it absorbs heat. The seas are rising at what appears to be an accelerating pace, lately reaching a rate of about a foot per century.
One of the authors of the new paper, Dr. Rahmstorf, had previously published estimates suggesting the seas could rise as much as five or six feet by 2100. But with the improved calculations from the new paper, his latest upper estimate is three to four feet.
That means Dr. Rahmstorf’s estimate is now more consistent with calculations issued in 2013 by the Intergovernmental Panel on Climate Change, a United Nations body that periodically reviews and summarizes climate research. The panel found that continued high emissions might produce a sea rise of 1.7 to 3.2 feet over the 21st century.
这意味着，拉姆斯托夫现在的估计和政府间气候变化专门委员会（ Intergovernmental Panel on Climate Change，简称IPCC）2013年发布的计算结果更趋一致。IPCC是联合国机构，定期回顾和总结气候研究。它发现，持续的高排放可能会导致海平面在21世纪上升53厘米到92厘米。
Dr. Rahmstorf said, however, that the rise would eventually exceed three feet — the only question is how long it will take. The recent climate agreement negotiated in Paris, if acted upon, will bring emissions down enough to slow the rate of sea-level rise in coming centuries, but scientists say the deal was not remotely ambitious enough to forestall a significant melting of the Antarctic and Greenland ice sheets.
On a geologic time scale, the recent, human-induced planetary warming has been quite sudden, and the huge ice sheets have only just started to respond.
The upper estimate of three to four feet of sea-level rise in the 21st century rules out any large contribution from Antarctica in the near term, but that finding is tentative, given that the ice covering the western part of that continent is already showing signs of instability. And recent studies suggest that the destruction of large parts of the Antarctic ice sheet may have become inevitable, even though that could take hundreds or thousands of years to play out.
“Sea level is going to continue going up for many centuries,” Dr. Rahmstorf said.