At many river deltas scientists are missing a major source of sea

first_img At many river deltas, scientists are missing a major source of sea level rise Email For coastal communities, the sea level rise propelled by melting ice and warming oceans is bad enough. But people living on the soft, compressible sediments of river deltas have another factor to contend with: sinking land. Scientists have traditionally inferred the sinking from tide gauge readings or measured it directly at GPS stations. But a team of scientists now says these methods significantly underestimate subsidence at many deltas and low-lying coastlines worldwide.In recent years, scientists at Tulane University in New Orleans, Louisiana, have shown that in the Mississippi River delta, fluffy, young sediments within a few meters of the surface are compacting rapidly. They estimate the effect more than doubles the region’s rate of sea-level rise to a total of 13 millimeters a year. Tide gauges and GPS stations miss that subsidence because they are anchored to deeper layers, which are less susceptible to compaction.The same mechanism is likely at play in many low-lying coastal areas worldwide, which host some 10% of the global population, the team argues in a paper published this week in Ocean Science. “Tide gauges are not measuring what we need,” says Torbjörn Törnqvist, a geologist at Tulane and co-author on the study. “We need to really rethink how we monitor these areas.” Click to view the privacy policy. Required fields are indicated by an asterisk (*) A surface elevation table is used to measure subsidence in a tropical swamp in Indonesia. Sign up for our daily newsletter Get more great content like this delivered right to you! Countrycenter_img By Paul VoosenJan. 30, 2019 , 12:10 PM SIGIT DENI SASMITO/CIFOR/FLICKR Country * Afghanistan Aland Islands Albania Algeria Andorra Angola Anguilla Antarctica Antigua and Barbuda Argentina Armenia Aruba Australia Austria Azerbaijan Bahamas Bahrain Bangladesh Barbados Belarus Belgium Belize Benin Bermuda Bhutan Bolivia, Plurinational State of Bonaire, Sint Eustatius and Saba Bosnia and Herzegovina Botswana Bouvet Island Brazil British Indian Ocean Territory Brunei Darussalam Bulgaria Burkina Faso Burundi Cambodia Cameroon Canada Cape Verde Cayman Islands Central African Republic Chad Chile China Christmas Island Cocos (Keeling) Islands Colombia Comoros Congo Congo, the Democratic Republic of the Cook Islands Costa Rica Cote d’Ivoire Croatia Cuba Curaçao Cyprus Czech Republic Denmark Djibouti Dominica Dominican Republic Ecuador Egypt El Salvador Equatorial Guinea Eritrea Estonia Ethiopia Falkland Islands (Malvinas) Faroe Islands Fiji Finland France French Guiana French Polynesia French Southern Territories Gabon Gambia Georgia Germany Ghana Gibraltar Greece Greenland Grenada Guadeloupe Guatemala Guernsey Guinea Guinea-Bissau Guyana Haiti Heard Island and McDonald Islands Holy See (Vatican City State) Honduras Hungary Iceland India Indonesia Iran, Islamic Republic of Iraq Ireland Isle of Man Israel Italy Jamaica Japan Jersey Jordan Kazakhstan Kenya Kiribati Korea, Democratic People’s Republic of Korea, Republic of Kuwait Kyrgyzstan Lao People’s Democratic Republic Latvia Lebanon Lesotho Liberia Libyan Arab Jamahiriya Liechtenstein Lithuania Luxembourg Macao Macedonia, the former Yugoslav Republic of Madagascar Malawi Malaysia Maldives Mali Malta Martinique Mauritania Mauritius Mayotte Mexico Moldova, Republic of Monaco Mongolia Montenegro Montserrat Morocco Mozambique Myanmar Namibia Nauru Nepal Netherlands New Caledonia New Zealand Nicaragua Niger Nigeria Niue Norfolk Island Norway Oman Pakistan Palestine Panama Papua New Guinea Paraguay Peru Philippines Pitcairn Poland Portugal Qatar Reunion Romania Russian Federation Rwanda Saint Barthélemy Saint Helena, Ascension and Tristan da Cunha Saint Kitts and Nevis Saint Lucia Saint Martin (French part) Saint Pierre and Miquelon Saint Vincent and the Grenadines Samoa San Marino Sao Tome and Principe Saudi Arabia Senegal Serbia Seychelles Sierra Leone Singapore Sint Maarten (Dutch part) Slovakia Slovenia Solomon Islands Somalia South Africa South Georgia and the South Sandwich Islands South Sudan Spain Sri Lanka Sudan Suriname Svalbard and Jan Mayen Swaziland Sweden Switzerland Syrian Arab Republic Taiwan Tajikistan Tanzania, United Republic of Thailand Timor-Leste Togo Tokelau Tonga Trinidad and Tobago Tunisia Turkey Turkmenistan Turks and Caicos Islands Tuvalu Uganda Ukraine United Arab Emirates United Kingdom United States Uruguay Uzbekistan Vanuatu Venezuela, Bolivarian Republic of Vietnam Virgin Islands, British Wallis and Futuna Western Sahara Yemen Zambia Zimbabwe Satellites are the main tools for monitoring the absolute changes in ocean height, which reflect the biggest drivers of sea level rise: melting ice and the expansion of warming water. But for people and ecosystems, the relative impact of rising or falling land is just as important. Some regions are still rebounding thousands of years after ancient ice sheets melted, lifting a colossal weight off Earth’s elastic mantle. Many more are subsiding. “It’s something we’ve been overlooking too long in sea level projections,” says Aimée Slangen, a climate scientist at the Royal Netherlands Institute for Sea Research in Yerseke and a lead author of the sea level chapter of the next United Nations climate report.Louisiana, for example, is sinking fast. Although compaction is the primary culprit, the extraction of groundwater, oil, and gas also play a role. Sediment washed down the Mississippi River once compensated for the subsidence, but levees and other engineered structures now shunt it out into the Gulf of Mexico. To monitor the sediment loss, the state over the past few decades has deployed a network of some 400 simple wetland-monitoring instruments, called surface elevation tables.The table, a metal arm that juts out parallel to the swamp’s surface, is anchored to a pole driven deep below. Twice a year, a series of pins are lowered from the table until they just touch the marsh surface—giving a regular measure of how fast the surface is sinking relative to deeper layers. Five years ago, when Törnqvist’s group began to use this network to divine the source of Louisiana’s subsidence, researchers realized the problem is not just sediment loss. Shallow soils, deposited in earlier centuries when the river ran free, are simply compressing. “Tide gauges were not capturing that,” says Molly Keogh, the Tulane graduate student who led the new work.The new paper lays out why. The region’s 131 tide gauges measure the tide in comparison with benchmarks anchored in deep sediments, often tens of meters down—”as close as we get to bedrock in Louisiana,” Keogh says. The region’s 10 GPS stations with known benchmarks were also anchored, on average, 14 meters deep in the mud. To both devices, the zone of the most compaction—a source of half the sea level rise—was invisible. The scenario could be true in other delta regions that also rely on tide gauges, casting doubt on estimates of regional sea level rise, says Mark Schuerch, a physical geographer at the University of Lincoln in the United Kingdom. “It’s quite innovative and quite exciting—or scary, really.”Figuring out the anchor depths of tide gauges elsewhere in the world will be a herculean task, warns Philip Woodworth, former director of the Permanent Service for Mean Sea Level in Liverpool, U.K., who reviewed the paper. Tide gauge records do not typically include the depth of their benchmarks; that knowledge, if it exists, is buried in country bureaucracies.Moreover, the rate of shallow compaction probably varies greatly from wetland to wetland. In some marshes, plants compensate for compaction by capturing new sediment with their roots. And in some regions, such as Bangladesh, compaction occurs more uniformly across shallow and deep layers, says Céline Grall, a marine geologist at Columbia University’s Lamont-Doherty Earth Observatory in Palisades, New York. “These assumptions are not true there.”Deploying elevation tables in deltas around the world could resolve those uncertainties, creating a global database, Törnqvist says. The tool is simple, cheap, and effective, and has already been used in more than 30 countries. For an area the size of coastal Louisiana, only 40 would be needed to keep track of subsidence—and determine how fast seas are truly rising. The millions of people living in the world’s deltas need to know the answer, Grall says. “That’s a legacy we should work on.”last_img read more