For the last 80 years, the coast of the U.S. state of Louisiana has been losing land to water in the Gulf of Mexico. The land has been disappearing through a natural process called subsidence.

　　But scientists now say Louisiana is sinking faster than they thought.

　　Worst-case scenario

　　A new study shows that the Louisiana coast is sinking at an average rate of nine millimeters – almost one centimeter – per year.

　　Torbjörn E. Törnqvist is a geologist at Tulane University in New Orleans. He is one of the writers the study, published June 14 in the Geological Society of America's GSA Today.

　　He says the new rate is higher than other recent studies have shown. And, he adds, the finding is not good news.

　　"That rate is about the same as what in previous studies has been cited as the worst case scenario. So the more pessimistic numbers that people have used in the past. Well, it turns out that those are the rates that are actually happening."

　　And the news gets worse. Törnqvist says that, in addition to the land dropping into the water, climate change is causing sea levels to rise at a rate of about 3 millimeters a year in the area.

　　He says the sea level rise is "almost certain to increase in the future," and at a faster rate by the end of the century.

　　The combination of the land sinking and the sea rising means the Louisiana coast "has one of the highest rates of sea level rise to the land anywhere in the world."

　　"You can look at maps or satellites images of parts of the coast and see what it looked like, 20, 30, 40, 50 years ago and compare that to what it looks like today and the differences are staggering in many cases. There are areas that were still solid marshland 50 years ago and now it's entirely open water."

　　For their study, the researchers used a new method. They combined GPS, or satellite system measurements, rods and instruments to measure both deep underwater and surface rates.

　　They measured 274 sites along the Louisiana coast. While they found the average loss rate was 9 millimeters a year, some areas lost more than 2 centimeters, and others almost none.

　　Researchers created a map showing where – and by how much – land is sinking along the coast.

　　Why is Louisiana so vulnerable?

　　Louisiana sits where one of America's largest river, the Mississippi, empties into the Gulf of Mexico. The river has been naturally dropping mud and sediment along the coast for probably thousands of years.

　　However, the coastline lacks strong bedrock, so the land washes away easily.

　　Jimmy Frederick works for the Coalition to Restore Coastal Louisiana (CRCL). He explains that human activities have changed the natural process of the river adding and removing sediment.

　　For example, the levees built to protect New Orleans and other cities changed the flow of sediment. Now the sediment moves farther into gulf waters instead of along Louisiana's coast.

　　The state of Louisiana is also rich in oil and gas. Half the nation's oil refining ability is in the state. Its busy ports carry goods up and down the river. Trucks then carry them across the country.

　　Frederick says extracting the oil and gas has meant cutting into the marshland and harming some areas along the coast.

　　At the same time, losing coastline affects the oil, gas and other industries – and, therefore, the U.S. economy.

　　When Hurricane Katrina hit Louisiana in 2005, oil production was shut down for three days. As a result, the price of gasoline nationwide rose an average of 46 cents.

　　What can be done?

　　Jimmy Frederick's organization, the CRCL, worked on a state plan to stop the land loss. The $50 billion plan aims to restore Louisiana's coast.

　　It also aims to protect the state from its frequent tropical storms and hurricanes. Just last week, Tropical Storm Cindy damaged the levee on Grand Isle, Louisiana, and left a highway covered in water.

　　The Louisiana state plan is to move sediment back into the wetlands. Frederick explains that, simply said, the engineers would cut holes in the levees.

　　"So that at times the river can then flow as it naturally would and replenish the wetlands, and that will help a couple of things. That will help keep up with sea level rise a little bit better because Louisiana, as you know, is sinking very, very quickly, because of our geology but also because of sea level rise."

　　When the projects are completed, they will add or maintain almost 1,300 kilometers of coastal land and wetlands.

　　The project is delayed until the U.S. government approves the plan.

　　The CRCL says help cannot come soon enough because, on average, Louisiana loses 91 meters of land to the gulf every hour. Since 1932, the state has lost over 3,050 kilometers.

　　Geologist Torbjörn E. Törnqvist says the most important way to fix the coastline is to do something about climate change. If we do not, he warns, "then it's going to be an unfixable problem," and sea levels will rise at much higher rates than we see today.

　　I'm Anne Ball.

　　Anne Ball wrote this story for Learning English. Kelly Jean Kelly was the editor. We want to hear from you. Write to us in the Comments Section.