In the past century, as the climate has warmed, sea level rise has accelerated. Scientists predict it will only increase, and they're studying changes in the ocean and land to better understand how and why the water is rising. "Changing Planet" is produced in partnership with the National Science Foundation.
Rising Sea Levels
ANNE THOMPSON, reporting:
The shoreline, where the land and the ocean meet; where 60 percent of the world’s population live and work within 60 miles of the coast; making rising sea levels a very big threat.
For centuries global sea levels have remained mostly constant. But over the past 100 years, as the climate has warmed, sea level rise has accelerated, rising by about 7 inches, or 17 centimeters.
And scientists predict it will only increase. Their models show that over the next 100 years the seas could rise anywhere from 7 inches to more than 3 feet, 18 centimeters to more than a meter, with potentially disastrous social and economic impacts.
Dr. BENJAMIN P. HORTON (University of Pennsylvania): If we get rates of sea-level rise greater than one meter, you're going to inundate many of the coastal areas on our planet causing health problems, socioeconomic problems, biological problems, even political instability.
THOMPSON: Dr. Ben Horton, at the Sea Level Research laboratory at the University of Pennsylvania, says the impacts of rising sea levels are already being felt.
Many island nations genuinely worry that their countries are at risk of disappearing altogether. To dramatize the problem, the Maldives government even held a cabinet meeting underwater.
In the United States, coastal communities are also worried, with many of its largest cities sitting right at waters edge. Boston, New York City, Washington, DC, Miami, New Orleans and Los Angeles are only some of the places that face the threat of greater storm surges, flooding, and coastal erosion.
Dr. HORTON: We’re trying to look at the globe and say well, where on our planet shall we be most worried about? Is it the Mississippi Delta? Is it the Nile Delta? Is it going to be Bangladesh with its huge areas of coastal lowlands with high population there? Is it some of the deltas around China?
THOMPSON: Scientists cite two main causes for rising sea levels: a warming climate that is heating the ocean and causing the volume of water to expand, and melting land-based ice sheets and glaciers that are adding to the total amount of water in the oceans.
Dr. DAVID HOLLAND (New York University): Sea level is rising; and of the sea level that we look at today, one third of that comes from warming of the ocean. The other two thirds come from adding water to the ocean.
THOMPSON: Scientists have long known that the warming atmosphere is causing ice sheets and glaciers to melt and flow toward the ocean. But recently, they have discovered that some ice sheets don’t just melt from the top.
Dr. David Holland, at New York University's Center for Atmosphere Ocean Science, studies marine ice sheets in Greenland and Antarctica. Marine ice sheets rest on the ocean floor and can melt from both above and below sea level.
Dr. HOLLAND: You can melt ice two ways. You can melt it from the top using the atmosphere or, turns out and more importantly for quick change, you can melt it from the bottom by ocean waters. We have warm waters that are near those ice sheets, and if those warm waters actually touch the marine ice sheets, the marine ice sheets melt, and you have big changes in sea level.
THOMPSON: As the marine ice sheets melt, the land-based ice behind them moves more quickly toward the sea, and this poses the greatest threat for rapid sea level rise.
To understand how and why warm ocean water is circulating to Antarctica, Holland devised a rotating water model. He uses ice and cold water for the polar region and warmer water to represent tropical waters, and adds blue and red dyes to represent cold and warm water.
Dr. HOLLAND: What we are really trying to understand is these warm ocean currents, will they actually touch the ice sheets more in the future or less, that-- that's the issue.
THOMPSON: NASA satellites have shown that since 1993, global sea levels are rising at an average of nearly 3 millimeters, or about point-12 inches, per year. That doesn’t sound like much. But when you add in other factors such as local gravity and ocean currents, sea level rise can vary, greatly influenced by the geology of the region.
Dr. HORTON: When we're thinking about sea level rise, we must also consider the land. And the land level changes will differ in relationship to ice age processes, sediment compaction, consolidation, ground water withdrawal, et cetera.
THOMPSON: Horton and his team take sediment cores from the salt marshes along the U.S. eastern shoreline to study historical sea levels. By analyzing the sediment and microscopic flora and fauna found in the cores, they can determine when sea levels changed dramatically.
Dr. HORTON: And if you look along the core, you've got changes in color that reflect changes in organic content. Each one of these changes marks a change in sea level whenever you move
THOMPSON: Horton uses the sediment cores to create a timeline that goes back thousands of years, long before sea levels were recorded by instruments, to gain an idea of how sea-levels and land-levels have changed.
Dr. HORTON: If we go back through our geological record, the coastline systems have always evolved. As a society, we have to learn to adapt to this dynamic nature of our coastlines. We cannot just say we're going to hold a line.
THOMPSON: Using the past to help people meet the challenges of the future, so we can plan and prepare for the changes in our planet.
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