Module 4: Understanding Sea Level Change



You have no doubt seen numerous references to sea level rise, in the media and elsewhere, in recent years. With 60% of the world’s population living within six miles of the coast, the current rates of sea level rise – 3.2 mm/yr., and a predicted of sea level rise of approximately 1 meter before the end of the 21st century - we know there will be serious consequences. Such phenomena as king tides, sunny day flooding, accelerated beach erosion, higher and more destructive storm surges, and salt water intrusion into freshwater wetlands and aquifers are a few of the effects that we are hearing about more and more frequently. As these effects persist, difficult questions about the future of some coastal communities will have to be addressed by municipalities, local governments, states, and the federal government, and are indeed already being addressed. In fact, there are many examples around the U.S. and the world of ways in which sea level rise is becoming a persistent problem for residents, and plans and policies to address the issues are being implemented.

In this module, we will examine sea level change at various temporal and spatial scales to gain a perspective and understanding of these current issues. In later modules, we will look at case studies in which sea level rise plays a major role in the daily lives of people in communities around the U.S. and the world and consider the implications for the future of these communities.

Required Video

Begin by watching the following 6 minutes 20 second introductory Rising Sea Levels video from NBC. Learn and make notes on the main takeaway points. These points will recur in this module.

Click for Rising Sea Levels video transcript.

Rising Sea Levels

ANNE THOMPSON, reporting:
The shoreline, where the land and the ocean meet; here 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 1.2 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.
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.

Learning Check Point

After watching the video, please take a few minutes to think about what you just learned, and then consider how you would answer the questions on the cards below. Click "Turn" to see the correct answer on the reverse side of each card.

As we discussed in Module 1, many coastal cities and smaller communities are increasingly vulnerable to coastal flooding, and sea level rise is a major concern for residents, businesses, and planners. The video mentioned multiple types of issues faced in a future with increasingly higher sea levels, including health problems and political unrest. In this module, we will explore science behind causes and effects of sea level change through Earth’s history and examine the recent sea level trends in the context of challenges facing coastal human communities, landscapes, and ecosystems at present.

The following is a NASA video (1:58) showing animation of sea level anomaly data. The data visualization introduced in the video demonstrates that in some areas sea level has risen, while in others it has fallen. Overall, the trend is a global increase in mean sea level, with an increase in the rate of sea level rise.

The current average rate of sea level rise of close to 3 mm per year does not sound like a lot, but it represents an approximate tripling of sea level rise rates since the beginning of the 20th Century. (1900 rate was approximately 1.4 mm/ year on average, now it is more than 3.4 mm/ year on average).

This visualization shows total sea level change between 1992 and 2014, based on data collected from the TOPEX/Poseidon, Jason-1 and Jason-2 satellites. Blue regions are where sea level has gone down, and orange/red regions are where sea level has gone up. Since 1992, seas around the world have risen an average of nearly 3 inches.
Click for transcript.

Hi! I'm Josh Willis, the project scientist for the Jason-3 missions to measure sea level rise from space.

In some ways, sea level rise is really simple. As water heats up, it takes up more room. This drives sea level rise and, in addition, as glaciers and ice sheets are melted, extra water is added to the ocean, just like when you turn on your faucet in the bathtub.

Over 90 percent of the heat trapped by greenhouse gases is being absorbed by the oceans. When that happens, sea water expands, and this helps drive sea level rise.

Hundreds of millions of people around the world live on coastlines that can be threatened by rising seas. This animation shows how sea levels have changed over the last 23 years. Globally, sea levels have gone up by about 6 centimeters during that time, but it doesn't happen all at the same speed everywhere. Some places are rising faster than others, and some places are even falling.

Orange and red colors mean that sea levels have gone up in these locations, and blue and white means sea levels stayed the same or actually fallen. We can see that most places in the ocean are orange, meaning sea levels risen over the last 23 years. In a few places, you can see blue where sea level has actually dropped. Here, we see the Gulf Stream. The red and blue indicate that this massive current has shifted slightly in the last 23 years off the west coast of the United States. We've seen sea levels actually drop. This is because waters there have been cooling because of something called the Pacific decadal oscillation. In the western Pacific, sea levels have been rising very rapidly. This is because of heat being pushed from east to west across the Pacific. Sea level rise is going to be a major impact of human-caused climate change and, here at NASA, we're doing everything we can to try and better understand it.