For most people, sea level rise is caused by melting ice sheets. It is so easy to visualize a glacier melting into the ocean. As it turns out, an equally important factor is the expansion of seawater as it warms. In this section, we explore these different mechanisms in some detail.
Growth and Melting of Ice Sheets
How are absolute changes in sea level caused? As we have seen, the most direct way is through the growth and melting of the major ice sheets, as discussed in the following video.
Video: NASA: A Tour of the Cryosphere 2009 (5:12)
NASA: A Tour of the Cryosphere 2009
[MUSIC]
NARRATOR: Though cold and often remote, the icy reaches of the Arctic, Antarctic, and other frozen places, affect the lives of everyone on Earth. We start our tour in Antarctica. Where they meet the sea, mountains of ice crack and crumble. The resulting icebergs can float for years. Ice shelves surround half the continent. They slow the relentless march of ice streams and glaciers, like dams hold back rivers. But the region is changing.
As temperatures increase, we see a growing number of melt ponds. As this heavy meltwater forces its way into cracks, ice shelves weaken and can ultimately collapse. After twelve thousand years, the Larsen B Ice Shelf collapsed in just five weeks. Offshore, sea ice forms when the surface of the ocean freezes, pushing salt out of the ice. The cold, salty, surface water starts to sink, pumping deeper water out of the way powering global ocean circulation. These currents influence climate worldwide. Most ice exists in the cold polar regions, but we see glaciers like these in the Andes, all over the world. Most are shrinking.
Here in North America, millions of people experience the cryosphere every year. Eastward moving storms deposit snow, like thick paint brushes. Mountain snow packs store water. Snowmelt provides three-quarters of the water resources used in the American West. Substantial winter snows produced a green Colorado in 2003, but drier conditions the previous year limited vegetation growth and increased the risk of fires. In the Rocky Mountains, there are patches of frozen ground called permafrost that never thaw.
These regions are unusual in the mid-latitudes, but farther north, permafrost is more widespread and continuous, covering nearly a fifth of the land surface in the Northern Hemisphere. Sea ice varies from season to season and from year to year. Data show that Arctic sea ice has shrunk dramatically in the last few decades. The effects could be profound. As polar ice decreases, more open water could promote greater heating. More heating could lead to faster melting, reinforcing the cycle. If this trend continues, the Arctic Ocean could be ice-free in the summer by the end of the century. These changes in ice cover are not limited to oceans.
Greenland's ice sheet contains nearly 10% of the Earth's glacial ice. Glaciers in western Greenland produce most of the icebergs in the North Atlantic. After decades of stability, Greenland's Jakobshavn ice stream, one of the fastest flowing glaciers in the world, has changed dramatically. The ice has thinned, and the front retreated significantly. Between 1997 and 2003, the glacier's flow rate nearly doubled to 5 feet an hour. These are just some of the cryospheric processes that NASA satellites observe from space. Continued observation provides a critical global perspective, as our home planet continues to change day to day, year to year, and further into the future.
Iceberg Images
The following videos describe melting of ice sheets on Greenland and Antarctica.
Video: "CHASING ICE" captures largest glacier calving ever filmed (4:41)
Transcript: Video: "CHASING ICE" captures largest glacier calving ever filmed
Speaker 1: I'm on the phone with Jim on one of our regular check-ins. Jim, just nothing's happening.
Speaker 2: Hey, Jim. It's going well. We had some serious bouts of wind, but other than that, things are fairly well set up here. We've got some continuous time-laps.
Speaker 1: It's starting, Adam. I think Adam is starting.
Speaker 2: Oh, wait, Jim. Jim, this is the big piece is starting to cast. Let me call you back. Hold on. Okay, bye.
Speaker 1: You're still going?
Speaker 2: Yeah. In that V section right there. Holy shit. Look at that big bird rolling. All four are running, right?
Speaker 1: Yeah.
Speaker 2: Look at that. You see? Look at the whole thing.
[Ice cracking noises and ocean water rushing]
Speaker 1: The calving face is 300, sometimes 400 feet tall. Pieces of ice were shooting up out of the ocean, 600 feet and then falling. The only way that you can really try to put it into scale with human reference is if you imagine Manhattan. And all of a sudden, all of those buildings just start to rumble and quake and peel off and just fall over and fall over and roll around. This whole massive city just breaking apart in front of your eyes. We're just observers. These two little dots on the side of the mountain. We watched and recorded the largest witness calving event ever caught on tape.
Speaker 1 at a podium: So how big was this calving event that we just looked at? We'll resort to some illustrations again to give you a sense of scale.
[Music]
Speaker 1: It's as if the entire lower tip of Manhattan broke off, except that the thickness, the height of it is equivalent to buildings that are two and a half or three times higher than they are.
[Music]
Speaker 1: That's a magical, miraculous, horrible, scary thing. I don't know that anybody's really seen the miracle and horror of that. It took 100 years for it to retreat 8 miles from 1900 to 2000. From 2000 to 2010, it retreated 9 miles. So in 10 years, it retreated more than it had in the previous 100.
Video: Antarctic Wilkins ice Shelf Collapse (No Narration) (2:20)
This process has been active over much of geologic time, all except for the very warmest time periods when there were no polar ice sheets. If we were to melt all of the ice on Antarctica and Greenland, we would see a sea level rise of almost 70 meters (Greenland would cause about 6 m of sea level rise, Antarctica about 60 m). This would take melting of the relatively stable interior of the ice sheets, which will take thousands of years to occur if modern warming rates continue unabated. However, there is much we do not understand about the behavior of the more dynamic areas of the ice sheets closer to the edges, and this imparts a great deal of uncertainty to any predictions of sea level rise in the coming centuries.
New research appears all the time that shows vulnerable parts of the Antarctic ice sheet, especially its shelves. Geologists are able to use radar instruments to image the base of the ice shelf and the seabed. Ice shelves refer to places where ice overlies seawater or bedrock that is below sea level. Recently, glaciologists have found places in West Antarctica where the underlying seabed is much smoother than expected, meaning that the glacier can advance readily under the right circumstances. Moreover, some of these places are vulnerable to being heated by warm ocean currents in the future. We presented the physical evidence for ice melting in Module 2, below are before and after photos from Alaska to remind you.
The second process that is causing sea level rise on human time scales is the physical expansion of seawater as a result of temperature increase. When materials are heated, they expand and, in the case of the oceans, this causes the surface of the water to rise. This thermal mechanism can cause absolute sea level changes on the order of millimeters and centimeters per decade. It varies geographically depending on how fast the ocean is warming in individual locations, and temporally depending on variations in ocean temperatures associated with climate oscillations such as El Niño . Hard as it is to imagine with all of the press attention over melting ice, but thermal expansion may actually cause more sea level rise in the 21st century.
The following video describes how satellites provide a very detailed picture of sea level change.
Video: How to Get NASA Sea Level Rise Data | Climate Analysis Tutorial (8:07)
Transcript: Video: How to Get NASA Sea Level Rise Data | Climate Analysis Tutorial
Narrator: Sea level may seem like just another number, but it's actually one of the clearest signals we have that Earth's climate is changing. With every millimeter of ocean rise, our planet is telling us something. Now, thanks to NASA's satellite observations, we can explore sea level data more clearly than ever before. In this tutorial, we're going to explore NASA's global sea level data, how it's collected, what it reveals about our warming world, and how you can access and analyze it yourself, whether you're a GIS analyst, student, or climate educator.
Let's begin with why sea levels are rising. Scientists identify two major drivers. The first is the melting of glaciers and massive ice sheets in places like Greenland and Antarctica. As this ice melts, it adds fresh water directly into the oceans. The second factor is thermal expansion. As water warms, it expands. This increase in volume causes sea levels to rise slowly but steadily. Now, here's a striking fact. Since 1993, global sea levels have risen by about 100 millimeters. That's 10 centimeters, roughly the height of a coffee cup. It may sound small, but this rise is happening across the entire surface of the world's oceans.And it's accelerating.
NASA tracks sea-level rise using satellite missions managed by the Goddard Space Flight Center. These satellites use altimeters to precisely measure changes in sea surface height over time. This data is incredibly valuable, giving a near continuous global view of ocean changes month by month, year after year. Let's look at what this data actually shows. The first graph presents satellite measurements from 1993 to the present. It shows a clear upward trend in global sea level. As of April 2025, the rise totals 99. 5 millimeters, with a margin of error of plus or minus 4 millimeters. While there are small seasonal fluctuations, the overall pattern is unmistakable. Sea level. Sea level is rising and the rate of change is increasing.
To understand longer term trends, NASA scientists combine satellite data with tide gage measurements that date back over a century. This is where the second graph comes in. The second graph extends from 1900 to 2018 and includes both coastal tide gage and satellite data. From 1900 to about 1950, sea level rise was slow and gradual. But after 1990, the trend steepens, showing a much faster rate of increase. This acceleration is largely due to intensifying ice melt and warming oceans, direct effects of human caused climate change.
This graph also shows the different factors that influence sea level rise or fall. You'll see plus signs where rising influences occur, like mountain glacier melt, Greenland and Antarctica ice sheet loss, and thermal expansion. You'll also see occasional negative influences, like major dam projects that temporarily stored water on land instead of in the ocean. Now, let's say you want to explore this data yourself. Start by visiting the NASA Climate Vital Signs page on sea level. The address is climate.nasa.gov/vitalsign/sea level. To download the full data set, you'll need to create a free Earth data account. Simply go to urs.Earthdata.nasa.gov, sign up with your email, and once your account is verified, return to the sea level page and click the Download Data button. The data set includes a range of variables listed across 13 columns. These columns are labeled as HDR1 through HDR13. The data starts with metadata like altimeter type and the cycle number of the satellite observation. It then includes the time of measurement, the number of observations used, and the variation in global means sea level, both with and without adjustment for land motion, also known as GIA or glacial isostatic adjustment.
Some columns show the raw sea level change, while others show smooth versions of the data using a 60-day Gaussian filter to remove short-term noise. There are also versions with annual and semi-annual seasonal signals removed to help clarify long-term trends. Once downloaded, you can import this data into a data visualization tool. Thanks to open access data from NASA, we now have the tools to explore, teach, and act on the truth that our oceans are rising. As GIS professionals, educators, and students, we have the responsibility the ability to use that knowledge for good. The sea is rising, the data is here, and you now have the tools to make sense of it.
In the past, the significant sea-level rise was caused by major episodes of volcanism that added crust in the ocean basins and displaced seawater towards land. This happened when processes deep in the interior of the earth caused seafloor spreading rates to increase and massive eruptions of volcanic submarine plateaus away from the ridge. These processes occur on very long or geological time scales and are not a factor today.