Graphing Sea-Level Trends

In this activity, students will use sea-level rise data to create models and compare short-term trends to long-term trends. They will then determine whether sea-level rise is occurring based on the data.

• For this activity, 130 years of sea-level measurements have been divided into 20 data sets for examination. Depending on class size, the steps in “Procedures” can be done by individuals or groups of students. Students will examine all 20 data sets to learn how scientists study data over short and long periods of time. Some groups or individuals may need to examine multiple data sets in order to model all 20 sets.
• If you plan to have students transfer data into spreadsheet software, have them use the text files to do so. Otherwise, if students don’t have the skills to transfer the data into a spreadsheet, create graphs using the CSV (comma separated values) files.
• Depending on students’ familiarity with spreadsheets, the activity may need to be split into two days: one day for entering the data into spreadsheets and one day for analyzing the data.

Using a variety of methods, scientists have concluded that global sea level – the average height of the sea surface across the planet – has varied substantially throughout history, especially in response to the ice ages. In recent history, starting around 7,000 years ago, sea level became quite steady, but over the last century, it’s been rising. Global tide measurements from tide gauges suggest the global sea level rose approximately 3.4 millimeters (0.13 inches) per year over the past century. The New York City area, alone, has experienced roughly a foot of sea-level rise in the past century. (That’s measured at a tide gauge near Battery Park just off the southern tip of Manhattan. While tide gauges are one way we measure sea level, changes in global sea level should not be confused with tides. Tides are the regular rise and fall of the sea surface and make it appear as if the water level of the ocean is always changing. But tides represent normal changes in the sea level that we expect to see daily and monthly.)

How do we know sea level is rising? Sea level is measured by monitoring stations on the shoreline and at sea. Satellites also collect data on the height of the sea level. There are more than 120 sea-level monitoring stations in the US and 240 additional stations worldwide. By looking at data from these stations over periods of 30 years or more, trends can be identified at individual stations and compared with other stations. This gives scientists useful information about local conditions. Those data can also be used to calculate the global average sea level and study it over time, giving scientists a picture of what’s happening to the ocean on a planet-wide scale. Sea level has been measured at some stations for more than a century, providing data about sea level going back to 1880.

It’s important to note that when sea level rises, the total amount of water on Earth isn't increasing. Instead, the volume of liquid that fills the ocean basins is growing, raising the elevation of the sea's surface and spilling ocean water onto low-lying land. The extra volume of seawater comes from two places:

• Melting ice sheets and glaciers on land add water to the sea. (See the “What’s Causing Sea-Level Rise: Land Ice Vs. Sea Ice” activity for more information about how melting ice contributes to sea level rise.) Of the total contributed by glaciers to rising sea levels, the Antarctic ice sheet may contribute about 10 percent, Greenland’s ice sheet adds about 30 percent¹, and the lower latitude small glaciers and ice caps, which make up only about four percent of the total land-ice area, have added as much as 60 percent since the 1990s (Meier, et al., 2007).

• The second, and less obvious, cause for rising sea level is thermal expansion. Water expands as it warms, so the more heat energy the ocean absorbs, the more space its water requires. It turns out the ocean can absorb a lot of heat, so as the atmospheric temperature rises, so does the ocean temperature.
  

Atmospheric temperature is rising on land, near the ocean’s surface and in the lowest layer of the atmosphere as a result of increased levels of CO₂. Since the beginning of the Industrial Revolution, the concentration of carbon dioxide in the atmosphere has increased by 39 percent. The majority of CO₂ from human activity is created by burning fossil fuels that we use for heating, electricity and transportation. Much of this CO₂ will stay in the atmosphere for thousands of years, trapping heat in the atmosphere as a result of the greenhouse effect.

So global sea level is currently rising as a result of both ocean thermal expansion and glacier melt, with each accounting for about half of the observed sea-level rise, and each caused by recent increases in global mean temperature. From 1961 to 2003, sea level rose due to thermal expansion and glacier melt (small glaciers, ice caps, ice sheets) at a rate of 1.11 millimeters (0.04 inches) per year². Between 1993 and 2003, the contribution to sea-level rise increased for both sources to 2.79 millimeters (0.11 inches) per year (IPCC AR4 WG1, 2007). So, not only is sea level rising, but also it is rising at a faster rate than in the past. Currently, it is rising at a rate of over three millimeters per year.

• Why is it important to look at sea level rise data over long periods of time?
  Short-term data might indicate GMSL is falling when it is in fact rising.
• What is causing the rise in GMSL that the data indicates?
  • Melting land ice. (Students may have misconceptions about how land ice contributes to sea-level rise and sea ice does not. Complete the “What’s Causing Sea-Level Rise? Land Ice Vs. Sea Ice” lesson for a concrete example.)
  • Thermal expansion. (Complete the "Thermal Expansion Model" lesson for a concrete example.)
• What is causing the melting of land ice and the thermal expansion of ocean water?
  A rise in land, air and sea temperature.
• What is causing the rise in land, air and sea temperature?
  An increase in CO₂ in the atmosphere. CO₂, a greenhouse gas, traps heat that would otherwise be able to escape into space, thus raising global temperatures.
• Where is the CO₂ coming from?
  Human sources of increased CO₂ include burning fossil fuels to generate electricity and power automobiles, planes, trains and other forms of transportation. Livestock (cattle, poultry, pigs, etc.) account for other sources of human-related greenhouse gases.
• We use energy to heat and cool our homes. The main source of that energy involves burning fossil fuels and releasing CO₂. What can be done to reduce the energy used in heating and cooling?
  Moderate thermostats to cut off heat at lower temperatures and cut off cooling at higher temperatures. Insulate homes as much as possible. Plant shade-producing plants and trees.
• We use electricity to power appliances. What can be done to reduce the amount of energy used in homes, workplaces or schools?
  Think about what’s being left on and being turned off when not in use. Consider how phantom charges impact energy use. Connect appliances drawing a phantom charge to power strips and turn them off when not in use.
• We use cars for the majority of travel. What can be done to reduce the amount of CO₂ released into the atmosphere when we travel and commute?
  We can take public transportation, carpool or use alternative transport such as bicycling and walking.
• Who else can make a difference?
  Students may consider contacting local, state and federal representatives.
• What else can be done?

• Complete the GMSL student record sheet.

In addition to causing sea-level rise, the increase in global temperatures linked to higher CO₂ concentrations leads to more heat waves and greater area affected by drought. We also see an increase in heavy precipitation events and flooding on land. Warm surface waters can damage coral reefs, reducing opportunities for fishing and tourism, and leave coasts vulnerable to storm surges and erosion.

How can you make a difference? Calculate your carbon footprint to find out just how much carbon you’re contributing to the environment. Then, create a plan of action to reduce your impact.

• Climate Change Lessons and Activities from JPL Education – These standards-aligned lessons feature NASA missions and science and explore the changes happening on Earth, from sea-level rise to global warming.
• NASA Climate Change website – Explore news, mission data, images and vital signs that track the changes happening on Earth.
• NASA Climate Change social media – Follow the latest news and updates from NASA Climate Change on: Facebook | Twitter | Instagram | YouTube
• NASA Sea Level Change Portal
• Images of Change – Compare satellite images taken at different periods to see how Earth is changing.
• Infographic: Sea-Level Rise
• Infographic: Earth's Carbon Cycle Is Off Balance
• Climate Kids website – Games, projects, and activities all about Earth and climate for children between the ages of 8 and 13.