This is how NASA measures rising sea levels from space

Climate change is an immediate threat to the future of humanity, but the global climate is so complex that it is often difficult to see the magnitude of the problem. One of the best ways we have to measure climate change may not be what you expect — because it’s not about measuring the land or the atmosphere. Instead, we need to measure the ocean to learn more about climate change.

Sea level rise is not only impacting coastal communities by reducing land mass, but also points to the broader problem of rising global temperatures. That means that sea level rise is of great importance to NASA, which not only looks at other worlds, but also monitors the Earth from space. A new sea level monitoring satellite, Sentinel-6 Michael Freilich, was launched in November 2020 and became the official reference mission for sea level rise in March this year, following the acquisition of its predecessor, Jason-3.

With Sentinel-6 taking on its new role and a twin successor, Sentinel-6b, waiting in the wings to take over when needed, we are set for the next 10 years of sea level measurements. You can even see for yourself where Sentinel-6 is above Earth right now by tracking it using NASA’s Eyes web application.

We spoke with Josh Willis of NASA’s Jet Propulsion Laboratory, project scientist for both Sentinel-6 and Jason-3, about measuring sea level rise from space.

Our Climate Change Scorecard

This map shows the sea level measured by the Sentinel-6 Michael Freilich satellite from June 5-15.  Red areas indicate areas where the sea level is higher than normal, and blue areas indicate areas where it is lower than normal.
This map shows the sea level measured by the Sentinel-6 Michael Freilich satellite from June 5, 2021 to June 15, 2021. Red areas are areas where the sea level is higher than normal and blue areas indicate areas where it is lower than normal. NASA Earth Observatory

Sea level rise is not only important to understand the changing oceans. It is also one of the most valuable tools we have for measuring climate change as a whole. “In a way it’s a scorecard,” Willis said. “It’s our scorecard for how we’re doing with the climate.”

That’s because much of the rising average temperature of the planet as a whole is reflected by the level of the oceans. There are three major man-made factors contributing to sea level rise: the melting of the ice sheets in Greenland and Antarctica, the melting of small glaciers elsewhere in the world, and ocean warming causing the ocean to expand. These factors each contribute about a third of the total increase, as more water is added to the oceans as glaciers and ice sheets melt, as well as expansion of the water due to rising global temperatures. Because the oceans cover such a large part of the world, they end up absorbing much of the excess heat generated by human activity.

“I see sea level rise as the clearest indicator of human interference in the climate,” Willis said. “Oceans cover two-thirds of the planet’s surface, they absorb 90% of this extra heat that is the whole cause of climate change, and they also absorb all the water that melts from the glaciers and ice sheets. So they really count everything in terms of how we’re changing the climate in the most general way.”

And the problem isn’t just that sea levels are rising. It is that the rate of that increase is also increasing.

A map showing 102.3 millimeters of sea level rise since 1993.
NASA

“The rate of rise of the oceans is not constant. It’s actually increasing,” Willis said. “In the early 20th century, the oceans were rising at a rate of about two millimeters per year. In the 90s or 2000s there were more than three. And now it’s four and five millimeters a year. The rate of increase has therefore more than doubled in the past hundred years. And it will increase faster and faster.”

30 years of continuous measurements

Part of the reason sea level rise data is so valuable is that it represents a long-term record collected since the 1990s. The first global measurements of the oceans from space began when the TOPEX/Poseidon mission was launched in 1992, followed by the three Jason satellites and then the first Sentinel.

To maintain consistent data that can be compared over the years, all missions in this series have been placed in the same orbit so that they have the same view of the oceans.

Whenever a new satellite took over from its predecessor, the two flew close together for months. This allowed for a very careful calibration to ensure that the data to date could be consistently traced across the five satellites.

“It’s a really great achievement in terms of our climate science record,” Willis said.

10 more years of measurements

An artistic rendering of the Sentinel-6 Michael Freilich satellite.
NASA

“We have this incredible record that is now 30 years old, and Sentinel-6 is built to extend that record for another 10 years,” Willis said. To accommodate these additional 10 years of observations, NASA has built not one but two satellites, both essentially identical, so that once the recently launched Sentinel-6 Michael Freilich comes to the end of its life in 2025, its twin Sentinel-6b it can take over. This will enable consistent records of sea level rise for a total of 40 years.

“It’s the first time we as a community have decided to do this for the long term — committing ourselves to take measurements of sea level rise from space, not just one satellite at a time,” he said. “The sea level isn’t going to stop rising any time soon. We won’t be able to stop measuring it, so we need to have this continuity of measurements across missions.”

If it appears that these satellites have a relatively short life expectancy compared to other satellite missions that can last for decades, it has to do with the altitude at which they operate. When the first sea-level rise satellites were launched, we didn’t have very good technology to determine the position of those satellites — and position data is important to get accurate sea-level readings. To make this possible, the satellites were launched into a very high orbit of 13,000 kilometers, where there is very little atmosphere and therefore very little protection from radiation.

Researchers want to keep sending satellites to the same orbit to ensure continuous measurements, but that means accepting that these satellites will be affected by radiation and will only last a relatively few years.

Instruments on the satellite

The accuracy of the measurements makes the Sentinel-6 satellite the internationally recognized instrument for measuring sea level rise. Researchers from all different fields and different countries agreed that the measurements from Sentinel-6 and its predecessors would be used as the standard measurement for sea level rise.

The tools on Sentinel-6 are relatively simple, conceptually at least. There is the radar, which sends radio waves to the surface to measure the distance between the satellite and the ocean, the positioning systems that provide information about the satellite’s altitude so that it can be subtracted from the sea level readings, and then another important instrument radiometer. called.

The radiometer measures the amount of water in the atmosphere by looking at the clarity of the ocean. The water in the atmosphere affects the radio waves emitted by the radar, so the radiometer must correct this and ensure a high degree of accuracy for sea level readings.

These three instruments, along with the consistent orbits, make Sentinel-6 the most accurate method we have for measuring sea level rise — and therefore it is accurate enough to be the international reference mission.

The changing oceans

Mayflower Autonomous Ship alone in the ocean
Oliver Dickinson for IBM/ProMare

The more complicated part of measuring sea level rise is how to interpret the data collected by the satellite. The oceans are not flat, so the satellite averages the readings over an area of ​​several square kilometers to make this possible.

But there are also other factors that influence sea levels. This includes weather, as changes in atmospheric pressure allow the sea to solidify when the pressure is low, the tides and ocean currents, and even the gravity of underwater mountains, causing peaks in sea level to appear above. The researchers using Sentinel-6 data to measure sea level rise will need to account for these other factors by considering atmospheric conditions data and maps of the ocean’s gravitational field.

However, all of these other effects could provide useful data for other areas of research. By looking at how much a measurement is averaged over a certain area, researchers can estimate how big the waves are and how strong the wind is. They can see in real time how currents move through the ocean because currents cause the ocean to tilt so that one side of the current is higher than the other. They can also detect debris or oil when it is spilled into the ocean.

The satellite will also continue to collect data as it passes over land, and this data can be used to monitor lakes and rivers.

All of the data collected by the satellite is publicly available and used by researchers around the world from a wide variety of fields. You can find the data on the JPL website or on NASA’s Earth data website.

The threat of climate change

With projects like Sentinel-6, we can immediately see how our climate is changing as a result of our activities as humans. We can see that not only are sea levels rising, but they are also rising faster and faster, and there is no evidence that this change will slow down or stop in the short term. There is an existential terror in that.

“If we look at what’s happening to the planet, it’s scary,” Willis said. “We have already pushed our climate into uncharted territory. And it gets more and more unknown every year.”

However, he does not despair of the future of mankind. Rather, he emphasizes that the future of our planet is in our own hands.

“There’s still room for hope because we can do something about this,” Willis said. “We know what the problem is, and we know as well as how to fix it. It’s not like there’s a giant meteor headed for Earth that’s going to wipe us all out. We can certainly do something about climate change, we just have to muster the will.”

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