Monday, December 4, 2023

This is how NASA measures rising sea levels from space

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

Sea level rise not only affects coastal communities by reducing landmass, but also indicates the broader issue of rising global temperatures. This means that sea level rise is very important for NASA, which not only looks at other worlds, but also controls 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 takeover of its predecessor, Jason-3.

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

We spoke with Josh Willis of NASA’s Jet Propulsion Laboratory, a project scientist for both Sentinel-6 and Jason-3, on how to measure sea level rise.

Our scorecard for climate change

This map shows sea level measured by the Sentinel-6 satellite Michael Freilich from June 5, 2021 to June 15, 2021. Red areas are regions where 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 for understanding the changing oceans. It is also one of the most valuable tools we have to measure climate change overall. “In a way, it’s a scorecard,” Willis said. “It’s our scorecard on how we’re doing with the climate.”

This is 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 rising sea levels: the melting of the ice sheet in Greenland and Antarctica, the melting of small glaciers elsewhere around the globe, and the warming of the ocean that is causing it to expand. These factors contribute about one-third each to the overall increase, due to more water being added to the oceans as glaciers and ice sheets melt, as does the expansion of the water due to rising global temperatures. Because the oceans cover so much of the globe, they end up absorbing much of the excess heat generated by human activity.

“I think sea level rise is the clearest indicator of human interference with the climate,” Willis said. “Oceans cover two-thirds of the planet’s surface, absorb 90% of this extra heat, which is the cause of climate change, and absorb all the water that melts from glaciers and ice sheets. So they really count everything as we change the climate in the biggest picture. “

And the problem is not just that sea level is rising. Is that the rate of this rise is also increasing.

Diagram showing 102.3 millimeters of sea level rise since 1993.

“The rate at which the oceans are rising is not constant. It’s actually increasing, ”Willis said. “In the early part of the 20th century, oceans were rising at a rate of about two millimeters a year. By the 1990s or 2000s, it was more than three millimeters. And now it’s four and five millimeters a year. So the the rate of increase has more than doubled in the last hundred years. And it will continue to grow faster and faster. “

30 years of continuous measurements

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

In order to maintain consistent data comparable over the years, all missions in this series have been placed in the same orbit so that they receive the same view of the oceans.

Whenever a new satellite took over from its predecessor, the two would fly close together for months. This allowed for a very careful adjustment to ensure that data could be tracked consistently across the five satellites so far.

“It’s a really amazing achievement according to our climate science record,” Willis said.

Another 10 years of measurements

The artist's interpretation of the Sentinel-6 Michael Freilich satellite.

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

“This is the first time that we as a community have decided to do this in the long run – to commit to making measurements of sea level of the cosmos, not just one satellite at a time,” he said. “Sea levels will not stop rising soon, we will not be able to stop measuring it, so we need to have this continuity of measurements across missions.”

If these satellites appear to have a relatively short lifespan compared to other satellite missions that could last for decades, this is related to the altitude at which they operate. When the first sea-level satellites were launched, we didn’t have such good technology to determine the position of those satellites – and positioning data is important to get accurate sea-level readings. To allow this, the satellites were launched into a very high orbit of 1,300 kilometers, where there is very little atmosphere and therefore very little protection from radiation.

Researchers want to continue sending satellites to this same orbit to ensure lasting measurements, but that means accepting that these satellites will be hit by radiation and will last only a relatively few years each.

Instruments on the satellite

The accuracy of its values ​​is what allows the Sentinel-6 satellite to be the internationally recognized tool for measuring sea level rise. Researchers from all different fields and different countries agreed that the measurements taken by Sentinel-6 and its predecessors would be used as the standard measurement for sea level rise.

The instruments on Sentinel-6 are relatively simple, conceptually at least. It is the radar that sends radio waves down to the surface to measure the distance between the satellite and the ocean, the positioning systems that give information about the altitude of the satellite so that it can be subtracted from sea level measurements, and then one more important instrument. . called a radiometer.

The radiometer measures the amount of water in the atmosphere by looking at the brightness of the ocean. The water in the atmosphere affects the radio waves sent by the radar, so the radiometer is required to correct this and ensure high accuracy for sea level measurements.

These three instruments, along with the consequent orbits, are what 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 values ​​over an area of ​​several square kilometers to allow this.

But there are other factors that also affect sea levels. This includes the weather because changes in atmospheric pressure allow the sea to dome up when pressure is low, the tides and sea currents, and even the gravity of underwater mountains, which make peaks appear at sea level above them. Researchers who use Sentinel-6 data to measure sea level rise should consider these other factors when considering atmospheric conditions data and maps of the ocean’s gravitational field.

All of these other effects, however, can provide useful data for other fields of research. By looking at how much reading has been averaged over a given area, researchers can estimate how big the waves are, and how strong the winds are. They can see how currents move across the ocean in real time, as currents cause the ocean to tilt so one side of the stream is higher than the other. They can also track debris or oil when it is spilled into the ocean.

The satellite also continues to collect data during land crossings and this data can be used to monitor lakes and rivers.

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

The threat of climate change

With projects like Sentinel-6, we can see firsthand how our climate is changing because of our activities as humans. We can see that not only are sea levels rising, but they are rising faster and faster, and there is no indication that this change will slow down or stop any time soon. There is an existential terror to that.

“As we look at what’s happening to the planet, it’s scary,” Willis said. “We have already pushed our climate into unfamiliar territory. And it is becoming 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 this is something we can do something about,” Willis said. “We know what the problem is, and we know almost how to fix it. It is not like a giant meteor heading for Earth that will wipe us all out. We can really do something about climate change, we just have to do it. “

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