This is how NASA measures sea level rise from space | Digital trends

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

Sea level rise not only affects coastal communities by reducing land mass, but also points to the larger problem of rising global temperatures. This means that sea level rise is of great importance to NASA, which not only looks to other worlds, but also monitors Earth from space. A new sea level monitoring satellite, Sentinel-6 Michael Freilich, has been launched in November 2020 and has become the official reference mission for sea level rise in March of this yearafter taking over from 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 on Earth right now, tracking it using NASA Eyes Web App.

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

Our dashboard for climate change

This map shows sea level measured by the Sentinel-6 Michael Freilich satellite from June 5, 2021 to June 15, 2021. Red areas are regions where sea levels are above 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 evolution of the 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 dashboard for how we’re doing with the climate.”

This is because much of the increase in average temperature for the planet as a whole is reflected in sea levels. Three major man-made factors contribute to sea level rise: the melting of ice caps in Greenland and Antarctica, the melting of small glaciers elsewhere in the world, and the warming of the ocean causing it to expand. These factors each contribute about a third of the total increase, due to the increase in water added to the oceans when glaciers and ice caps melt, as well as the expansion of water due to the increase in global temperatures. As 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 climate,” Willis said. “The oceans cover two-thirds of the planet’s surface, they absorb 90% of that extra heat that’s causing climate change, and they also absorb all the water that’s melting from glaciers and ice caps. So , they really matter everything in terms of how we change the climate in the most comprehensive way.”

And the problem is not just that the sea level is rising. It is that the rate of this increase also increases.

A graph showing 102.3 millimeters of sea level rise since 1993.

“The rate of rise of the oceans is not constant. It’s actually on the increase,” Willis said. “At the beginning of the 20th century, the oceans were rising at a rate of about two millimeters per year. In the 90s or 2000s, it was more like three. And now it’s four and five millimeters a year. Thus, the growth rate has more than doubled over the past hundred years. And it will continue to increase faster and faster.

30 years of continuous measurements

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

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

Each time a new satellite took over from its predecessor, the two would fly together for months. This allowed for very careful calibration to ensure data could be plotted consistently across all five satellites to date.

“It’s really an incredible achievement in terms of the record of our climate science,” Willis said.

Another 10 years of measurements

Artist's impression of the Sentinel-6 satellite Michael Freilich.

“We have this incredible record that now stands for 30 years, and Sentinel-6 was designed to extend that record for another 10 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 recently launched Sentinel-6 Michael Freilich reaches the end of its life in 2025, its twin Sentinel-6b will be able to take over. . This will allow a total of 40 years of consistent records of sea level rise.

“This is the first time that we have decided as a community to do this for the long term – to commit to measuring sea level rise from space, and not just one satellite at a time,” said he declared. “Sea levels aren’t going to stop rising anytime 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 could 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 such good technology to determine the position of these satellites – and positioning data is important for getting accurate sea level readings. sea. For 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 against radiation.

Researchers want to keep sending satellites into that same orbit to ensure continued measurements, but that means accepting that those satellites will be battered by radiation and last relatively few years each.

Instruments on the satellite

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

Sentinel-6’s instruments are relatively simple, at least conceptually. There’s the radar, which sends radio waves up to the surface to measure the distance between the satellite and the ocean, the positioning systems which give information about the satellite’s altitude so that it can be subtracted from the measurements of the sea ​​level, and then another important instrument called a radiometer.

The radiometer measures the amount of water in the atmosphere by observing the brightness of the ocean. Water in the atmosphere affects the radio waves sent by the radar, so the radiometer must correct for this and ensure a high level of accuracy for sea level measurements.

These three instruments, together with the coherent orbits, make Sentinel-6 the most accurate method we have for measuring sea level rise – and that’s why it’s accurate enough to be the international reference mission. .

The changing oceans

Mayflower autonomous ship alone in the ocean
Oliver Dickinson for IBM/ProMare

The most 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 readings over an area of ​​several square kilometers to account for that.

But other factors also affect sea level. This includes weather, as changes in atmospheric pressure allow the sea to rise when pressure is low, tides and ocean currents, and even the gravity of the mountains beneath. -marines, which cause spikes in the sea level above them. Researchers using Sentinel-6 data to measure sea level rise must account for these other factors when considering atmospheric conditions data and ocean gravity field maps.

All of these other effects may, however, provide useful data for other areas of research. By looking at the average of a reading over a given area, researchers can estimate the size of the waves and the strength of the winds. They can see how currents move through the ocean in real time, as currents cause the ocean to rock, so one side of the current is higher than the other. They can also track debris or oil as it is dumped into the ocean.

The satellite also continues to collect data as it passes over the earth and this data can be used to monitor lakes and rivers.

All data collected by the satellite is publicly available and is used by researchers around the world in a wide variety of fields. You can find the data at JPL website Or on NASA Earth Data Website.

The threat of climate change

With projects like Sentinel-6, we can directly see how our climate is changing due to our human activities. We can see that not only is sea level rising, but it is rising faster and faster, and there is no indication that this change will slow down or stop anytime soon. There is an existential terror to this.

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

However, he does not despair of the future of humanity. Rather, he insists that the future of our planet is in our hands.

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

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