Climate change is an immediate 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 we can 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.
Rising sea levels not only affect coastal communities by reducing land mass, but also point to the wider problem of rising global temperatures. This means that sea level rise is of great importance to NASA, which not only looks at other worlds, but also observes the Earth from space. A new sea level monitoring satellite, Sentinel-6 Michael Freilich, was launched in November 2020 and became an official reference mission for sea level rise in March this year, after taking over from its predecessor Jason-3.
With Sentinel-6 taking on its new role and double successor Sentinel-6b waiting on its wings to take over 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 above the Earth right now by tracking it with the help NASA’s Eyes web application.
We spoke with Josh Willis of NASA’s Jet Propulsion Laboratory, a scientist in both the Sentinel-6 and Jason-3 projects, about how to measure sea level rise from space.
Rising sea levels are not just important for understanding the changing oceans. It is also one of the most valuable tools we have for measuring climate change in general. “In a way, it’s a scorecard,” Willis said. “This is our map of how we deal with climate.”
This is because much of the rising average temperature on the planet as a whole is reflected by the level of the oceans. There are three major man-made factors that contribute to rising sea levels: the melting of ice sheets in Greenland and Antarctica, the melting of small glaciers elsewhere on the globe, and the warming of the ocean, causing it to expand. These factors contribute approximately one-third to the overall rise due to the addition of more water to the oceans when glaciers and ice caps melt, as well as the expansion of 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 rising sea levels are the clearest indicator of human intervention in the climate,” Willis said. “The oceans cover two-thirds of the planet’s surface, they absorb 90% of that extra heat, which is the whole cause of climate change, and they also absorb all the water that melts from glaciers and ice sheets. So they really take everything into account in terms of how we change the climate in the greatest possible way. “
And the problem is not just that sea levels are rising. That is, the pace of this rise is also growing.
“The rate of rise in the oceans is not stable. It’s actually increasing, “Willis said. “In the early 20th century, the oceans were rising at a rate of about two millimeters a year. By the 1990s or 2000s, it was more like three. And now it’s four and five millimeters a year. So the growth rate has more than doubled in the last hundred years. And it will continue to grow faster and faster. “
Part of the reason why 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 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 are placed in the same orbit so that they get the same view of the oceans.
Each time a new satellite takes over from its predecessor, the two fly close to each other for months. This allowed very careful calibration to ensure that the data could be tracked sequentially in the five satellites so far.
“It’s really an amazing achievement for our climate research,” Willis said.
“We have this incredible record, which is already 30 years long, and the Sentinel-6 was created to extend that record by another 10 years,” Willis said. To allow these additional 10 years of observations, NASA has built not one but two satellites, both essentially identical, so that after the recently launched Sentinel-6 Michael Freilich ended its life in 2025, its twin Sentinel-6b could will take over. This will allow a total of 40 years of consecutive sea level rise records.
“For the first time, we as a community have decided to do this in the long run – to commit to measuring sea level rise from space, not just one satellite at a time,” he said. “The sea level will not stop rising soon, we will not be able to stop measuring it, so we must have this continuity of measurements in the missions.”
If these satellites appear to have a relatively short lifespan compared to other satellite missions that may last for decades, this is due to the altitude at which they operate. When the first sea level satellites were launched, we did not have such good technology for determining the position of these satellites – and positioning data are important for obtaining accurate readings at sea level. To allow this, the satellites were launched into a very high orbit of 13,000 kilometers, where there is very little atmosphere and therefore very little radiation protection.
Researchers want to continue sending satellites into the same orbit to provide continuous measurements, but this means accepting that these satellites will be hit by radiation and will last only a few years each.
The accuracy of the readings is what allows the Sentinel-6 satellite to be an internationally recognized instrument for measuring sea level rise. Researchers from different fields and different countries have agreed that measurements made by Sentinel-6 and its predecessors will be used as a standard measurement for sea level rise.
Sentinel-6’s tools are relatively simple, at least conceptually. There is radar that sends radio waves down to the surface to measure the distance between the satellite and the ocean, positioning systems that provide information about the height of the satellite so that it can be taken out of measurements at sea level, and then another important an 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 must correct this and provide a high level of accuracy for measuring sea level.
These three instruments, along with successive orbits, are what make Sentinel-6 the most accurate method we have for measuring sea level rise – and therefore accurate enough to be an international reference mission.
The more complex 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 a few square kilometers to allow this.
But there are other factors that affect sea levels. This includes weather, as changes in atmospheric pressure allow the sea to rise when the pressure is low, tides and ocean currents, and even the gravity of the underwater mountains, which cause peaks to rise above them. Researchers using Sentinel-6 data to measure sea level rise should take these other factors into account, taking into account atmospheric weather data and ocean gravity maps.
However, all these other effects can provide useful data for other areas of research. By looking at the average reading for an area, researchers can estimate how big the waves are and how strong the winds are. They can see the currents moving across the ocean in real time, as the currents cause the ocean to tilt so that one side of the current is higher than the other. They can also track debris or oil as it spills into the ocean.
The satellite also continues to collect land-based data, and this data can be used to monitor lakes and rivers.
With projects like Sentinel-6, we can directly see the way our climate is changing because of our human activities. We see that not only sea levels are rising, but that they are rising faster and faster, and there is no indication that this change will slow or stop any time soon. There is existential horror in this.
“As we watch what happens to the planet, it’s scary,” Willis said. “We have already taken our climate to unfamiliar territory. And every year it becomes more and more unknown. ”
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 it’s something we can do something about,” Willis said. “We know what the problem is and we know almost how to fix it. It’s not like there is a giant meteor heading for Earth that will destroy us all. In fact, we can do something about climate change, we just need to gather the will. “
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