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As a SciShow viewer, you can keep building your STEM skills with a 30 day free trial and 20% off an annual premium subscription at Brilliant.org/SciShow. Can you spot the difference between these two pictures?

You’re probably saying “Stefan, they’re the same picture”, and in this case, you’d be right. But while the ocean’s color looks blue to our eyes, there’s a whole rainbow of colors we can’t make out. The ocean’s color is tied to its health, and a single, unique satellite has detected that it's changing.

And of course, climate seems like it’s to blame. [♪ INTRO] Water isn’t really blue, of course. What we perceive as the ocean’s color is a function of the light it absorbs and reflects, and blue is what ends up getting reflected to our eyes. But that absorption and reflectance can change depending on the ocean’s properties.

After all, It’s not like the ocean is only water. Particles in the water, like sand or silt, will bring in more red hues. And phytoplankton growing in the water are naturally green thanks to chlorophyll, so when they’re growing in abundance, the ocean looks greener.

So when the ocean’s color changes, it’s for a reason. Even though the changes are generally too subtle for our eyes to pick up, it still means there’s something going on. Scientists are especially interested in the amount of that green from chlorophyll.

Since phytoplankton form the base of the ocean’s food web, scientists hypothesize that “green-ness” can stand in for the health of the ecosystem. But collecting that data is a lot harder than going “yeah, looks green!” To get these data, we’d have to have some way of viewing the entire ocean at once, and measuring its color on a regular schedule. Since the ocean is enormous, the only way we can really do that is by satellite observations.

But while we do have a lot of satellites up in space, they all have different instruments and detectors, which means their datasets aren’t always compatible with each other. And while we can sometimes correct for that using math, every correction introduces more error. Which we’re not crazy about, because chlorophyll data is already messy.

It’s based on phytoplankton abundance, and even without climate change or any other trends, that varies a lot from year to year and between seasons. This matters because the more variable a parameter naturally is, the longer you need to measure it to start seeing larger trends that mean anything statistically. And when it comes to chlorophyll, researchers estimate we’d need 30 years of data, all collected by the same instrument and in the same format, to get anything useful out of it.

And stuff in Earth’s orbit doesn’t tend to last that long, with shoutouts to notable exceptions like the Hubble Space Telescope. But that leads to a conundrum: We’d like to know how chlorophyll levels change to understand the ocean’s health, but that… might not be a knowable thing. Not until the next satellite plus thirty years, anyway.

Unless you could find something a heck of a lot cleaner to measure than chlorophyll. A UK- and US-based research group say they’ve found just such a metric: a parameter called remote-sensing reflectance, or Rrs. Rrs is a measurement of the wavelengths of light that bounce off the ocean surface and return to the satellite.

It sort of averages several colors, from red to blue, and produces a single number that’s tied to the ocean’s overall color. Kind of like a hex code for seawater. Unlike chlorophyll, which only corresponds to one wavelength of light, Rrs collects data from multiple wavelengths.

Single wavelengths tend to be more variable than multiple wavelengths, so Rrs data tends to be less messy than chlorophyll data. And the team figured out that you can get good results with “only” about 20 years worth of data. As luck would have it, we do have Rrs satellite data going back 20 years, thanks to an instrument called the Moderate Resolution Imaging Spectroradiometer, or MODIS, aboard NASA’s Aqua satellite.

This is one of the oldest oceanography satellite programs still up and running. Over the years, it’s been used to measure sea level rise, global ice cover, land cover changes, and more. The Aqua satellite is one of a pair that make up the overall mission, and it’s been up there since 2002.

Its mission was planned to last only six years, but now NASA’s hoping they can make it to 2026. And one of the parameters MODIS-Aqua collects is Rrs! Which means we go from not really knowing when or if we’ll be able to give the ocean a checkup based on its color, to having the data sitting right there, thanks to MODIS.

After looking at the Rrs data, the researchers realized that the ocean has been getting steadily greener since 2002, but it’s hard to say exactly why that’s been happening. The green color isn’t directly related to sea surface temperature, at least not according to statistics. But interestingly enough, the color change does line up with what climate models have said we should expect to see.

Those climate models predicted a change of color over about 50% of the ocean’s surface, which is exactly what the Rrs data is showing. That means that even though we aren’t sure exactly how, this change in color is probably related to climate change. This makes sense, because we know by now that climate change is more than just rising temperatures.

A lot of different parameters are changing at once, and any one of those parameters or any combination of them could be responsible for the color change. It does seem pretty likely that the green-ification is related to phytoplankton and plankton ecosystems in general. But it’s hard to see that from a satellite.

So if what we learn is so broad, why do we actually care that the ocean’s turning green? Well these findings can tell us two main things. They can tell us where ocean communities are changing, which tells us which parts of the ocean need a bit more attention.

With this information, we can start to make predictions about the health of fisheries in the affected areas, or where we might need to set up protected areas. They also tell us that the amount and distribution of plankton is changing over parts of the ocean. That could be a bit worrying, because those plankton are responsible for storing a lot of carbon.

The ocean has already taken up about a quarter of our carbon emissions, which means that without the ocean, the climate crisis would be a lot worse than it already is. So we need to keep an eye on any change in the ocean’s carbon storage system. This change in ocean color could be a warning sign, providing clues about where we should be taking closer looks.

It’s also a big screaming “we were right” that suggests our computer models are correctly projecting how the climate crisis is going down. And we wouldn’t have known about any of it if not for Aqua’s MODIS, an instrument that kept recording for way longer than anyone expected. And not only did that instrument keep collecting data for so long, but it kept sending the data back to us on Earth!

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