Picture it – you’re an 18th Century sailor navigating the ocean south of Java when, out of nowhere, the sea is lit up by a luminescence so bright it lights up the night sky overhead. Billows of rolling black clouds appear lit from beneath making the sky appear to be hewn in two by a streak of brilliant white light. Four hundred years ago, it would have all seemed quite… well, apocalyptic.

Today, we call this phenomenon ‘milky seas’, but more than 300 years ago these were occurrences that would go on to be passed through stories and folklore.

Admittedly, researchers in the 21st Century still don’t know much more about these peculiar happenings than sailors and ship captains did all those centuries ago. However, thanks to the tenacity and pioneering foresight of a pair of scientists from Colorado State University, we may just be on the pathway to better understanding this most awe-inspiring natural phenomenon.

Based on 400 years of historical documents, including eyewitness reports from ship logs and diary entries spanning the centuries alongside modern day satellite imagery, Justin Hudson, a doctoral student in atmospheric science at Colorado State University has compiled the world’s most comprehensive global database of ‘milky sea’ events to help cast new light on a subject most ironically shrouded in the veil of mystery.

So, what are milky seas? It’s a pertinent question that – given the rare nature of their occurrences and the minimal research conducted into the phenomenon to date – is the most critical to answer. The simplest conclusion can be drawn from those years of eyewitness accounts, that this rare ocean occurrence is a ‘vast, steady glow’ – something similar to bioluminescence – but blankets (according to some accounts) vast swathes of the ocean, as much as 100,000-square-kilometres – the size of Iceland – and can last for weeks.

“We strongly suspect – based on characteristics of the light and how long it lasts – that these are caused by bacteria,” said Hudson. “But it’s a question of, how do you take some of the smallest organisms on Earth and have them engage in some behaviour that’s half the size of the state of Colorado?”

Milky seas have been notoriously impossible to predict. Even today, with advances in technology and satellite technology, just when the next one will occur is information that continues to elude researchers. 

While they have been documented in eyewitness accounts for the past 400 years, only one sample of ‘milky sea’ ocean water has ever been collected. And that was by accident, in 1985.

“According to the data we have on that, they found this bioluminescent bacterium called Vibrio harveyi around there, which lines up pretty well with what the glow of this milky sea looks like,” said Hudson. “So, that’s why we strongly suspect this.”

Hudson’s database merges 240 credible accounts dating back to the 1600s with modern, low-light satellite imagery, giving scientists their first real opportunity to forecast where and when the phenomenon will next appear.

So, what does that tell us? Where do milky seas occur most frequently? Analysis shows that almost 60% of all documented displays occurred in a triangle surrounded by Somalia, Yemen’s Socotra Island, and the Maldives, while a second cluster appears to span Indonesia’s Java and Banda Seas.

The team also found a clear statistical link between milky-sea sightings and two major climate oscillations: the Indian Ocean Dipole and the El Niño-Southern Oscillation. It’s been spotted that when those patterns push warm, nutrient rich surface water into the region, the odds of a glow rise sharply.

“The best way to describe a milky sea to a modern viewer would be if you imagine those glow-in-the-dark stars that children put on their ceiling in their bedroom – that kind of green, pale, even glow in every direction where you look on the ocean,” said Hudson. “Imagine you’re on a boat in the middle of the night, there’s no lights, only the stars above. Then suddenly the ocean, which normally was very, very dark compared to the sky, just becomes this bright green-white colour. And everywhere you look is just that even green.”

This description differs from plankton’s flickering blue pinpoints found along many coastlines; instead milky seas appear to emit a uniform white or pale-green light – bright enough, according to some accounts, to read a logbook by and in some cases, to be visible from space.

In fact, the study’s co-author Professor Steven Miller has tracked these glows from orbit for over a decade. Talking to Interesting Engineering, he has described his work as ‘bridging knowledge across the centuries.’

“Sailors have described these waters since the 17th Century, but we’ve lacked a way to bridge folklore and physics,” he said. 

The database – comprising entries from the Marine Observer journal and century-old merchant-ship logs, aims to therefore forge that bridge by offering modellers ‘time-stamped positions and weather notes’ that can be cross-checked against satellite detections.

Better understanding milky seas is – according to Hudson – a critical issue. 

“A couple of things that I think drive why we should look into them is that the bacteria that we suspect causes milky seas, this Vibrio harveyi is known as a pest species and it can negatively infect fish and crustaceans,” he explained. “For people who farm those organisms, having a Vibrio infestation could wipe out their stocks.

“In the region where milky seas happen, fishing is one of the main economic activities.”

An alternative theory suggested by Hudson comes with rather more positive connotations, and it’s to do with the Earth’s carbon cycle. 

“One level of the Earth’s carbon cycle involves how bacteria break down certain chemicals or compounds in the water or the soil, and how they move that to the next part of the carbon cycle,” he continued. 

“We have no idea what milky seas mean in terms of that movement or partitioning of carbon in the ocean. It could mean absolutely nothing. It could mean this is actually a really good event in terms of capturing more carbon and sequestering it in the ocean, or it could be a massive carbon release.”

The hope now is that the database will be used to trigger a coordinated, rapid-response cruise. Instruments could measure oxygen levels, plankton species, nutrient chemistry, and the genetic fingerprint of any luminous bacteria. 

“The next step is trying to use this database to predict when milky seas will occur,” said Hudson. “Even being able to say that next year is a good time to look for milky seas in this region would be a massive step forward in our understanding.”

Hudson and Miller’s co-authored paper – From Sailors to Satellites: A Curated Database of Bioluminescent Milky Seas Spanning 1600 – Present – has been published in the scientific journal Earth and Space Science.

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