JOHN YANG: It sounds like science fiction.

A 5,000 mile long belt of seaweed mitt weighing more than 11 million tons is sloshing around to the Atlantic Ocean.

When some of it reaches Florida, it threatens to wreak havoc in the coastal waters and on the beaches, but it is very real.

It's called The Great Atlantic Sargassum belt, so big it can be seen from space spanning the tropical Atlantic from West Africa to the Caribbean.

Earlier I talked with Ajit Subramaniam, an oceanographer at Columbia University's Lamont Doherty Earth Observatory.

AJIT SUBRAMANIAM, Oceanographer at Columbia University's Lamont Doherty Earth Observatory: So I guess it was a seaweed that grows entirely on the surface of the ocean it has never attached to land, and the Sargasso Sea is called the Sargasso Sea because of the prevalence of Sargassum in the northern part.

I guess Sunbelt is a new population of Sargassum that seems to have developed since about 2011.

We have been seeing there and satellite imagery before that.

And then we saw this explosion of a popular new population about them.

But seems to basically slosh back and forth between the coast of West Africa and the Yucatan, a Mexican coast, on the other side of the Caribbean, on an annual basis.

JOHN YANG: Is this one bed of seaweed, is it multiple plants?

AJIT SUBRAMANIAM: Actually, it is made up of individual strands.

So when you're out on a ship, we often will see a band of Sargassum maybe a couple of meters wide, so let's say 10 feet wide, and that stretches sort of disconnected, but into the horizon.

So you have these streaks that are continuous only to about 200 meters or 30 meters, each one, but then they line up one behind the other because of the wind.

JOHN YANG: And you say that this was first seen in imagery in 2011.

And it's grown a lot since then, why did it suddenly appear?

And why is it growing so fast?

AJIT SUBRAMANIAM: There are a couple of theories on that that have been published.

One has to do with the change in circulation and a deep mixing which brought nutrients to the surface in 2010. that seems -- that could have initiated this new population.

The other theory is that it is changes in agricultural or land use patterns in the Amazon River basin that has increased the flow of nutrients coming out with the river.

I personally think the boat may be partially right.

But I do not know that either one explains it completely.

So for me, it is still a little bit of a mystery as to what caused the new population.

But it is obvious it is there.

And it has been growing since.

JOHN YANG: And I also understand that in the open sea there can be benefits from this is that right?

AJIT SUBRAMANIAM: That's right.

So in the Sargasso Sea, for example, where this population has existed since before the times of Christopher Columbus, this has seen as habitat for fish.

When we are out in the ocean, we often see fish like mahimahi that are hanging out under these big rafts of Sargassum.

So, it's a very active ecosystem.

I've basically sometimes compared it to an upside down coral reef in that it's a hotspot of biological activity.

JOHN YANG: What are the threats if it gets when it gets closer to land?

AJIT SUBRAMANIAM: There are just multiple threats.

One is that when it washes up on beaches and in Barbados, I've seen piles of Sargassum five feet high and you do not want to go to the beach when it is covered with Sargassum, both because it really smells very badly when it rots.

But then people have now done studies to show that pregnant women are affected by the hydrogen sulfide that is produced due to the clotting of a Sargassum.

Methane is produced when it rots.

And that is a very potent greenhouse gas.

You also have environmental damage because while in the process of cleaning up Sargassum you have heavy trucks going on the beach and damaging the rather delicate environment that beaches.

There is increased erosion when Sargassum was washing up on the beaches and gets washed away.

It's been suggested that catching turtles have a difficult time finding their way through the massive Sargassum into the beach, and therefore, it might be affecting turtle populations.

JOHN YANG: Is there anything that can be done about this?

AJIT SUBRAMANIAM: The idea of dealing with this is to try and prevent it from beaching.

The (INAUDIBLE) is where you put it, you know, because you don't have enough land area to then go dump it someplace.

I have been working with colleagues.

We've been working on this idea that if we can collect the Sargassum when it is still in deep water offshore and sink it, then we are actually coming up with a nature based solution and basically mitigating against climate change.

Because when the Sargassum grow, they do photosynthesis, which basically means they take up carbon dioxide from the atmosphere and convert it into biomass.

And if you think that biomass to depths greater than maybe 2,000 meters, you're taking the carbon dioxide out of circulation for about 100 years from the atmosphere.

JOHN YANG: We've seen a lot of discussion about seaweed replacing plastics, and they're starting to farm it in the oceans.

Are there any ecological concerns about farming in the ocean?

AJIT SUBRAMANIAM: If you are doing this in a way to mitigate against climate change or replace plastic from seaweed, you just want to make sure that you're doing some sort of lifecycle analysis where you're not spending more carbon in harvesting, less amount of carbon, you know, I always say you can spend 100 kilos of carbon to get 10, get rid of 10.

And so that lifecycle analysis is something that needs to be done.

And people are doing research on that.

But I do not know that.

That's a simple fact that you can actually do this in a carbon efficient way if your objective is to mitigate against climate change.

JOHN YANG: Ajit Subramaniam of Columbia University's Lamont Doherty Earth Observatory.

Thank you very much.

AJIT SUBRAMANIAM: Thank you pleasure.