Seaweed farming is the fastest-growing force in global food production today. But while giant marine algae can produce tons of fuel and sustenance, scientists have recently pointed out that its ability to offset the climate crisis is largely unknown.

To sink just 1 gigaton of carbon emissions a year, recent simulations suggest massive seaweed farms would have to cover 1 million square kilometers of the ocean's most productive areas.

For comparison's sake, about 1.6 million square kilometers of US land is currently used for agriculture.

According to a new study led by researchers at the University of California, Irvine, to achieve a sweep of seaweed this extensive would require current coastal farming efforts to expand by 370-fold.

And that would only get us a fraction of the way towards our international climate goals, which rely on between 4 and 13 gigatons of carbon removal from the atmosphere each year to limit warming below 1.5 °C above pre-industrial levels.

Moreover, as seaweed farms swell in size, their yields may diminish. Researchers estimate that even if seaweed farms took over a quarter of the world's coasts, the resulting carbon sink could max out at 3.5 gigatons per year.

That's because not all coastlines are equally productive. Plus, high-density, intensive farms of seaweed in once fertile areas can deplete the surrounding ocean of nitrate, possibly "straining the inventory of near-surface ocean nutrients and disrupting the natural biological carbon pump," the researchers say.

Compared to smaller farms, where nitrate remains abundant, larger farms could deplete the potential of seaweed harvest worldwide by a median of 90 percent.

And that's just considering one nutrient crucial to algae survival; there are probably other limiting factors, too.

The new simulations are based on idealized scenarios, researchers admit, but even these simplified estimates provide some possible constraints of seaweed farming.

"The purpose of this work is not to advocate for the widespread deployment of seaweed farms over a substantial fraction of the global oceans, as we expect this would come with unacceptable trade-offs to ocean health, but rather to assess the geographic distribution and potential of offshore seaweed farming to produce harvestable biomass at climate-relevant scales," the researchers explain.

In 2021, scientists at the US National Academy of Sciences, Engineering, and Medicine estimated that seaweed farming around the nation's coasts could extract about 0.03 gigatons of carbon annually.

The new study suggests it is well below the practice's full global potential. But even then, that lower target would require increasing the current seaweed cultivation area by over tenfold in the most productive regions.

Nor do any of these estimates examine what happens to all that seaweed once it is harvested.

For instance, how do these swathes of floating greenery impact fishing, shipping, traffic, and marine protected areas?

Some companies have proposed simply sinking the seaweed once it's grown to ensure its carbon remains trapped in the ocean. But what will that do to deep ocean ecosystems?

At the moment, we simply have no idea. And there are concerns that such a strategy could exacerbate hypoxic, or 'dead', zones. So while the new study suggests seaweed has the potential to contribute to a multi-prong approach for carbon drawdown, many potential problems are left to assess.

"Given that there remain many unknowns and hurdles for large-scale seaweed farming, our analysis suggests that refinement of the global seaweed cultivation potential warrants investment in future research," the researchers write.

Companies that claim to offset their emissions with seaweed are hiding behind a climate solution that hasn't been thoroughly thought through.

The study was published in Nature Communications Earth & Environment.