What could feed humanity if a catastrophe like nuclear war blotted out the sun? One of the best options, scientists say, is seaweed.
If nuclear war broke out, burning cities and forests could emit 150 million tonnes of soot, according to previous research, dimming the sun. Temperatures could drop by 9°C (16°F) and global food production from agriculture could decline 90 per cent in the first year of nuclear winter.
Florian Ulrich Jehn at the Alliance to Feed the Earth in Disasters and his colleagues modelled how well seaweed would fare around the world under this scenario. They found that seaweed would still be able to grow up to 13 per cent per day in places such as the east Pacific.
Within nine to 14 months, seaweed cultivated on ropes between buoys could meet up to 15 per cent of the food currently eaten by humans, 10 per cent of animal feed and 50 per cent of biofuel production, according to the simulation. Expanded seaweed cultivation could avert up to 1.2 billion deaths from starvation, the team estimates.
“We will need food, and we will need a lot of it because our current food system won’t work any more,” says Jehn. “[Seaweed] is definitely one of the top candidates for a quick upscaling.”
The tropics and some of the subtropics would still have enough warmth and light for the growth of some land crops as well as seaweed, which is typically limited by nutrient availability. In a nuclear winter, cooling surface waters would sink and force nutrient-rich deep waters to rise. That would vastly expand the area suitable for growing seaweed like Gracilaria tikvahiae, a red alga that is already farmed for food in Asia.
An asteroid or comet impact or a large volcanic eruption could similarly disrupt our food systems by occluding sunlight. For example, there were widespread crop failures across the northern hemisphere in 1816, known as the “year without a summer”, after the eruption of Mount Tambora in present-day Indonesia.
The best preparation is to start producing more seaweed now, says Jehn.
A 2023 United Nations report found that farmed seaweed is a low-carbon source of protein and other nutrients that could boost food security. But it also warned that seaweed can absorb dangerous levels of heavy metals in polluted waters. And while a 10 per cent seaweed diet provided beneficial amounts of iodine, excessive intake “has the potential for adverse health effects”, the report said.
Halley Froehlich at the University of California, Santa Barbara, says that although additional seaweed production could make our food system more resilient, it is still highly uncertain how many people it could feed after a nuclear war. Scaling up would require massive amounts of spores to seed the ropes for the farms, as well as human labour that may not be available.
“Some seaweeds might thrive under eutrophic [nutrient-rich] and radioactive conditions, but it’s much harder for people,” says Froehlich.