Atom Bombs, Ocean Mixing, and a Poisoned Fishing Crew
How the modern science of global warming got its start
On March 1, 1954, the Japanese vessel Daigo Fukuryū Maru (Lucky Dragon 5) was fishing near the Marshall Islands in the southern Pacific Ocean, about 90 miles from Bikini Atoll. The US Navy was enforcing an exclusion zone downwind of Bikini, and Lucky Dragon was staying clear. At 6:45 AM, the Castle Bravo thermonuclear bomb test was detonated on a reef just offshore from Namu Island.
Castle Bravo was supposed to produce a 6 Megaton yield, equivalent to the explosion of 6 million tons of TNT. The bomb's designers miscalculated, and the actual yield was 15 Megatons. It created far more radioactive fallout than expected and launched it further into the atmosphere. Winds changed direction and got stronger during the test. The fallout traveled far outside the exclusion zone.
For the crew of Lucky Dragon, at 6:45 the sky lit up like another sun on the horizon. 7 minutes later the sound blast hit. Two hours after that, fine ash started falling and fell for three hours. It covered the boat. They brushed it off their skin and out of their hair. The crew didn't know what it was, but they hauled their fishing gear and moved further from Bikini to continue fishing. Retrieving the gear took time. They were exposed to radioactive fallout for several hours after the ash stopped falling.
By the next day, the entire crew felt ill and decided to return to port. The trip took 2 weeks. They reached dock in Yaizu port on March 14 suffering from nausea, headaches, burns, bleeding around their teeth. Their hair was falling out, their skin was discolored and bruised. They were immediately diagnosed with acute radiation sickness and admitted to hospital. Most of the crew eventually recovered, some with scars and lingering symptoms. On September 23, Radio Operator Aikichi Kuboyama died. It is reported that his last words were "Please make sure that I'm the last victim of a nuclear bomb."
Japan responded with anger and fear. They were still dealing with the nuclear legacy of Hiroshima and Nagasaki. They were an ocean nation, tied by economy and culture to the sea. Running deep was a fear that the ocean itself might be radioactive, and the seafood that was central to much of their diet and culture might be poisonous.
The United States' responses didn't help. Japan asked for the composition of the fallout cloud, to get some idea what isotopes the men had been exposed to. They asked what materials were in the cloud and how far the fallout had spread, because there were other fishing boats in the area, and because they were worried about radioactive fallout in the fish they might bring home. The US considered this to be classified information and didn't respond. The conversations became tense.
Japan kept pressing. Their central concern (after 'you bombed one of our fishing boats!') boiled down to, "How do we know the fish we're catching and eating are safe." The US response, stripped to its essence, was "The ocean is very very large. It'll mix into the deep ocean and not be a problem." Japan said, "Prove it." Eventually, the US agreed to study how radiation from nuclear tests became mixed into the deep seas.
This was a critical moment in the movement toward banning nuclear bomb testing. It also, it turns out, launched our modern understanding that CO₂ can cause global warming.
Radioactive carbon and the chemistry of the oceans.
In 1954, Roger Revelle was a powerhouse scientist. He was in the process of transforming the Scripps Institution of Oceanography, then a quiet campus on the coast of California, from a little marine biology laboratory into the major research center it still is today. He collaborated with everybody, had fingers everywhere in the Institution's research.
Revelle had recently hired Hans Suess and Harmon Craig, who were looking at radioactive carbon in the atmosphere and in plants. Radioactive carbon, C¹⁴, is continuously created by interactions of nitrogen with sunlight in the air, and continually breaks down because it is radioactive. These processes balance, so the amount of radioactive carbon in the air stays the same over time, year to year, century to century. Buried carbon, including oil and coal reserves, starts with the same amount of radioactive carbon as the air, but loses C¹⁴ as it decays over time. The CO₂ from burning fossil fuels is tens or hundreds of millions of years old and has no remaining radioactive carbon.
Suess and Craig, working with the US Air Force, had developed methods for detecting and counting the rare radioactive C¹⁴. They could tell how old something was since the last time it absorbed carbon from the atmosphere, by measuring how much of the radioactive carbon had decayed. They could tell how old dead trees and long-buried animal bones were by looking at how much radioactive C¹⁴ was still left in them.
They noticed that young wood from trees, everywhere they looked, had less radioactive carbon than expected. This could only happen if there was less C¹⁴ in the air. And THAT could only be caused by burning fossil fuels. The carbon from oil and coal has no radioactive carbon remaining in it. When it burns it makes CO₂ with no radioactive C¹⁴, and adds it to the air. Trees get that CO₂ from the air and use it to make new wood. By measuring the unexpected reduction in C¹⁴ in young wood from the trees, they were indirectly measuring how much of the carbon in the trees came from oil and coal emissions. This was one of the first confirmations that human emissions were changing the chemistry of the atmosphere.
One of Revelle's interests was trying to understand the carbon chemistry of the oceans. This is extremely complicated, involves dozens of different carbon-containing chemicals that change from one to another. Ocean mixing is a key part of the equation. CO₂ from the air and at the ocean surface has to be carried by moving water before it can interact with carbon deeper in the ocean.
No one knew how fast this mixing happens, and Revelle was keenly interested in finding out. He thought that by following the extra 'stable' CO₂ that Suess and Craig had found in the modern atmosphere, as it moved from the air into the ocean, they could perhaps measure how fast that happened, and then how fast the ocean surface mixed with deeper water.
Between them, these three men had the perfect set of skills to see how radioactive fallout from nuclear tests would mix into the deep oceans. In 1955 the United States hired Revelle, and his new associates Suess and Craig, to monitor the aftermath of an undersea nuclear test explosion several thousand feet deep in the south Pacific ocean. They completely expected that Revelle would see the radioactive fallout rapidly mixing into deep waters. They would be able to reassure the Japanese, continue thermonuclear testing without the pressure they were getting from other nations, and everything would be normal again.
What does this have to do with global warming?
We've known that CO₂ is a greenhouse gas since the experiments of John Tyndall in 1859, 160 years ago. By the 1950s, many scientists had worked on the science, and it was generally accepted that increasing CO₂ in the air would cause the planet to warm.
Almost no one thought warming would happen though, for one simple reason — the oceans are very, very large. CO₂ moves freely back and forth between the air and the oceans. The oceans mix, moving the CO₂ to deeper water. We could keep emitting large amounts of CO₂ into the air, it was thought, and the oceans would absorb it. The amount in the air would change only a little, and slowly. If CO₂ in the air can't increase, then it can't cause warming. There seemed to be no problem.
Revelle was aware of this. He knew of the work that showed CO₂ is a greenhouse gas, and that showed how much the earth would warm if there were more of it. He knew how much humans were adding to the atmosphere. He knew from Suess' and Craig's work on excess 'stable' carbon in trees, that human emissions had already detectably changed the earth's atmosphere. He also thought like everyone else - but he wasn't sure - that ocean mixing would likely remove excess CO₂ to the deep oceans.
Everything converged on understanding ocean mixing.
Understanding ocean carbon chemistry requires understanding ocean mixing. The question of whether fossil fuels will cause the planet to warm is in large part the question of how fast the ocean surface mixes with deep ocean waters. Revelle was deeply interested in the question of mixing. And the US government, because of their need to reassure the Japanese government after the Daigo Fukuryū Maru nuclear accident, had just hired Revelle to measure ocean mixing.
What they found changed the science of climatology. As we came to understand their results, they let us know that CO₂ emissions and the risk of global warming were a serious problem.
What Revelle found.
What Revelle and his team did was measure radiation levels from the surface to hundreds of feet deep, around the blast zone. What they found was simple. Near the site of the explosion, as one would expect, the radiation levels were chaotic and mixed. The bomb blast itself stirred up the ocean. But outside the blast zone, radioactive fallout spread through the ocean in very thin layers, hundreds or thousands of feet under the surface, but sometimes only 1-2 meters thick, and extending out from the blast site for hundreds of square kilometers.
Imagine a large bowl full of water. Put a single drop of red food coloring halfway down, right in the middle, representing the radiation from an undersea blast. Now take your fist and punch from the surface tight through that drop of red. In the middle, where you punched, the coloring will mix rapidly. Further out, it will mix in tendrils and twists, curling and turning, up and down from top to bottom. This is what everyone, including Revelle, expected to see.
Now imagine that same bowl of water, but this time when you punch it, the red dye spreads in flat paper-thin sheets, with completely clear water between, never moving out of that sheet or mixing with any other water in the bowl. No one, including Revelle, expected this. But that, as impossible as it would be in your imaginary bowl, is how the ocean mixes. Sideways, in flat sheets. Not up and down. Revelle observed that there is very little vertical movement in the oceans.
He quickly realized his result means that CO₂ from the air only mixes into the surface ocean waters, where it can easily evaporate back into the air. It can’t easily mix to the deeper ocean. The amount of CO₂ in the air, to nearly everyone's surprise, must be increasing as we emit more and more of it from burning fossil fuels.
Revelle at the same time was also working with Craig, following the excess 'stable carbon' in the air, to measure the movement of CO₂ into the oceans. They and others found that on average, a molecule of CO₂ stays in the atmosphere only 10 years before it gets absorbed in the seas. But more importantly, their work on carbon chemistry showed that once CO₂ goes into the ocean, most of it very quickly goes right back to the air. Surface waters can only hold a limited amount of CO₂.
Together, these results told us that less CO₂ than expected gets absorbed into the ocean surface waters. Far less CO₂ than expected moves from the ocean surface to deep waters. The CO₂ stays in the atmosphere, where it can trap heat. And that means global warming, unexpectedly, is a problem.
Revelle calculated expected rates of CO₂ increase, and by 1960 started saying publicly that we might see the effects of global warming as soon as the end of the century. Among the people who listened was one of his students, Al Gore. And, it turns out, Roger Revelle was right.
Measuring CO₂
Following very quickly after this, scientists realized they needed to know how much CO₂ is actually in the air. In 1958, Charles Keeling founded the CO2 Instrument at the Mauna Loa Observatory in Hawaii. The instrument takes continuous CO₂ measurements from the well-mixed air high on the mountaintop, in the middle of the Pacific Ocean. Despite occasional threats to funding, it has been running continuously for 62 years now. It is perhaps the most important experiment in all of climate science, and its existence follows directly, by way of Roger Revelle, from the radiation poisoning of the crew of the Daigo Fukuryū Maru.
It is sensitive enough to see the earth's' breathing', an annual cycle of CO₂ increasing and decreasing as plants in the northern hemisphere grow and decay from season to season. Over those 62 years — almost exactly my lifetime — the Keeling observatory has documented an increase in the earth's atmospheric CO₂ from 315 to 415 PPM. This is what that looks like:
The Daigo Fukuryū Maru Memorial
Science is often strange. Reality has a way of making itself known. Enough people were paying attention to ocean and carbon chemistry, that we eventually would have known these things. But the tragedy of the crew of the Daigo Fukuryū Maru, and the pressure from their experience that led to direct measurements of these key ocean processes, certainly clarified and sped the process. It also added greatly to the international pressure that eventually ended nuclear tests.
The Daigo Fukuryū Maru was kept isolated until 1976, when the radioactivity on board had decayed enough to make it safe for viewing. It is now in the Tokyo Metropolitan Daigo Fukuryū Maru Exhibition Hall, maintained by the Japanese government and open to the public as a memorial to the effects of nuclear warfare. Perhaps, if you get the chance, visit and pay respects to both the tragedy of that day, and to the science it helped launch.
