Sunday, April 3, 2011

Roger Revelle

The investigation into atmospheric CO2 and its interaction with the oceans is a long and complex story. Dozens of oceanographers and chemists contributed in this work, and the chemistry is also very complex. However, Roger Revelle is the central figure in this field of research. At least I consider him so. At any rate, he was the first to show that mankind's addition of CO2 to the atmosphere would not be absorbed by the oceans quickly, and was able to work out why.

This had been a question since Arrhenius. It was known since the Challenger Expedition of the 1870s that the oceans contain large amounts of CO2, and that oceans are alkaline world-wide. The Challenger expedition showed that the oceans contained ~50 times as much CO2 as the atmosphere. One of the main objections to anthropogenic global warming is that the oceans are alkaline---and should therefore absorb CO2 easily.

But proponents of anthropogenic global warming, such as Arrhenius, Alfred Wallace, and Callendar raised an interesting question. If the oceans really could absorb all the anthropogenic emissions of CO2 easily, why didn't the oceans absorb all the CO2 that is in the air now? In other words, since the oceans hold 50 times as much CO2 as the atmosphere, why didn't the oceans just absorb the 51st molecule, and then have the Earth freeze into a snowball?

This was a nagging question for oceanographers, but that scientific field was consumed by another controversy. As I wrote in a previous blog entry, the oceans have contained roughly the same salt concentration as today for billions of years. Once it was realized that the Earth was billions of years old, the main question for oceanographers was how do the oceans get rid of their salt? Even now some aspects of that question have not been solved, although we now have a broad picture of how salt can be evaporated and buried under sediments in shallow seas and estuaries. But during much of the 20th century, the salt question was the major question in oceanography.

Roger Revelle (1909-1991) was an oceanographer with the Scripps Institute of Oceanography. During the mid 1950s he was part of a team studying how fast the oceans 'turn over', the seawater at the surface sinking to the depths and deep ocean waters rising to the surface. This was suddenly an important question. The Japanese were in an uproar over nuclear testing and radioactive pollution of their Pacific fishing grounds.

There had been rising anxiety in Japan already about nuclear fallout (Japan had great nuclear anxiety in any case from their experience with the atomic bombing of Hiroshima and Nagasaki less than 10 years earlier.) In 1954 two incidents occurred.
The first, and most serious, was the irradiation of the Daigo Fukuryƫ Maru (q.v) and later that year, the release of the movie Gojira, which we know as Godzilla, rushed into production after the Daigo Fukuryƫ Maru incident.

The United States Navy rushed a study to find out how fast the oceans 'turned over' and carried radioactive fallout to the depths. Revelle and his team determined that the oceans turned over over several hundred years (that is a bit wrong---we no know that the oceans turn over in about 3,000 years). That data showed the oceans turn over fast enough to absorb and remove CO2 from anthropogenic emissions. And yes, 3,000 years is fast enough also to dissolve most CO2 in the oceans and keep atmospheric CO2 from rising much.

But Revelle went further. The question of why the oceans didn't absorb all CO2 nagged at him. And his research in the field gave him knowledge and access to a tool oceanographers didn't have. The nuclear tests in the Pacific created lots of radioactive carbon isotopes. Carbon isotopes as great as C-22 and as low as C-8 were created. Most of these had a half life of microseconds or less. But C-11 (carbon 11) has a half-life a little over 20 minutes. Revelle didn't use the C-11 created by nuclear explosions---almost all would be gone in a couple days--too quick to visit an explosion site, with lots of other longer-lived radioisotopes around. But C-11 did give him an idea---create CO2 using C-11 and see how it interacted with ocean water. It was radioactive enough to be very easily traceable, but not too fast to decay immediately. And also, in a day or two almost all the C-11 would be gone. So it wasn't a disposal hazard.

The chemistry he found was amazingly complicated. Seawater is not just salt, it is a complex soup of many thousands of chemicals dissolved within it. And it also has living organisms. So there are thousands of reactions that CO2 can make with the different chemicals in seawater.

It had been suspected that the oceans had a buffering mechanism. What Revelle found was that in many cases, CO2 combined with chemicals in the seawater and created volatile compounds that promptly evaporated back into the air. Once back in the atmosphere, the CO2 would encounter free oxygen, or be dissociated by ultraviolet light, and create CO2. When he raised CO2 concentrations slightly, to 350 ppm or 400 ppm in the atmospheric samples over the tanks of seawater, molecules containing the radioactive C-11 were returned back to the air in significant amounts, while significantly less C-11 remained in the seawater. C-11 decays too quickly for longer studies, so Revelle switched to C-14, with a half-life of 5,730 years. In 1955-56 he determined that when CO2 in the atmosphere increased, about half of what the oceans absorbed would be evaporated out via volatile organic compounds within a year.

In a paper he co-authored with Dr. Hans Seuss (1909-1993) (no, not that Dr. Seuss) Revelle wrote in a few sentences at the end that assuming that CO2 emissions stayed at 1957 levels, CO2 would rise in the atmosphere about 40% (to 440 ppm) over the next few centuries and stabilize.

This was mind-blowing. 440 ppm was a big rise! And certainly enough to warm the Earth's climate considerably! Revelle was not a climatologist or meteorologist, and did not realized the implications of what he had written. But others did.

It created a big scientific controversy. Unlike today, it played out in scientific circles and was largely unreported to the public. Objections were made, and then refuted. Perhaps the C-11 (with a half-life of 20 minutes, it is very radioactive) was killing the seawater microbes and they were releasing volatile organic compounds when they died. But research by others using C-14 quickly showed this was not the case. By 1960 Revelle's work was accepted.

One of the scientists Revelle worked with was Dr. Charles Keeling. Keeling, whom we all know, was inspired by Revelle to create his famous Mauna Loa carbon dioxide measuring observatory. It also combined with other work that I will be talking about in another blog entry, about how CO2's apparent saturation in its IR bands was not really saturated after all to show that increasing CO2 in the atmosphere would result in global warming. This has been the consensus for the past 50 years, and is the consensus now.

Revelle's guess about future CO2 concentrations in the atmosphere was a gross underestimate. He assumed that CO2 emissions would remain close to their 1957 levels. At the time, that was not a ridiculous assumption.

During the previous generation, there had been two terrible world wars. Continuous, progressive, exponential economic growth had not been the reality. In 1950, industrial production in the Soviet Union, Germany, France, Italy and Japan was lower than it was in 1913! There was no real reason to expect that the future would be different from the past. What Revelle didn't realize was that beginning in the mid 1950s the world had entered an unprecedented economic boom---with CO2 emissions more than doubling between 1957 and 1972. From 1973 to the late 1990s, emissions slowed in their rate of increase, but did not stop rising. And the growing boom in China and India has caused CO2 emissions to rise more rapidly in the past 15 years than during the pause from the mid 70s to the mid 90s.

In short CO2 emissions rose on an annual basis

1957-1973 6%
1974-1997 2%
1998-2011 3.5% (4%+ 2005-2010, despite the recent recession)

The combination of unprecedented economic growth, Revelle's CO2 chemistry work, Keeling's Curve, the discovery that IR absorption by CO2 was not saturated led by 1965 to a scientific conference in Boulder, CO about anthropogenic global warming--the first scientific conference with that as the main topic. I'll blog about that soon.

Revelle's work was also incomplete. He identified some of the main chemical pathways CO2 dissolved in the oceans returns to the atmosphere in volatile compounds. But many other oceanographers and chemists have been working since to identify other pathways--there are tens of thousands of them! Individually, most are trivial, but together they make a significant fraction of carbon dioxide atmospheric return. They vary by differences in temperature, local differences in chemicals dissolved in the oceans, and of course, different organisms present in the surface waters.

Also, some of the major chemical reactions that return CO2 to the atmosphere had already been discovered during the 1940s and 1930s. But it wasn't realized that they were a major part of the flux of carbon between the atmosphere and oceans. What Revelle largely did was discover that these reactions returned a large amount of carbon to the atmosphere quickly. There is a distinction between discovering a chemical reaction and discovering its magnitude and importance.

One alarming thing is that this return of carbon dioxide to the atmosphere is a powerful feedback. The warmer surface temperatures rise, the more quickly organic compounds evaporate and return to the atmosphere. Also, as temperatures rise, more compounds become able to evaporate.

Research shows, and is unanimous in agreement, that for each 1 C° rise in the surface ocean temperature, the return rate of CO2 to the atmosphere increases by 7%.

When I said unanimous, I was not quite right. All models show an increase from 6-8% per degree Celsius. What's really interesting is that if the increase is closer to 8% then more CO2 will be returned to the atmosphere, warming it faster, and cause more CO2 to be emitted by the oceans. This has big implications on temperature and oceanic acidification. It results in a damned either way situation. A slightly faster CO2 evaporation rate will result in faster warming, but less oceanic acidification. A slower CO2 evaporation rate will result in less warming, but more acidification. A difference between 6.2% and 7.8% is pretty close agreement, but running the calculations out to 2100 and beyond results in quite different states for the atmosphere and the oceans. As always, we need more research!

Roger Revelle

I will add further information in this blog entry later on some of the chemical pathways of CO2 return to the atmosphere.

1 comment:

  1. Remember my entry on Lewis Fry Richardson?

    I talked about how he modeled casualties of war, and found that war casualties follow a power law. That's been updated with research about how casualties vary through time after unusually deadly events.