Deciphering climate messages via the heart of the atom

Vemork Hydroelectric Plant at Rjukan, Norway, which Hans Suess advised on heavy water production, telling Nazi Germany it couldn’t make heavy water quickly enough for military use. His expertise with heavy water was part of an interest in nuclear science that led him to become a pioneer in carbon dating.

When Hans Suess chose to study physical chemistry, he went nuclear, apparently overturning two generations of family tradition. Hans was born in 1909, just as his father Franz succeeded his grandfather Eduard as a geology professor at the University of Vienna. Hans got his PhD from the same university in 1936, but in studying heavy water he was set to aid the historic advances in nuclear science of the time. Yet a transatlantic scientific coincidence would bring him back to more environmental science, and see him help pioneer radiocarbon measurements. With that expertise, Hans showed humans were raising atmospheric CO2 levels, and revealed another surprising source of variations in climate.

The common theme to these achievements was how neutrons and protons combine in an atom’s nucleus. For example, hydrogen atoms found in conventional water have just a single proton in their nuclei. In heavy water, some of these atoms are replaced by a rarer form of hydrogen, known as deuterium, whose atoms have an extra neutron in their nuclei. That gives heavy water properties that can help nuclear reactors, which Nazi Germany notoriously hoped to exploit to make nuclear weapons.

With Hitler’s armies occupying Austria just two years after Hans finished his PhD, his expertise brought him to the attention of the Nazi regime. They called him in to advise a hydroelectric power plant in Vemork, Norway, that was making heavy water. Hans visited several times, reporting that it couldn’t make heavy water quickly enough for military use. Allied forces destroyed it in 1943 anyway, in audacious raids fictionalised in the film “Heroes of Telemark”.

Alongside working with heavy water, Hans studied why the chemical elements exist in the amounts that they do. The answer laid in how stable different numbers of protons and neutrons are when they come together in nuclei. He continued this work after the Second World War in West Germany, helping develop the “Nuclear Shell Theory” explanation, which other scientists won the Nobel Prize for Physics for in 1963. Suess missed out on this acclaim partly because two teams came up with the explanation at the same time. But when the other team, based at the University of Chicago, invited him to visit, Hans’ life changed course towards unravelling the secrets of Earth’s history.

The carbon dating game

A modern carbon-dating setup used to date samples more than 40,000 years old.

Willard Libby, also at the University of Chicago had just invented carbon dating and was just writing his first paper on his method, which would also win him a Nobel Prize. Carbon dating relies on the fact that as well as carbon-12, the dominant and stable form, there’s carbon-14, which is heavier and unstable as it has two extra neutrons. Carbon-14 slowly decays to nitrogen and would soon run out, if cosmic rays from space didn’t keep reacting with nitrogen in the atmosphere to make it. That keeps a fairly steady amount in the air, and so when trees are taking up CO2 to grow, they contain specific ratios of carbon-12 to carbon-14. But when trees die, the ratio in their wood progressively decreases, at around 14 atoms per minute per gram of pure carbon. That regular tick can be used as a clock to work out the time since the tree died.

Hans learned the method from Willard, and set up a lab at the US Geological Survey in Washington, DC, to develop and use it in his own research. His early results came from stumps or logs of trees in the northern US knocked over by sheets of ice that advanced across the country around 20,000 years ago. But when looking at more modern samples, he found a strange effect: the ratios of the types of carbon had been changing. He soon realised that part of the explanation was coal burning. Having taken a very long time to form, coal has no carbon-14, and so neither does the CO2 that it makes when burned. That reduces the ratio of carbon-14 to carbon-12 in air, giving direct evidence that burning fossil fuels adds carbon to the atmosphere. And although radiocarbon dating was still establishing its reliability, the finding lent weight to the idea that human fossil fuel burning was warming the planet, championed by Guy Callendar.

New school thinking

Hans Suess helped understand why and how much radiocarbon dating varied when compared to ages given by counting tree rings. Image from Suess, 1965, used via Creative Commons license, see reference below.

Meanwhile, an oceanographer named Roger Revelle had worked out that the oceans can absorb CO2 from the air. He was also busy growing the University of California, San Diego (UCSD) out of his research institute, the Scripps Institution of Oceanography. Seeing the power of radiocarbon dating, in 1955 he invited Hans to join him as a professor at the new university. Together they worked out that oceans only absorb a limited amount of the CO2 humans add to the air by burning fossil fuels. Their findings made Roger and Hans keen to know more about “the large scale geophysical experiment” burning coal had kicked off. And, powered by their enthusiasm, they helped find the cash that let Charles Keeling go on to do detailed measurements on CO2 in air.

At UCSD, Hans also continued his carbon dating work, particularly on the puzzle of why wrong ages were still regularly cropping up. He resolved this problem by comparing trees’ radiocarbon ages against the age given by counting tree rings. Comparing them showed that carbon-14 production varies regularly, usually rising by about one percent in about 20 years, then decreasing again over a century. That affects the carbon dating ratio, with trees growing when more carbon-14 is being made appearing younger. By 1980, Hans had taken his tree-based calibration curve back 8,000 years, making carbon dating a precise tool for dating organic materials, and tracking carbon’s movement through ecosystems.

The curve also helped confirm a controversial possible cause for the variations in carbon-14 production: changes in the Sun’s activity. A strong piece of evidence came from two periods in the 15th and 17th centuries, during a cold interval known as the ‘Little Ice Age’. This period had been especially hard to carbon-date, and Hans showed that’s because carbon-14 production was particularly high. If the Sun was less active – a shocking idea at the time – perhaps its magnetic field was weaker, letting more cosmic rays through into our atmosphere and turning more nitrogen into carbon-14. Records showed far fewer sunspots than normal during this period, giving the quiet Sun idea extra backing. The cooler temperatures during that time could also have been because the Sun was giving out less heat. So despite seemingly turning away from studying the Earth at an early age, by his retirement Hans had helped highlight two important influences on our climate: CO2 and solar activity.

Variation of carbon-14 levels in the atmosphere over the last 8000 years, as determined by Hans Suess. The bottom axis shows the date, on a scale of hundreds of years BC and AD, and the variation in carbon-14 concentration is along the vertical scale in parts per thousand. Image from Suess, 1980, used via Creative Commons license, see reference below.

Further reading:

Spencer Weart’s book, ‘The Discovery of Global Warming’ has been the starting point for this series of blog posts on the key players in the history of climate change.

The US National Academies Press has published biographical memoirs of Hans Suess by his coworkers.
Suess, H. (1954). U. S. Geological Survey Radiocarbon Dates I Science, 120 (3117), 467-473 DOI: 10.1126/science.120.3117.467
Suess, H. (1955). Radiocarbon Concentration in Modern Wood Science, 122 (3166), 415-417 DOI: 10.1126/science.122.3166.415-a
Stuiver, M; Suess, H. E. (1966). On The Relationship Between Radiocarbon Dates And True Sample Ages Radiocarbon, 534-540
Suess, H. E. (1980). The Radiocarbon Record in Tree Rings of the Last 8000 Years Radiocarbon, 22, 202-209