Headline: Remembering Paul Crutzen: A Grand Scientist who took on the Grand Challenges of the Anthropocene

Paul Crutzen
Wolfgang Filser (archive of the Max-Planck-Society, Berlin-Dahlem)

Paul Crutzen would sometimes become disturbed by where a discussion was heading, and with a wry half-grin would impatiently throw in a completely unanticipated intervention to shake things up – often with far-reaching consequences. Sometimes he made these interventions figuratively, through his numerous ground-breaking scientific publications.

Other times his interventions were literal, most famously at a meeting where he repeatedly heard people use the geological term characterising our time as the “Holocene”, and suddenly blurted out: “We’re not living in the Holocene; we’re living in the Anthropocene!” That once seemingly strange term – “Anthropocene” – has now become a widely-used characterisation of our current epoch, indicating the impact of humans on the Earth system globally and likely lasting for geological timescales.

Paul Crutzen, who left us last month, on 28 January 2021, aged 87, was certainly one of the most important Earth system scientists of the Anthropocene.

I had the incredible privilege and pleasure to interact with him closely for nearly 30 years, starting as a PhD student, then as a postdoc and research group leader, and later still visiting him regularly after I took a position as director at the IASS in Potsdam.  I got to visit him for the last time about three weeks before he passed away, and we talked about the plight of the world with the COVID-19 pandemic, and the vision of a “healthy Anthropocene”.

Paul’s work covered a wide range of topics. His earliest work focused on stratospheric ozone chemistry, through which he helped develop an understanding of the potential effects of human activities, including supersonic aircraft, on the Earth’s ozone shield. His work also provided a basis for understanding the Antarctic ozone hole, for which he was awarded the Nobel Prize in Chemistry in 1995, together with Mario Molina and Sherry Rowland.

Paul contributed substantially to understanding air pollution from various sources, like ocean-going ships and forest fires, and in polluted regions like Southern Asia. He also addressed numerous other issues, like the threat of a “nuclear winter” and the impacts of emissions of nitrous oxide (N₂O) – a potent greenhouse gas – from agro-biofuel production.

And of course, there was his landmark essay in Climatic Change in 2006 considering stratospheric aerosol injection geoengineering, seen by many as the key paper breaking the taboo that many felt against researching and discussing this deeply controversial idea. This was another of Paul’s very unanticipated interventions.

Paul and I reflected back on the story of that paper in one of his last scientific papers.

Prior to his 2006 essay, he had repeatedly heard policy makers pitting air pollution against climate change. It was already well known at that time that sulfur dioxide (SO₂) and subsequently produced sulfate particles have substantial impacts on human health, ecosystems, visibility, structures and monuments. Nevertheless, policy makers would say that they couldn’t afford to reduce their SO₂ emissions, because that would “unmask” more of the global warming due to increasing greenhouse gas levels, which was being partly reduced (“masked”) by reflective sulfate particles and their impacts on clouds.

We wrote:

At that time, which was during the run-up to the fourth assessment report of the IPCC [2007], with the rapidly growing awareness and acceptance of the seriousness of global warming and its anticipated impacts, policy makers were hesitant to do anything that would exacerbate global warming.

This contrast between beneficial and harmful effects was the main premise for Paul’s essay in Climatic Change in 2006, in which he wrote:

This creates a dilemma for environmental policy makers, because the required emission reductions of SO₂, and also anthropogenic organics (except black carbon), as dictated by health and ecological considerations, add to global warming and associated negative consequences, such as sea level rise, caused by the greenhouse gases.

Paul thus deliberated on the possibility of hypothetically shifting the SO₂ pollution injections from the Earth’s surface to the stratosphere (above about 20 km altitude), which would produce sunlight-reflecting sulfate aerosol particles. These particles would in turn cool the climate, compensating at least partly for the reduced cooling if surface-level SO₂ emissions were stopped.

Despite the fact that Paul emphasised that this would not be something to consider lightly – only as an absolute last resort, if all other measures to reduce global warming were failing – a draft of his paper nevertheless resulted in a tremendous backlash from colleagues who were deeply concerned that such a discussion might get in the way of or at least distract from efforts towards reducing carbon dioxide (CO₂) emissions.

Paul definitely worried about that. But he also felt that the efforts to reduce climate change – focused primarily on reducing CO₂ emissions – were getting in the way of efforts to help directly save human lives by reducing a main contributor to air pollution, namely SO₂ emissions.

He took this debate very seriously, and modified his paper extensively in numerous iterations with me and several other colleagues, but he decided to still push through with it. Interestingly, the narratives and counterarguments took various unexpected twists and turns afterwards, so that the original policy dilemma that irked Paul eventually made way for other narratives and dilemmas (more about this is in our 2017 paper).

Paul’s effort certainly seemed to open up the field of research on climate geoengineering, as was shown nicely in a study by Oldham et al. More about the developments in the field since that time can be read in a 10-year anniversary special issue of the journal Earth’s Future in 2016.

But was “breaking the taboo” really worth it?  In reflecting on it for our 2017 paper, Paul and I came to the conclusion that:

… given the balance of results of model studies over the last decade … and the challenging directions that this implies both for future research and also for socio-political aspects, especially public perception and the development of good governance principles, we have to conclude that the overall verdict is still out. The responsibility still resides with the scientific community to conduct research and engage in the broader dialogue in a responsible way, so that whatever the outcome, historians will hopefully look back and conclude that it was indeed of value – and in that sense a moral imperative – to begin carefully investigating this topic at this point in our history.

Of course, this is where C2G comes in. Along with many other worldwide efforts focusing on climate geoengineering, it is taking the responsibility seriously to help develop the understanding and governance frameworks that are needed to ensure that “breaking the taboo” was indeed worth it.

Paul Crutzen made many significant contributions like this during his lifetime – truly a great Anthropocene scientist. But he was also much more than that. He was a very enthusiastic, kind, and generous person who will be fondly remembered by all those who knew him. I feel that I speak for many colleagues who will miss Paul immensely, and we will be forever grateful for our many collective years of fruitful collaboration with him – and I know there are many who join me in extending our sincere condolences to his family, especially his wife Terttu and his daughters and grandchildren, and to his many friends and colleagues around the world.


This blogpost was published first on the website C2G.

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