Svensmark: Supernova explosions control life and climate on Earth

Illustration: Greg Stewart/SLAC National Accelerator Laboratory

Powerful explosions from dying stars, also called supernovae, have for billions of years controlled the conditions for complex life on Earth and played a far greater role in our climate than we have thought. Henrik Svensmark, astrophysicist and senior researcher at DTU Space, says just that in a short, straightforward, and controversial way in his latest study.

“Humans, through their curiosity, have a desire to understand their role in relation to the universe. These results carry a surprising realization that the universe has set the conditions for life on Earth—also for us humans. It’s a realization that changes our whole perception of our significance in relation to space,” he said for Ingeniøren.

Henrik Svensmark’s new study “Supernova Rates and Burial of Organic Matter” was published on Wednesday in the peer-reviewed scientific journal Geophysical Research Letters. In the study, he describes a correlation between fluctuations in the number of supernovae and the amount of organic matter buried in the ocean floor.

Henrik Svensmark also presents further evidence for his and his colleagues’ theory that there is a link between cosmic rays, the formation of clouds, and the climate on Earth.

The theory is much-debated and controversial because it questions whether greenhouse gases are as relevant to climate change as the vast majority of the world’s climate scientists and the UN’s Intergovernmental Panel on Climate Change (IPCC) assess based on decades of massive international research—or whether cosmic rays may also be important.

Despite a close to 100 percent agreement among researchers that climate change is primarily caused by man-made greenhouse gas emissions, and the fact that the IPCC in the fall determined that virtually all global warming is caused by humans, Henrik Svensmark has for more than two decades worked tirelessly to prove his theory of the role of space.

The universe controls life on Earth

The new study describes correlations between fluctuations in the number of exploding dying stars—the so-called supernova rate—and a number of parameters that are linked to life and climate on Earth throughout billions of years. Henrik Svensmark uses the supernova rate as a proxy to express how much cosmic radiation has reached Earth within a given period of time.

Supernovae emit enormous amounts of cosmic radiation consisting of high-energy charged particles (about 90 percent protons). When charged particles hit Earth’s atmosphere, it is ionized. Henrik Svensmark’s research therefore suggests that they play a crucial role in life and climate. We will return to that part.

On geological time scales, which are periods of millions of years, there are fluctuations in the supernova rate and the resulting cosmic radiation. The amounts of organic matter buried in marine sediments also vary markedly over time. The graphs to the right show the development of cosmic ray intensity at the top, calculated by Henrik Svensmark, and the amounts of organic matter over the past 3.5 billion years at the bottom.

At the top is a curve of cosmic ray intensity over the last 3.5 billion years, which Henrik Svensmark has calculated from star formation data, open cluster data (a group of stars that are gravitationally bound to each other), and changes in solar evolution. The bottom curve shows the fraction of carbon buried in the sediments. The gray band represents one sigma uncertainty. Above the top graph, glacial periods are marked in blue. Illustration: Henrik Svensmark

Henrik Svensmark finds a correlation between the supernova rate and organic matter buried in marine sediments. Although a correlation does not necessarily entail a causality, he believes that the curves overlap so perfectly that it can be no coincidence.

“The amount of biomass at any given time aligns incredibly well with the changes in the supernova rate. There is no evidence in the natural sciences, only probability, but having said that, so many things fit so incredibly well together that the probability of a coincidence is very small,” Henrik Svensmark says.

A controversial climate theory

Henrik Svensmark takes the correlation with buried organic matter in his latest study several steps further and examines correlations between the supernova rate, global temperatures, nutrients, and the accumulation of oxygen in the atmosphere over time. And here we return to the controversial theory.

About 25 years ago, Henrik Svensmark and his colleague Eigil Friis-Christensen first presented the theory that cosmic-ray ionization in the atmosphere causes cloud formation. The explanation, according to the theory, is that the charged particles assist the formation of aerosols, which are airborne particles and the trigger of clouds in the sky.

Water does not like to bind to itself, but instead settles on the surface of aerosols and condenses to become droplets and eventually clouds. More clouds over a longer period of time lead to a colder climate because they reflect the energy of the sunlight back into space.

The most controversial part of the theory is the assumption that cosmic rays form aerosols that can form clouds. Experiments from both DTU and the European Organization for Nuclear Research (CERN) have previously shown that the ionizing radiation can form small aerosols a few nanometres in size in an atmosphere containing water vapour, ozone, and sulphur dioxide.

Critics of Henrik Svensmark’s climate theory, however, strongly doubt that many of these small aerosols are able to grow to a size of at least 50 nanometres, which is required for them to function as condensation nuclei for cloud formation. The criticism is supported, among other things, by computer simulations conducted by Colorado State University in the USA in 2009.

Henrik Svensmark believes to have refuted this point of view with a study published in Nature Communications in 2017, which points out that cosmic rays can also accelerate the growth rate of aerosols. However, Svensmark’s 2017 study has been criticized, among others by a CERN researcher, for having a too small effect.

But back to his latest study—if it is the case that cosmic rays from supernovae affect Earth’s climate through aerosol and cloud formation, then the dying stars also regulate the conditions for life on Earth.

Supernovae and life on Earth

Life on Earth and the total biomass at any given time depend on the supply of nutrients such as iron, phosphorus, nitrogen, and carbon. The amount of nutrients at a given time has been mapped by scientists over time by studying the content of a number of nutrients in pyrite (also called fool’s gold) embedded in black shale sedimented on the ocean floor.

On the basis of data on the content of nutrients in the last 500 million years, Henrik Svensmark finds another correlation with fluctuations in the supernova rate (see graph below).

The blue curve shows changes in supernova rate and the gray area shows this with an uncertainty of one sigma. Variations in the amount of nutrients in the oceans are shown as blue dots, with the statistical uncertainty (upper and lower quartile) displayed as vertical gray lines going through them. Illustration: Henrik Svensmark

This correlation leads back to the amount of organic matter buried in marine sediments, which Henrik Svensmark uses in his study as a proxy for the production of an important prerequisite for complex life: oxygen.

Because when the biological systems supply large amounts of nutrients, it is possible to have a large amount of biomass. A large amount of biomass means that more dead organic matter, all other things being equal, falls to the seabed and is buried in sediments. Conversely, when some nutrients are supplied to the biological systems, there is less biomass and less organic matter is buried, Henrik Svensmark explains.

“The interpretation must be that supernova rates affect the climate, resulting in marked changes in mixing of nutrients between the atmosphere and the oceans. A cold climate will entail a large temperature difference between the equator and the poles and therefore intense mixing. Therefore, a cold climate will supply significantly more nutrients to the biological systems, while a warm climate will entail less mixing and supply a smaller amount of nutrients,” he says.

In addition, Henrik Svensmark points out that organic matter buried in sediment produces oxygen. Because if organic matter is not buried there, and microorganisms begin to decompose it, then the photosynthetic generation of oxygen will ‘be reversed’, as the oxygen and the organic matter again become CO2 and water, says Henrik Svensmark.

“But if the organic matter is buried in sediments, the reaction cannot be reversed. Therefore, the burial of organic matter in sediments is also an indication of the production of oxygen. Supernovae have thus had indirect control over oxygen production, and oxygen is the foundation of all complex life, including us humans,” he says and concludes:

“Results with such far-reaching consequences will probably be perceived as controversial by some, but empirical data underpin the entire causal chain from supernovae to the climate to the delivery of nutrients to control over bioproductivity. The supernova rate has an effect on the intensity of cosmic rays that reach Earth, and we now have a theory that seems to explain how cosmic rays connect to the climate.”

It must be emphasized here that there is no scientific consensus on whether cosmic rays cause the formation of aerosols and thus clouds, which is how Henrik Svensmark links the supernova rate to the climate. But the correlations themselves are attracting attention on the other side of the Earth.

Geology professor: “Amazingly strong” correlation with glacial periods

Among the researchers who have been involved in the peer review of Henrik Svensmark’s study is Claudio Gaucher, a professor at the Department of Geological Sciences at the University of the Republic in Uruguay. According to him, the correlations found by Henrik Svensmark are “astonishingly strong” and “the probability of pure coincidence is unusually small”.

“This is a potential paradigm shift in the doctrine of climate change throughout history, and it changes our understanding of it. In particular, his hypothesis explains the occurrence of glacial periods very well, both in terms of the causes of global cooling and the times of the glacial periods,” he said for Ingeniøren.

Svensmark’s results show that Earth has been free of glaciation during periods of very little cosmic radiation, while the opposite is also true, he says (this can be found in the graph earlier in the article).

“The longest glacial periods in Earth’s history correspond to the maxima of cosmic radiation, but the less intense glacial periods also match periods of increased cosmic radiation,” he says, adding that Henrik Svensmark’s results will probably lead to further research into the relationship between cosmic rays and climate change.

“It’s imperative to confirm whether Earth’s climate is affected by cosmic rays and on what time scales. Is it only in the long term or are short-term fluctuations also associated with the intensity of cosmic radiation? A confirmation of Svensmark’s ideas will lead to nothing less than a paradigm shift that corresponds to the emergence of plate tectonics in geology or evolution in biology.”