Scientific Method / Science & Exploration

Hints of cosmic ray-climate link in sediment core from Japan

But the correlation only holds during some of the times examined.

Clouds over Osaka Bay, Japan.

The idea that galactic cosmic rays play a large role in Earth’s climate may, at first blush, sound like a 1950s sci-fi premise, but it continues to capture attention. Some of that attention comes from folks desperate to find a climate control knob that makes anthropogenic greenhouse gas emissions seem insignificant, but some comes from researchers curious to test the hypothesis.

That hypothesis states that cosmic rays (high energy charged particles that enter our solar system) facilitate the nucleation of cloud droplets when they hit Earth’s atmosphere. If the incoming flux of cosmic rays increases, so too does cloud cover. Since clouds reflect sunlight back into space, more of them should mean a cooler climate (although that depends in part on the clouds' altitude). The solar wind shields the Earth from cosmic rays, greatly reducing the number that make it through, so the incoming flux ebbs and flows with solar activity.

There are, however, many issues with the idea. Cloud cover is also controlled by global temperature in important ways. An ongoing experiment at CERN has shown that charged particles can induce the formation of tiny atmospheric particles, but it doesn't necessarily follow that those particles can grow to the size of cloud condensation nuclei and influence the number of cloud droplets. And while some correlations between cosmic rays and global temperature in recent history looked pretty strong, notable exceptions have cast doubt on a causal connection.

Researchers have tried to test the hypothesis by finding natural experiments in the climate records. Unfortunately, those can be difficult to interpret, since solar activity can influence both cosmic rays and the amount of sunlight Earth receives. Since the Earth’s magnetic field also fends off some cosmic rays, variations in magnetic field strength are attractive tests because they aren't connected to solar activity in any way.

One such study looked at a marked weakening of Earth’s magnetic field about 41,000 years ago called the “Laschamp event”. Though the magnetic field was about 90% weaker, no climate response is seen in ice core records.

Climate records in the mud of Osaka Bay

A recent paper tries again with a similar strategy. Using a unique sediment core from Osaka Bay in Japan that goes back three million years, the researchers compare five interglacial periods 680,000 to 1.08 million years ago. During two of them, longer-lived magnetic pole reversals occurred that included several thousand years of a very weak magnetic field. Because the climate was warmer at these times than during the Laschamp event, the researchers hoped that any cooling signal resulting from the increase cosmic ray flux would stand out more clearly.

The sediment core is much different from the usual ocean sediment core climate records, which normally use measurements of oxygen isotopes in the shells of single-celled foraminifera. In the Osaka Bay core, temperature has to be inferred using the species of plant pollen and diatom plankton that are present.

The link between these species and climate comes via sea levels. Because of an underwater ridge at the mouth of Osaka bay, the area is shaped a bit like a bathtub. When sea level is low, the river feeding the bay makes it a freshwater body; when sea level rises above the ridge, the bay becomes salty. This transition can be seen in the chemistry of the sediment as well as the diatom species living at the time.

In comparing the five interglacial periods, the two that coincided with magnetic pole reversals did appear different from the others. While the peak in reconstructed temperature and sea level lined up during the other three interglacials, the warmest temperatures appeared to lag behind the highest sea level by several thousand years during the magnetic pole reversals.

Because the increase in cosmic rays would be greatest near the equator, and minimal at the poles, the researchers interpret this as the regional climate being cooled by increased cloudiness, even as the ice sheets are melting back and raising sea level. The researchers suggest that this could be why the climatic effects of cosmic rays aren’t seen in ice core records.

Zooming in on those two interglacials, the researchers looked to see how closely changes in reconstructed temperature  tracked magnetic field strength (based on magnetic measurements of the core). There appeared to be a tantalizing correlation over some periods, but a disconnect during others.

Problems and weak correlations

While this adds a couple more intriguing (if partial) correlations to the stack, we still lack a mechanism that clearly links cosmic rays to cloud formation. And several components of the study’s results are less than satisfying.

Raimund Muscheler, a researcher at Lund University in Sweden who has studied the cosmic ray hypothesis, told Ars that the idea that cosmic rays would mainly affect the tropics was problematic. “[T]his seems to contradict our present understanding of the climate system. We believe that climate change is usually enhanced towards the poles.” While Muscheler thought the motivation for the study was reasonable, the results weren't a slam dunk. “I agree that the climate system might react differently depending on the state of the climate system and depending on duration of the forcing,” he said. “However, it is a problem if one sometimes seems to see a correlation between [cosmic ray] flux and climate and sometimes it is absent.”

Jürg Beer of the Swiss Federal Institute of Aquatic Science and Technology (who, along with Muscheler, were part of the group that looked at the Laschamp event) also expressed skepticism to Ars. Beer argued that the Laschamp event was sufficiently long for a cosmic ray impact on climate to have appeared, yet there was nothing obvious in the record. Beer also cautioned that uncertainties in the type of climate reconstructions available from the Osaka Bay core make it difficult to draw firm conclusions.

While the correlations in the study don’t seem to support the notion that cosmic rays are the primary drivers of climate change, as Henrik Svensmark originally hypothesized, there’s enough there to help sustain interest in the general idea. But finding erratic correlations can only get you so far, while demonstrating a valid mechanism by which cosmic rays would influence climate would go a long way toward validating the hypothesis.

Proceedings of the National Academy of Sciences, 2013. DOI: 10.1073/pnas.1213389110  (About DOIs).

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