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

Clouds over Osaka Bay, Japan.

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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.