New study confirms Sun/Cosmic-Ray climate connection

A new study published in Nature Scientific Reports by researchers at the Danish National Space Institute at the Technical University of Denmark (DTU) and The Hebrew University of Jerusalem suggests that the Sun’s activity in screening cosmic rays affects clouds and, ultimately, the Earth's energy budget with concomitant climatic effects.

This research, by Henrik Svensmark, Jacob Svensmark, Martin Bødker Enghoff, and Nir Shaviv supports 25 years of discoveries that point to a significant role for cosmic rays in climate change. In particular, it connects observable variations in clouds and Earth's energy budget to Copenhagen laboratory experiments and theory, showing how cosmic rays help make aerosols and accelerate their growth to become cloud condensation nuclei and ultimately clouds.

Eruptions on the Sun screen the Earth from galactic cosmic rays – energetic particles raining down on our planet from exploded stars. “The Sun carries out fantastic natural experiments that allow us to test our ideas about cosmic ray effects on the atmosphere," professor Henrik Svensmark, lead author of the study told the GWPF.

Solar explosions produce magnetised gas that sweeps past the Earth reducing the cosmic ray flux reaching us. These events are called Forbush decreases taking their name from the American physicist Scott E. Forbush, who first noticed them more than 80 years ago. They lead to a temporarily lower production of small aerosols - molecular clusters in the air - that normally grow to seed the water droplets of low-level clouds. This, in turn, reduces the cloud cover which is known to affect climate.

The recent breakthrough is that the effect on the Earth's energy budget has been quantified using satellite observations from the CERES instrument on NASA's Terra and Aqua satellites.

The observational data indicate that the Earth absorbs almost 2 Watts per square metre additional energy within 4 to 6 days of the cosmic-ray minimum. Such a large effect is a major surprise since the general consensus of the climate community, recently expressed in the 2021 released IPCC report AR6 (chapter 7.3.4.5), is that, "... the GCR [galactic cosmic rays] effect on CCN [cloud condensation nuclei] is too weak to have any detectable effect on climate and no robust association was found between GCR and cloudiness. ... There is high confidence that GCRs contribute a negligible ERF [effective radiative forcing] over the period 1750 to 2019."

These new results show the IPCC’s conclusion will need to be reassessed. Two Watts per square metre can be compared with the IPCC report’s estimate of solar effective radiative forcing over the period 1750 to 2019 of only 0.01 Watts per square metre (obtained by only considering solar irradiance changes).

“We now have simultaneous observations of decreases cosmic rays, aerosols, clouds, and the energy budget” says professor Nir Shaviv.

The solar effects in this study are too short-lived to have a lasting effect on the climate, but they point the way to research that could uncover how the mechanism works on longer time scales. “It could be that the effect of carbon dioxide in the atmosphere – the so-called climate sensitivity – might be smaller than is deduced from climate models when this effect is taken into account,” adds professor Svensmark.

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