Tor Vergata Study Reshapes Climate Science: The Sun's UV Swings Are Stronger Than Assumed
A thousand-year analysis uncovers unexpected UV variability and its impact on Earth's atmosphere.
The Sun's ultraviolet (UV) radiation plays a crucial role in shaping Earth's atmosphere. It drives the chemical reactions that create and maintain the stratospheric ozone layer, regulates the thermal structure of the upper atmosphere, and even affects the atmospheric drag experienced by satellites in low Earth orbit. To understand how Earth's atmosphere has evolved over centuries, scientists must first understand how the Sun's UV output has changed over time.
Reconstructing a Millennium of Solar Activity
A team of researchers from Tor Vergata University of Rome, working in collaboration with the National Solar Observatory and the Institute of Computational Life Sciences, has now reconstructed a full millennium of solar UV variability. Their work provides the most detailed long-term picture to date of how changes in solar activity have influenced Earth's atmospheric chemistry.
The study, first authored by Raffaele Reda of the Department of Physics, is the result of more than a decade of collaboration coordinated by Prof. Francesco Berrilli at Tor Vergata. The research team also included climatologists from Sapienza University of Rome, CNR/ISAC, University of Trento, HAO/NCAR, and the University of Exeter.
The Hidden Variability of the Sun
The Sun is not a static star. Its magnetic field intensifies and weakens over an approximately 11-year cycle, producing the well-known rise and fall of sunspots. Beyond this regular rhythm, however, the Sun also undergoes longer-term variations. Historical records reveal extended periods of unusually low activity-known as Grand Minima-such as the Spörer Minimum and the Maunder Minimum, during which overall solar output declined significantly.
Until now, estimates of UV radiation during these Grand Minima were highly uncertain. To address this gap, the team combined two independent sources of information: records of past solar magnetic activity derived from radioisotopes preserved in tree rings, and a global archive of solar images. Using an empirical model that links UV emission to magnetic structures on the Sun's surface, they reconstructed four UV spectral bands that are critical for ozone chemistry.
Implications for Ozone and Climate
The results show that UV radiation decreases substantially during Grand Minima, altering the conditions that regulate ozone formation in the stratosphere. The most striking finding concerns the middle ultraviolet (MUV) band, spanning 180-300 nm. Previous models suggested only modest long-term variability in this range. However, when the Sun's "quiet" magnetic network-small-scale magnetic features far less visible than sunspots-is fully accounted for, the reconstruction reveals significantly stronger fluctuations.
This finding highlights the crucial role of small-scale solar magnetism in controlling UV emissions. Because MUV radiation directly influences both ozone production and stratospheric heating, these results have important implications for climate modelling and for our broader understanding of how Earth's atmosphere has evolved over the past thousand years.