How do fluctuations in the Sun's brightness affect the climate on Earth? And what role does our star play in current climate change? To answer these questions, a look into our star’s past is necessary. In this endeavor, the most meaningful parameter is the total solar irradiance. It indicates how much radiation from the Sun in all wavelengths reaches Earth’s atmosphere. The total solar irradiance can only be measured directly with the necessary precision in space – and thus only since the beginning of the space age about 50 years ago. For the centuries before that, researchers rely on proxies such as the number of dark spots covering the Sun. These have been recorded for hundreds of years and are still carefully observed and documented. Historical solar observations and modern data can then be combined to reconstruct the total solar irradiance far back into the past.
In the award-winning publication in the journal Solar Physics, Theodosios Chatzistergos from the Max Planck Institute for Solar System Research (MPS) takes a critical look at a widespread reconstruction. Several such reconstructions exist. They are based on different proxies, data sets, and methods and are, in a sense, comparable to different retellings of a historical event that rely on different sources and narrative styles.
The reconstruction now under investigation dates from 1993. According to it, the total solar irradiance increased significantly since the middle of the 20th century, as has the global temperature on Earth. All other reconstructions – including many more modern ones – show no such increase. How can this be?
In search of answers, Theodosios Chatzistergos takes on the laborious task of carefully examining and reproducing the assumptions, methods, and results of the 1993 reconstruction. He shows, that the discrepancy between this reconstruction and all other published results can be traced to methodological shortcomings and inconsistencies in the 33-year-old work. In his now distinguished study, he extends the old reconstruction to the present day using more recent measurements and thus produces an improved and updated version that aligns well with direct irradiance observations.
As it turns out, the reconstruction now comes to a similar conclusion as all others: solar irradiance decreased slightly over the second half of the 20th century, in clear contrast to the sharp rise indicated in the 1993 publication.
The award-winning publication not only makes a valuable contribution to the current climate debate. It also shows how much science benefits when previously published results are revisited and reproduced by other researchers – an often laborious but valuable task.
About laurate and award
Dr. Theodosios Chatzistergos studied astronomy and astrophysics at the National and Kapodistrian University in Athens (Greece) and at Queen Mary University in London (England). Since completing his doctorate at MPS in 2017, he has been working on reconstructing historical fluctuations of the Sun’s brightness. To this end, he evaluates among other things historical photographs of the Sun from the 20th century and hand drawings from the 19th century. In addition, he has succeeded in compiling the first complete chronicle of the surface area coverage of solar faculae, particularly bright areas on the Sun's surface, from 1892 to 2021, based on 300,000 images of the Sun from more than 40 archives worldwide. In 2024, the European Society for Space Weather and Space Climate awarded him the Alexander Chizhevsky Medal for this achievement. In 2022, Theodosios Chatzistergos received the Distinguished Young Scientist Award from the Scientific Committee on Solar-Terrestrial Physics (SCOSTEP), a committee of the International Science Council (ISC).
The Kees de Jager Prize has been awarded annually since 2021. It is named after Cornelis “Kees” de Jager, the founding editor of the journals Solar Physics and Space Science Reviews.
Original Publication
Theodosios Chatzistergos: A Discussion of Implausible Total Solar-Irradiance Variations since 1700, Solar Physics (2024) 299:21
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