Nir Joseph Shaviv (Hebrew: ניר יוסף שביב; born July 6, 1972) is an Israeli‐American physics professor. He is professor at the Racah Institute of Physics of the Hebrew University of Jerusalem. He is known for his solar and cosmic-ray hypothesis of climate change which disagrees with the scientific consensus on human-caused climate change. In 2002, Shaviv hypothesised that passages through the Milky Way's spiral arms appear to have been the cause behind the major ice-ages over the past billion years. In his later work, co-authored by Jan Veizer, a low upper limit was placed on the climatic effect of CO2. His best known contribution to the field of astrophysics was to demonstrate that the Eddington luminosity is not a strict limit, namely, that astrophysical objects can be brighter than the Eddington luminosity without blowing themselves apart. This is achieved through the development of a porous atmosphere that allows the radiation to escape while exerting little force on the gas. The theory was correctly used to explain the mass-loss in Eta Carinae's giant eruption, and the evolution of classical nova eruptions.
Education and career Shaviv started taking courses at the Israel Institute of Technology in Haifa at age 13. After a 3-year service in the IDF Unit 8200, he received in 1994 a Master of Science in physics and a doctorate during 1994–96. During 1996–99, he was a Lee DuBridge Prize Fellow at Caltech's TAPIR (Theoretical Astrophysics) group. During 1999–2001, he was in a postdoctorate position at the Canadian Institute for Theoretical Astrophysics. In 2001–2006, he was a senior lecturer at Racah Institute of Physics at the Hebrew University of Jerusalem. In 2006–2012, he was an associate professor, and full professor since 2012. Between 2008 and 2011, he was the head of the faculty union of the Hebrew University, and he served as the chairman of coordinating council of faculty unions between 2010 and 2014. In 2014, he became a member of the Institute for Advanced Study in Princeton, and served as the chairman of The Racah Institute of Physics between 2015 and 2019.
Scientific research
Eddington luminosity limit In 1999, Shaviv has shown that inhomogeneities in stellar atmospheres reduce the effective opacity and thus increase the Eddington luminosity. Shaviv later showed that atmospheres are inherently unstable as the Eddington luminosity is approached, that these atmospheres will develop continuum driven winds that explain the appearance of eta-Carinae and classical nova eruptions. In 2010, Shaviv made the prediction that Type IIn supernova should have super-Eddington outbursts before the main supernova explosions since the super-Eddington states can naturally explain the circum-stellar material present around the supernova at the time of explosion (Giving the narrow lines observed in the spectrum, i.e., the “n” in the Type IIn). Such precursors were later detected with the Palomar Transient Factory, making them the first systematically detected supernova precursors.
Cosmic rays and climate Shaviv has been one of the proponents of a cosmic ray climate link. In 2003 he has shown that the cosmic ray flux over the past billion years can be reconstructed from the exposure ages of Iron meteorites, that these flux variations are expected from spiral arm passages, and they correlate with the appearance of ice age epochs on Earth. In a later work with Ján Veizer, it was demonstrated that the temperature reconstruction over the Phanerozoic correlates with the cosmic ray flux, but it does not correlate with the CO2 reconstruction, thus placing an upper limit on the effects of CO2. This prompted several reactions by the climate community and rebuttals by Shaviv and his colleagues. He has also shown that the cosmic ray climate link explains part the faint young Sun paradox, since the slowly decreasing solar wind will give rise to a cooling effect that compensates the solar irradiance increase. Moreover, long term star formation activity in the Milky Way correlate with long term climate variations. In a more recent work with Andreas Prokoph and Ján Veizer, it was argued that the reconstructed temperature has a clear 32 million year oscillation that is consistent with the Solar System's motion perpendicular to the galactic plane. The oscillation also appears to have a secondary modulation consistent with the radial epicyclic motion of the Solar System.
Solar variation and climate sensitivity Because the existence of a significant cosmic ray climate link implies that solar variability will also have a large effect on the climate, Shaviv advocated the idea that natural climate variations play a significant role in 20th century climate change. Moreover, if solar activity increase over the 20th century contributed to warming in addition to the anthropogenic forcing, then the overall climate sensitivity should be lower than advocated by standard scenarios which do not include solar forcing. In 2008, Shaviv used the oceans as a giant calorimeter to quantify the solar radiative forcing. He found that the peak to peak variations are close to 1 W/m2, significantly more than can be expected from the changes in the solar irradiance. In 2011, he published a paper with Shlomi Ziskin arguing that the solar variability explains about half the 20th century warming, with the other half attributable to anthropogenic forcing. Shaviv's solar hypothesis has been disputed by Mike Lockwood and Claus Froehlich (1936–2019) in an analysis of the Sun's output over the last 25 years. They argue that the Sun's activity has been decreasing since 1985 while global temperatures have continued to rise. Shaviv argues that Lockwood and Froehlich's analysis is flawed for a number of reasons. Firstly, while sunspot activity declined after 1985, cosmic ray flux reached a minimum in 1992 and contributed to warming during the 1990s. Secondly, Shaviv argues that short term variations in radiative forcing are damped by the oceans, leading to a lag between changes in solar output and the effect on global temperatures. While the 2001 maximum was weaker than the 1990 maximum, increasing solar activity during previous decades was still having a warming effect, not unlike the lag between noon and the hottest hour of the day. Later quantitative modeling showed that indeed there is no discrepancy. The perceived "hiatus" in the early 2000s is a natural consequence of the decreased solar activity.
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