During the discussion of how dynamical air-sea coupling in the tropical Pacific and solar variability interact from a bottom-up perspective, several participants remarked on the wealth of open research questions in the dynamics of the climatic response to TSI and spectral variability.
According to two presenters on paleoclimate, there is a need to study the idiosyncrasies of each key proxy record. Yet they also emphasized that there may be an emerging pattern of paleoclimate change coincident with periods of solar activity and inactivity, but only on long timescales of multiple decades to millennia. Several speakers discussed the effects of particle events and cosmic-ray variability. These are all areas of exciting fundamental research; however, they have not yet led to conclusive evidence for significant related climate effects.
The Effects of Solar Variability on Earth's Climate: A Workshop Report
The key problem of attribution of climate variability on the timescales of the Little Ice Age and the Maunder Minimum were directly addressed in several presentations. Several workshop participants remarked that the combination of solar, paleoclimatic, and climate modeling research has the potential to dramatically improve the credibility of these attribution studies.
On September , , experts in solar physics, climate models, paleoclimatology, and atmospheric science assembled at the National Center for Atmospheric Research NCAR in Boulder, Colorado for a workshop to consider the Sun's variability over time and potential Sun-climate connections. While it does not provide findings, recommendations, or consensus on the current state of the science, The Effects of Solar Variability on Earth's Climate: A Workshop Report briefly introduces the primary topics discussed by presenters at the event.
As context for these topics, the summary includes background information on the potential Sun-climate connection, the measurement record from space, and potential perturbations of climate due to long-term solar variability. This workshop report also summarizes some of the science questions explored by the participants as potential future research endeavors. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website. Jump up to the previous page or down to the next one.
Also, you can type in a page number and press Enter to go directly to that page in the book. Switch between the Original Pages , where you can read the report as it appeared in print, and Text Pages for the web version, where you can highlight and search the text. To search the entire text of this book, type in your search term here and press Enter. Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available. Do you enjoy reading reports from the Academies online for free? Please visit specific meetings for exact scheduling.
Virtual participation is made possible via zoom. Please email Serena Criscuoli for details. A dedicated Journal Club will introduce students and scientists to the topic of the focus meeting. The Journal Club is usually scheduled on the same day and in the same room of the Focus meeting, and it starts 30 minutes ahead. To receive information about upcoming focus meetings, please subscribe to the mailing list. Tom Ayres, CU. What do we know about stars concerning their possible impacts on planets and habitability, specifically focusing on low-mass dwarfs?
Regner Trampedach, SSI. Giulio Del-Zanna, Atomic Astrophys. Cambridge, UK. Anil Pradhan, Ohio State Univ. Judge et al , Sol Phys This anti-correlation is the basis for using 14 C and 10 Be deposited in tree rings and ice cores as a proxy measure of solar activity dating back thousands of years. In addition to the year GCR cycle, there is also a year variation related to the polarity of the solar magnetic field. These variations can be understood through the physics of charged-particle motion in turbulent electric and magnetic fields associated with the solar wind plasma.
Even during the Maunder Minimum when sunspots were scarce, there were still polarity reversals, and there were still enhancements of GCRs in the year cycle such that a stronger magnetic field yielded smaller GCR flux. The highest flux of GCRs on record occurred during the most recent solar minimum.
The radioisotope record in 14 C and 10 Be provides information on the varying concentration of these isotopes in the biosphere over past millennia. This information can be translated into a time series showing the variation in flux of the GCRs mainly responsible for the formation of these isotopes. The resulting GCR flux variation has been used in Sun-climate studies in two separate ways. It can be used directly to study the possible effect of GCR variation on atmospheric electrification and cloud formation.
As pointed out in the presentation by Giacalone, recent findings from laboratory measurements at CERN have brought more attention to studies of cosmic ray effects. He also described the various influences causing GCR flux variation on timescales from days to millennia. Sudden changes in the solar magnetic field or in the resulting heliospheric structures account for the rapid changes.
The and year cycles are caused by more gradual variation of the heliospheric field strength and complexity, and in the number of solar eruptive events over the sunspot cycle. Supernova eruptions and other changes in the interstellar medium play a role on millennial timescales because of the large distances between stars and the diffusive transport of the GCR population.
Nearby supernovae can create short-lived pulses, but these are exceptionally rare. Shapiro, W. Schmutz, E. Rozanov, M. Schoell, M. Haberreiter, A. Shapiro, and S. Nyeki, A new approach to the long-term reconstruction of the solar irradiance leads to large historical solar forcing. Astronomy and Astrophysics , Foukal, A new look at solar irradiance variation, Solar Physics , Giacalone also pointed out that, although the four basic processes of particle diffusion, convection, drifts, and energy changes have been known for 50 years, the transport coefficients needed to calculate their effects are still poorly known.
However, it is now accepted that none of the processes included in the classic Parker transport equation can be neglected; it is a complex system. This complexity limits the accuracy of attempts to use the isotope record to derive the record of solar irradiance. The solar modulation of cosmic rays is caused by both the relatively gradual evolution in the background solar wind and by the pulses associated with coronal eruptions, through the complex processes referred to. Much of the variation in irradiance at least the year variation originates from the emission from plasma heated by the dynamics of the near-surface magnetic field with contributions from both the open and closed field components both during quiescence and during flarings.
This differential behavior makes it difficult to use radioisotopes to generate more than a rough estimate of variation in TSI and ultraviolet flux, such as those shown by workshop presenter Raimund Muscheler. Giacalone showed the evidence for a low-level year cycle in radioisotopes during the extended Maunder Minimum of solar activity during the 17th century.
Such isotopic evidence currently provides the best chance of determining how much the solar magnetic field decreased during that period and therefore how much the Sun dimmed. He discussed how the differences between galactic cosmic rays and anomalous cosmic rays ACRs could be seen during the most recent solar minimum. The GCR intensity was the highest measured by spacecraft, but ACRs had a lower intensity compared to previous solar minima. He suggested one possible interpretation was that fewer ACRs were being produced at the termination shock of the heliosphere.
This difference is useful for determining the physics of cosmic ray transport in the heliosphere. The most common isotopes used in these studies are 10 Be in ice cores and 14 C in tree rings, and they can be used together to separate changes in solar activity from differences in climate. The challenge is to detect reliable signals from these data sets for a particular time period. An estimate of the geomagnetic field intensity is necessary along with isotope measurements to determine the solar modulation.
In the atmosphere itself, circulation and climate also affect the deposition of the isotopes.
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Muscheler explained that CO 2 remains in the atmosphere for approximately years and then has a long residence time in the large reservoir of the ocean where it can continue to exchange with the atmosphere and the biosphere before being trapped in tree rings. This leads to a dampening of the smaller-scale variations. Because of this residence time in a large reservoir, 14 C is a less direct proxy for cosmic rays, but it does have the advantage of being less subject to concerns about geographic influences than is 10 Be.
Muscheler stated that even if changes in climate cause changes in the carbon cycles of the biosphere and the ocean, the 14 C in the atmosphere changes at the same rate and so does not obscure a solar signal. Even though climate influence is unlikely to be a major influence, a model of the carbon cycle is needed to calculate 14 C production rates to make an assumption about the carbon cycle.
Variations in 14 C in the atmosphere alone do not provide a signal of the production of 14 C. Muscheler discussed how 10 Be is produced by spallation processes in the atmosphere through reactions with nitrogen and oxygen. These reactions require high-energy particles. These SPE contributions are relatively short-lived and are generally believed to be undetectable in the climatological record because they are obscured by other short cycles, such as the changes in 18 O caused by yearly temperature changes.
Muscheler continued by explaining how 10 Be is produced in the stratosphere, becomes attached to aerosols, and is then sensitive to stratosphere-troposphere exchange processes before being deposited and trapped in ice cores. High-latitude locations such as Greenland or Antarctica have little shielding, and so the solar signal is relatively strong. At low latitudes there is little variation with solar cycle due to the stronger geomagnetic shielding.
A still unresolved issue is how to go from a local measurement with some random variability to a globally representative value. Muscheler summarized 10 Be as a relatively direct proxy for cosmic rays with significant noise associated with location and climate influences. Muscheler discussed two commonly used data sets from the past 1, years for 10 Be. There is a disagreement between researchers when looking at the records from the past 50 years.
Others claim that their reconstruction indicates a period of high solar activity in the past 60 years that is unique in the past 1, years. Muscheler suggested that the difference between the recent records is caused by one of them having been influenced by non-solar-related climate change. Muscheler discussed evidence of long-term changes in solar activity over the past 10, years. There are, however, uncertainties engendered by the comparison of the 10 Be to the 14 C record. These differences may be due to changes in 10 Be transport, snow accumulation rates, carbon cycle uncertainties, or.
Muscheler stated that further research would be required to understand these differences. Muscheler pointed out that there is good agreement between 10 Be and 14 C on the scale of the , , and year solar cycles and that those signals can be clearly seen. On the other hand, there is no evidence of sustained periods on the order of 1, years of low solar activity in either the 10 Be or the 14 C record.
This can be said with some confidence for the records going back over the past 10, years; however, characterization any farther back than that is more complicated because of the influence of climate change during the last ice age on the 10 Be record. Based on the proposed link between increased GCR flux and cloudiness, one might have expected that the late 20th century would be cooler than the early 20th century—a state that was not observed. Another audience member pointed out that it is necessary to be careful about the scale of the solar activity minima; minima on the scale of the heliosphere are not appropriately grouped with those on the scale of a hundred kilometers.
The relationship between the large- and the small-scale field of the Sun is not known. Muscheler agreed that in his radionuclide data, only the solar modulation of GCRs can be clearly seen. The frequency of grand minima Maunder Minimum-like occurrences is difficult to extract from the geophysical proxy record.
Lubin pointed out that the early pre-Hipparcos estimates of Maunder Minimum analog frequency gave estimates that are too large. Instantaneous activity measurements of the hydrogen and potassium spectral lines HK are suggestive but not conclusive for identifying Maunder Minimum analog candidates; the result depends strongly on the chosen inactive threshold. Very low activity may be seen with an old star nearing the end of its main sequence lifetime. However, the historical Maunder Minimum most likely did involve very low HK activity and weak cycling compared with the present-day Sun.
Baliunas, and R. Jastrow, Evidence for long-term brightness changes of solar-type stars, Nature , Wright, Do we know of any Maunder Minimum stars? The Astrophysics, Journal , Judge, and S. Lubin, D. Tytler, and D. Kirkman, Frequency of Maunder minimum events in solar-type stars inferred from activity and metallicity observations, The Astrophysical Journal Letters L32, Instrument meteorological records rarely extend back more than years; therefore, a long-term perspective on solar forcing must rely on the records provided by paleoclimate archives—principally, ice cores, lake and marine sediments, stalagmites, corals, and tree rings.
Within each of these natural archives, a number of parameters can be measured and their relationship to climate assessed through calibrations with overlapping instrument data. In this way, paleoclimate proxies extend the record of past climate over past millennia. Bradley, University of Massachusetts. Paleoclimate archives also provide an index of past solar activity, through the record of changes in cosmogenic isotopes recorded in tree rings and ice cores.
Solar Variability and Terrestrial Climate
In particular, variations in the cosmogenic isotopes 10 Be and 14 C indicate changes in the production rate of these isotopes. Thus, isolating solar magnetic effects on the production rate of 10 Be and 14 C requires that changes in the geomagnetic field strength be removed, leaving the heliomagnetic signal as a residual. Furthermore, although attempts have been made to calibrate changes in 10 Be in terms of variations in total solar irradiance, 19 it is still debatable how variations in cosmogenic isotope production relate to changes in total or spectrally distributed irradiance.
Bradley noted that, despite these limitations, paleoclimatologists have generally accepted that the record of 10 Be or 14 C anomalies provides an index of changes in TSI, and have often sought to correlate paleoclimatic records with these data. The results have been mixed. However, when other nearby proxy records were examined, there was no evidence for a similar relationship to solar forcing, leaving open the question of whether the Belukha record is superior to the others, or whether the observed relationship is only of local significance.
Bradley discussed how a number of high-resolution ice core records have noted a strong relationship between 14 C anomalies and changes in oxygen isotopes in the stalagmite carbonate.
For example, Neff et al. Steinhilber, J. Beer, and C. Eichler, S. Olivier, K. Henderson, A. Laube, J. Beer, T. Papina, H. Schwikowski, Temperature response in the Altai region lags solar forcing, Geophysical, Research, Letters L, Neff, S. Burns, A. Mangini, M. Mudelsee, D.
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Fleitmann, and A. Matter, Strong coherence between solar variability and the monsoon in Oman between 0 and 6 kyr ago, Nature , Bradley asserted that it is noteworthy that all these studies focus on changes in the hydrological cycle of each region, rather than changes in temperature. This points to the possibility that if there is a solar effect on climate, it is manifested in terms of changes in the general circulation, rather than in a direct temperature signal.
Bradley noted that in fact, despite the serious limitations in terms of statistical significance of most of the published paleoclimate studies that claim to find a solar signal in the records, there is nevertheless a clear geographical pattern in the overall signal that emerges when all records are mapped out. Specifically, periods of high cosmogenic isotope production which might be related to reduced irradiance appear to be associated with weaker monsoon rainfall in Oman, India, and southern China.
There is also evidence for colder conditions at high latitudes, more extensive sea-ice in the North Atlantic, and wetter and colder conditions in western Europe, suggesting a general expansion of the polar vortex and a southward displacement of the westerlies when solar activity is low. There is a corresponding displacement, or seasonal shift in the intertropical convergence zone, affecting rainfall distribution in Central and South America and equatorial Africa.
However, he asserted that it is clear that the current evidence for solar forcing from paleoclimate is very limited, and most records do not provide the necessary resolution or signal strength to detect a solar signal if it is present. Bradley suggested that further studies could be designed to address this question in a more rigorous and systematic manner. Some of these records are claimed to resolve timescales fine enough to spectrally resolve signals at the year-cycle period.
North described efforts to do this in oxygen isotope data from the Dye-3 core from Greenland and the Taylor Dome cores from Antarctica, both of which revealed a weak year signal. He described how further research, involving additional well-dated records and band pass filtering, may further elucidate the temporal evolution of such signals in relation to the long-term record of solar forcing.