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Sun, our home star, is a variable star. The variations in the appearance and energy output from the Sun are denoted as “solar activity”.  These variations range from milliseconds to decades, centuries, millennia, and beyond. The variations with time scales of up to a few solar rotation periods (~27 days) are referred to as “space weather”, and have significant impacts on the space-borne and ground-based technological systems. Longer-term solar activity variations are defined as “space climate”, and are supposed to be related to the terrestrial (and maybe other planets’) global climate variations.

A number of solar activity manifestations can affect the Earth’s atmosphere. The total solar irradiance (TSI) is the main energy source for the terrestrial system. TSI is dominated by visible light whose relative variations are only a fraction of a percent, but with the greatest absolute magnitude of change. With decreasing wavelength, the relative variations increase. Radiation in different spectral ranges affects different parts of the Earth’s atmosphere.

The Earth is also constantly exposed to the Sun’s corpuscular radiation as it together with the whole Solar System is inside the ever expanding solar atmosphere known as the “solar wind” – a flow of charged particles with embedded magnetic fields from the solar corona. On top of it, transient structures like Coronal Mass Ejections (CMEs) and High Speed Solar Wind Streams (HSSs) ride whose interactions with the terrestrial magnetic field lead to geomagnetic storms.

High energy particles from CME or HSS associated shocks, or (mostly) from their interactions with the Earth’s magnetosphere, precipitate in the high latitude atmosphere leading to increased ionization, enhanced production of compounds which affect the ozone balance, radiative heating and cooling, and finally changes in atmospheric dynamics and large-scale circulation modes like the North Atlantic Oscillation governing the weather over most of the Northern hemisphere.

The solar wind magnetic field modulates the flux of galactic cosmic rays, high energy particles coming from outside the Solar system. Due to their high energy, they penetrate deep into the atmosphere. Their variations are found to be strongly correlated with the atmospheric global electric circuit, cloud cover, albedo, and infrared opacity, determining the Earth’s energy balance.

In this review, after a short introduction to solar activity, I will describe its above mentioned agents, and explain the suggested mechanisms by which they influence the atmosphere and climate. I will present observational evidences of such influences, highlight the recent advances, and the still unsolved questions and uncertainties.  

How to cite: Georgieva, K.: Solar influences on the atmosphere and climate, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-704, https://doi.org/10.5194/ems2022-704, 2022.

Visual sunspot observations form the longest scientific record of solar activity, spanning over four centuries. Long term solar studies  are crucial to predict the future evolution of the solar cycles and at improving our understanding of the solar influence on Earth climate change.

As an important step for the ongoing sunspot number recalibration throughout the entire solar physics community, (Lefevre et al., 2018), SN Version 2 was released in July, 2015 (Clette et al., 2016), which helped in shedding new light to long-term solar variations and instabilities of the 11-year solar cycle. However, uncertainties remain and errors in past historical data need to be further revised using the data at our disposal  for a robust long-term series.

In 1843, Professor Rudolf Wolf  who coined the term “Sunspot Number”, founded a journal called the "Mittheilungen der Naturforschenden Gesellschaft in Berne" where he published yearbooks with all of his findings, including sunspot observations as far back as Galileo (Wolf, 1861). The journal was maintained from 1848 (Wolf, 1848) until his death (Wolf and Wolfer, 1894). The Sunspot records collected by him from his European colleagues and his auxiliary observers are all tabulated in this journal. The Royal Observatory of Belgium (https://www.astro.oma.be/en/), particularly the WDC-SILSO, conducted a mission between 2017 and 2019 to digitize all the data contained in the published Mittheilungen.

In this study we exploit this database along with other available recounts of various observers to identify scale discrepancies or inhomogeneities that happened along the Sunspot number series over time. We also introduce statistical techniques to implement confidence bands or errors on daily Sunspot Numbers, an information that existing versions lack. The long-term aim is a complete reconstruction of the Sunspot Number series from the available raw data instead of a recalibration.

How to cite: Bhattacharya, S., Lefevre, L., Clette, F., and Jansen, M.: Diagnosing and calibrating the multi-century Sunspot Number Series, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-403, https://doi.org/10.5194/ems2022-403, 2022.

Not only the climate in the troposphere is changing, also at higher altitudes long-term trends have been observed. The global mean middle and upper atmosphere have been cooling, resulting in atmospheric contraction and a decline in thermosphere density at fixed height, mainly driven by the increase in atmospheric CO₂ concentration. The secular variation of the Earth’s magnetic field is an additional driver of long-term change in the upper atmosphere, causing trends that are strongly location-dependent. While magnetic field changes are most important for the ionosphere, they also affect the temperature and wind structure throughout the thermosphere, mainly via changes in the strength and geographic distribution of Joule heating. Simulations with the Whole Atmosphere Community Climate Model eXtension (WACCM-X) suggest that perturbations induced by magnetic field changes in the lower thermosphere climate can further propagate downward via vertical dynamical coupling. Our results show a significant response in the zonal mean temperature and zonal wind in the Southern Hemisphere (SH) middle- to high-latitude troposphere, stratosphere, and mesosphere of up to ±2 K and ±2 m/s, as well as regionally significant changes in Northern Hemisphere (NH) polar surface temperatures of up to ±1.3 K, in December-January-February. In the SH, changes in gravity wave filtering in the thermosphere induce a change in the residual circulation that extends down into the upper mesosphere, where further changes in the zonal mean wind climatology are generated, together with changes in local planetary wave generation and/or amplification and gravity wave filtering. This induces an anomalous residual circulation that extends down into the troposphere. The NH middle atmosphere response is zonally asymmetric, consisting of a significant change in the positioning and shape of the upper stratospheric polar vortex, which is dynamically consistent with the surface temperature response. While the details of the lower and middle atmosphere responses may not be entirely accurate due to model limitations, the results from our simulations do indicate that dynamical coupling within the atmosphere can conceivably result in upper atmosphere processes having a significant effect on surface climate.

How to cite: Cnossen, I., Lu, H., and Liu, H.: Long-term change in the upper atmosphere and a possible effect on surface climate, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-461, https://doi.org/10.5194/ems2022-461, 2022.

Using an intermediate complexity climate model we investigate the so-called snowball Earth transition. For certain values (including its current value) of the solar constant, the climate system allows two different stable states: one of them is the snowball Earth, covered by ice and snow, and the other one is today's climate. In our setup, we consider the case when the climate system starts from its warm attractor (the stable climate we experience today), and the solar constant is changed according to the following scenario: it is decreased continuously and abruptly, over one year, to a state, where only the Snowball Earth's attractor remains stable. This induces an inevitable transition, or climate tipping from the warm climate. The reverse transition is also discussed. Increasing the solar constant back to its original value in a similar way, in individual simulations, depending on the rate of the solar constant reduction we find that either the system stays stuck in the snowball state or returns to a warm climate. However, using ensemble methods i.e., using an ensemble of climate realizations differing only slightly in their initial conditions we show that the transition from the snowball Earth to the warm climate is also possible with a certain probability which depends on the specific scenario used. From the point of view of dynamical systems theory, we can say that the system's snapshot attractor splits between the warm climate's and the snowball Earth's attractor. Despite the limitations of an intermediate complexity climate model, all this reveals that incautious geoengineering (e.g. overdone solar radiation shielding) could even result in a snowball-like climate.

How to cite: Herein, M., Haszpra, T., and Kaszas, B.: When the Earth goes white: the Snowball Earth attractor, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-394, https://doi.org/10.5194/ems2022-394, 2022.