The Influence of Spectral Solar Irradiance and Energetic Particle Precipitation on Climate

Abstract

Solar activity has been driving changes in Earth’s climate throughout history. However, since the 1970s, the emissions of greenhouse gases by human activities have become the dominant factor of climate change. Today, global warming is one of the main challenges of the modern society. On centennial time-scales, the solar contribution could still be important for climate. A factor closely related with solar activity is energetic particle precipitation. The impact of energetic particles on atmospheric composition and climate is relatively new area of research. Our aim is: (i) to investigate the influence of solar activity on terrestrial climate during the long term solar changes and (ii) to investigate the impact of energetic particle precipitation, specifically electrons, on atmospheric chemistry and climate. For these purposes we are using SOCOL3-MPIOM chemistry-climate model with interactive ocean. Measurement data from atmospheric monitoring stations shows a significant temperature increase During the early 20th century (1910 – 1940). This period coincided with an increase in both greenhouse gases and solar activity. To determine the main driver for the temperature increase we conducted a comprehensive model study. We considered separately solar UV radiation, solar visible and infrared radiation, energetic particle precipitation, greenhouse gases, ozone precursors, and volcanic eruptions. Globally, our results suggest that the surface warming was mostly induced by increase in concentrations of greenhouse gases. In Europe, however, this temperature increase may have been dominated by an increase of ozone precursors emissions (CO and NOx). The solar radiation in visible and infrared wavebands produced a smaller, yet detectable contribution in temperature trends, especially around Labrador Sea. In 1970s, some human-emitted substances were found to be depleting ozone layer. This was confirmed by observations and an ozone hole over Antarctica was found leading to the prohibition of ozone depleting substances emissions in 1987. Since the ozone layer is the protective shield of Earth against solar UV radiation, a thinning ozone layer is harmful to living beings. At the same time, however, solar UV radiation is responsible for producing ozone. Recent observations of the Sun show that solar activity is gradually decreasing and it has been hypothesized that the Sun might enter a new grand solar minimum in the 21st century. The change in solar radiation might impact the atmospheric chemical composition, temperature and regional climate. In order to investigate the effects of reduced solar activity on these variables, we conducted a model study covering the 21st and 22nd centuries that assumes the Sun will enter a phase of grand solar minimum. Focusing at the end of 21st century, we found that an unusually strong grand solar minimum enhances cooling of the stratosphere and mesosphere due to the presence of high concentrations of the greenhouse gases. We find that the global warming leads to an acceleration of the meridional circulation from tropics to poles and combined with the lower rates of ozone production due to the reduced solar activity, stratospheric ozone concentrations decrease over tropics. Though, based on the ban of ozone depleting substances ozone recovery is expected to happen within the 21st century, we show that total ozone would not recover globally to the levels before the ozone hole as long as grand minimum lasts. This could lead to increase of UV radiation reaching the surface with potential implications for Earth’s ecosystem. Energetic particles constantly bombard the Earth and, as mentioned above, their precipitation into the Earth’s atmosphere is another climate forcing related to solar activity. As part of these particles, energetic electrons can initiate cascades of chemical reactions of which some lead to the production of odd nitrogen oxides and odd hydrogen oxides (NOx and HOx). Due to their low energy (< 30 keV) auroral electrons produce NOx only above the mesosphere while continuously precipitating from magnetosphere. In the polar winter, NOx can descend inside the polar vortex down to the stratosphere where it depletes ozone. Only a recent advance in climate modeling allows implementing this effect of the auroral electrons, which motivated us to analyze their impact on atmospheric chemistry during the Southern Hemispheric winter. Results indicate that around 90% of winter NOx in the polar regions in the upper stratosphere comes from the precipitation of auroral electrons. In accordance with satellite observations, ozone anomalies of around 30% in the mesosphere and around 15% in the stratosphere are found during winters with intense electron precipitation. Electrons of higher energies (middle energy; 30 – 300 keV), precipitate from Earth’s outer radiation belt. They are able to penetrate to lower altitudes than auroral, producing NOx and HOx directly in the mesosphere. Satellite observations of higher energy electron fluxes became available only recently. In order to investigate their impact on atmospheric chemistry and climate, we have included them in our model. We confirm that these electrons, during geomagnetically active periods, significant amounts of NOx and HOx are produced and thus induce ozone depletion. Ozone anomalies in the mesosphere and stratosphere can reach up to 15% and 8%, respectively compared to climatological mean state. Furthermore, by depleting ozone they induce changes in atmospheric temperature and dynamics. Particularly during winters with high electron precipitation, we found anomalous mesospheric warming and stratospheric cooling in both Northern and Southern polar regions. The change in atmospheric temperature and dynamics leads to impact on the surface temperature during boreal winter: cooling around North Pole and warming over continental Asia. Using a single model, we made substantial advances in understanding the influence of solar radiation and energetic electron precipitation on Earth’s chemistry and climate. Nevertheless, we suggest, more models shall include these processes and continue broadening our knowledge about solar influence on climate.