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The Mysterious Relationship between the Universe and the Weather

Shinsuke Imada (Professor, Department of Earth and Planetary Sciences)

For us, the Sun is the source of life, bringing light and heat to the Earth. The radiative energy from the Sun warms the Earth’s surface, and this heat circulates through the atmosphere and oceans, giving rise to the seasons. Without the Sun, life on Earth would not exist. In this sense, the Sun can truly be regarded as a magnificent entity that nurtures life on our planet.

The Sun is by no means constant. Dark spots called sunspots appear and disappear on its surface, and the number of these spots fluctuates over an approximately 11-year cycle, known as the “solar cycle.” When the number of sunspots increases, the Sun enters a period of heightened activity called the “solar maximum,” during which explosive phenomena such as solar flares and coronal mass ejections occur more frequently. These events disturb the Earth’s magnetic field, producing auroras and causing disruptions in communication systems. This periodic variation of the Sun plays a crucial role as a rhythm that influences the Sun–Earth environment.

But does this solar rhythm also affect Earth’s climate? While an increase in sunspots raises the emission of ultraviolet and X-ray radiation, the brightness of visible light changes very little. Therefore, although variations in solar activity can influence high-altitude regions such as the stratosphere, their direct impact on the Earth’s surface, where we live, is thought to be small. However, a weakened solar activity allows more cosmic rays from space to reach the Earth, which may indirectly affect cloud formation. Such fluctuations in solar activity could subtly alter the Earth’s climate over long timescales (Figure).

To investigate solar cycle activity and its impact on the Earth’s environment, we formed a research team in Japan and conducted a comprehensive analysis under the project titled “Prediction of Solar Cycle Activity and Elucidation of Its Impact on the Earth’s Environment.” First, using ground-based observations and satellite data from missions such as Hinode, we studied changes in the Sun’s magnetic field in detail and developed a new numerical model to predict long-term variations in the solar magnetic field based on flows and turbulence on the solar surface. Using this model, we forecasted the next solar cycle, suggesting it could be several tens of percent weaker than the current cycle.

Furthermore, we used the Meteorological Research Institute’s Earth system model (MRI-ESM2.0) to analyze the relationship between solar activity and stratospheric variations. The results indicated that changes in solar activity could alter stratospheric temperatures, which, through interactions between the atmosphere and oceans, might slightly modulate surface climate. Additionally, by analyzing data such as carbon-14 in tree rings and beryllium-10 in Antarctic and Greenland ice cores, we clarified the relationship between solar activity and climate changes over the past 200–500 years. Comparisons with paleoclimate data also suggest that periods of low solar activity may have influenced precipitation patterns in Japan via increased humidity and changes in sea surface temperatures.

These findings demonstrate that, while the direct influence of solar activity on Earth’s climate is limited, it can exert indirect effects on the climate system through stratospheric chemical changes and cosmic ray interactions. Understanding these solar variations and the delicate, profound connection between the Sun and the Earth represents a crucial step toward more accurately predicting future climate change.

Gray et. al., 2010, from the PSTEP Webpage. Impacts on weather and climate through various elements of solar activity variability.