When solar activity diminishes and the heliosphere less holds back galactic rays, the planet's climate becomes noticeably cooler.
Cosmic rays affect the Earth's atmosphere, causing increased cloud formation and general cooling of the planet. These data explain the unexpected fluctuations in the earth's climate in the Middle Ages and early modern times (in scale they even exceeded the current global warming). For example, in Russia at the beginning of the 17th century, snow and frost regularly occurred in the summer months, which caused famine and Troubles. A related article was published in Nature Communications.
It is known from historical and paleoclimatic data that in 1000-1300 AD the climate was noticeably warmer than usual, and in 1400-1700, on the contrary, much cooler. It is also known that the last event coincided with a sharp decrease in the number of sunspots, that is, with a decrease in solar activity. However, the specific mechanisms that could explain the connection between such outwardly distant phenomena as spots on a luminary and the climate of its planet remained unclear for a long time.
The authors of the new work experimentally and using mathematical models show what may underlie such a connection. They conducted experiments in which the air in an isolated chamber was bombarded with particles similar in energy and mass to particles of cosmic rays. In astrophysics, elementary particles and atomic nuclei moving with high energies in space are called cosmic rays. Some of them have lower energies (those that move from the Sun), others are galactic cosmic rays, the energy of which is high enough to sometimes break through the protection of the solar heliosphere, inside which the Earth is located.
In the course of experiments, particles knocked electrons out of atoms in air molecules, thereby ionizing them (turning them from neutral atoms into ions having an electric charge). Then ions, due to electrostatic forces, help to vigorously form air aerosols from sulfuric acid and water molecules and remain stable for a long time to evaporation. This, as well as secondary collisions with new ions that increase their stability, help aerosol centers grow to sizes of tens of nanometers. As soon as they reach this level, water vapor from the atmosphere begins to rapidly condense on them, forming droplets. When this happens, the ground observer sees a cloud forming.
Of course, for this, there must already be water vapor in the atmosphere, however, in conditions without an external ion flux, cloud formation occurs much less frequently, and stable cloudiness is formed much longer. Since the time from cloud formation to rain in both scenarios is very similar, the total duration of the shadowing of the Earth's surface by tropospheric clouds in the scenario with ions is much longer than without them. Due to their white color, the clouds reflect most of the visible sunlight into space, thereby cooling the planet's surface.
The authors of the new work note that as the magnetic activity of the Sun increases (namely, it is responsible for the spots on it), the magnetic bubble of the heliosphere reflects galactic cosmic rays much more efficiently. But particles coming from the Sun, due to their much lower energy, cannot cause accelerated formation of clouds. Therefore, during the period of low solar activity, the Little Ice Age of 1400-1600 happened. On the contrary, since then and until the beginning of this century, solar activity has increased, which further accelerated global warming.
Interestingly, according to calculations, with a nearby supernova explosion, the process of cloud formation will be super-intense and will quickly lead to the cooling of the planet on an even larger scale than during the Little Ice Age. This may explain some of the unexpectedly sharp and seemingly unreasonable cooling in the Earth's past.
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