Effects of the Sun’s trajectory through the galaxy on Earth’s climate over the past 10 million years

 In its travel through the Milky Way, the Sun traverses a variety of Galactic
environments, including dense interstellar clouds. Astronomical effects on
Earth’s past climate have been limited to 10,000-year scales variations in
Earth’s orbital parameters while our recent studies suggest that
longer-term climate shifts that occur every few million year may be linked
to compression of the heliosphere (the “cocoon” formed by the solar wind)
when the Sun crosses dense clouds as it travels through the Milky Way.
During such periods Earth was exposed to increased radiation and large
amounts of hydrogen, potentially altering its climate. These events are
consistent with independent 60Fe records indicating nearby astrophysical
encounters at ~2–3 and ~6–7 million years ago (Ma), as well as 10Be
anomalies near ~10 Ma that may reflect prolonged exposure to enhanced
radiation during a cold cloud crossing.  A convergence of recent advances
across astronomy, space physics, and paleoclimate creates an unprecedented
opportunity to rigorously test this hypothesis. We now have high-precision
astrometry from the Gaia mission that allows one to reconstruct the Sun’s
trajectory through the Galaxy and to identify, with remarkable accuracy,
the interstellar structures it has encountered over the past ~10 Ma. Major
theoretical and modeling advances now enable quantitative predictions of
how the heliosphere evolved during these encounters. In this talk I will
discuss our recent work that show that during such periods, Earth was
exposed to increased radiation and large amounts of hydrogen. I will
discuss our preliminary results that show that the increase in hydrogen
augmented mesospheric water vapor, leading to increased formation of both
polar mesospheric clouds and polar stratospheric clouds. The amount of
radiation that Earth experiences from such events depends on the duration
of the crossing and the amount of compression of the heliosphere, with
implications for Earth’s climate. I will discuss our results as well that
indicate that high temporal 10Be signal in ocean records and ice cores can
distinguish between alternative scenarios such as supernova explosions and
cold cloud crossings.