In its motion through the galaxy the sun may have suffered a number of encounters with dense interstellar clouds for which the number density of molecular hydrogen H2 is >103 cm-3. Several authors have shown that this is sufficient density to stop the solar wind inside the earth's orbit. The earth's atmosphere would be subjected to an interstellar H2 flux of >7¿109 cm-2 s-1 for periods of ~105 years. We have examined by means of simple scaling arguments several consequences for the earth's atmosphere: (1) the ionospheric F-region would largely disappear; (2) the water vapor content of the middle atmosphere would be greatly enhanced, reducing the mesospheric ozone concentrations and thereby lowering the average temperature and altitude of the mesopause; (3) as a result of (2), widespread mesospheric ice clouds would occur, increasing the planetary albedo; (4) the resultant radiative cooling at the surface may have been sufficient to ''trigger'' an ice age.