SETI Institute: 189 Bernardo Avenue #100, Mountain View
Throughout the past century, while greenhouse gas (GHG) abundances have been steadily increasing and influencing Earth’s climate, the Sun has remained relatively bright and quiescent. Solar cycles have been steadily active, with instantaneous sunspot numbers at solar maximum exceeding 100 in every cycle since 1893 (Cycle 13). The climate warming we have experienced since the beginning of the modern industrial era cannot be attributed to the Sun. However, the recent minimum between Cycles 23 and 24, and NASA predictions of a substantially lower sunspot number at the 2013 solar maximum, suggest that the Sun’s recent bright and quiescent period may be ending. Both autocorrelation studies of recent solar cycles, and studies of solar analogs in nearby field stars, suggest a >40% chance of the Sun entering a new Maunder Minimum sometime in the Twenty First Century. During the historical Maunder Minimum (1645-1715), meteorological data from Europe and proxy records from global oceans indicate a substantially cooler climate, attributable to decreased solar irradiance. In our lifetime, we may therefore see a period of solar dimming in conjunction with increasing GHG abundances. A new Maunder Minimum would not entirely offset the projected GHG-induced warming (the GHG radiative forcing is at least three times larger than best estimates of the solar irradiance decrease). Instead, the complex interactions between radiative balance and atmospheric dynamics yield unusual regional patterns of pronounced warming versus cooling. This seminar will address the physical basis of climate change in the context of both GHG and solar variability, and will also extend the discussion to the influence of stellar variability upon habitable zones.