An investigation of the Archean climate using the NCAR CCM.

Abstract

The Archean (2.5 to 3.8 billion years ago) is of interest climatically, because of the "Faint-Young Sun Paradox", which can be characterized by the Sun's reduced energy output. This lower energy output leads to a frozen planet if the climate existed as it does today but, the geologic record shows that water was flowing at the earth's surface 3.8 billion years ago. Energy Balance Models (EBMs) and one-dimensional radiative-convective (1DRC) models predict a frozen planet for this time period, unless large carbon dioxide concentrations (CO$\sb2$) exist in the Archean atmosphere. The goal of this thesis is to explore the Archean climate with the National Center for Atmospheric Research (NCAR), Community Climate Model (CCM). The search for negative feedbacks to explain the "Faint-Young Sun Paradox" is the thrust of this study. This study undertakes a series of sensitivity simulations which first explores individual factors that may be important for the Archean. They include rotation rate, lower solar luminosity, and land fraction. Then, these climatic factors along with higher (CO$\sb2$) concentrations are combined into a set of experiments. A faster rotation rate may have existed in the Archean. The faster rotation rate simulations show warmer globally averaged surface temperatures that are caused by a 20% decrease in the total cloud fraction. The smaller cloud fraction is brought about by dynamical changes. A global ocean is a possibility for the Archean. A global ocean simulation predicts 4 K increase in global mean surface temperatures compared to the present-day climate control. Experiments with the following conditions are then combined: rotation rate corresponding to a 14-hr day, 8 $\times$ CO$\sb2$, zero land fraction and decreased solar luminosities of 10, 15, and 20%. The results show that a very equitable climate exist up to a 15% decrease in the solar luminosity. As the solar constant is reduced by 20% the simulated climate becomes unstable with sea ice existing near the equator. The conclusions are: (1) large CO$\sb2$ concentrations in the Archean atmosphere are not necessary to obtain an equitable climate; (2) a faster rotation rate causes a large reduction clouds. (3) Atmospheric dynamics are large components of the climate system and cannot be ignored.