The faint young Sun problem

For more than four decades, scientists have been trying to find an answer to one of the most fundamental questions in paleoclimatology, the `faint young Sun problem'. For the early Earth, models of stellar evolution predict a solar energy input to the climate system which is about 25% lower than today. This would result in a completely frozen world over the first two billion years in the history of our planet, if all other parameters controlling Earth's climate had been the same. Yet there is ample evidence for the presence of liquid surface water and even life in the Archean (3.8 to 2.5 billion years before present), so some effect (or effects) must have been compensating for the faint young Sun. A wide range of possible solutions have been suggested and explored during the last four decades, with most studies focusing on higher concentrations of atmospheric greenhouse gases like carbon dioxide, methane or ammonia. All of these solutions present considerable difficulties, however, so the faint young Sun problem cannot be regarded as solved. Here I review research on the subject, including the latest suggestions for solutions of the faint young Sun problem and recent geochemical constraints on the composition of Earth's early atmosphere. Furthermore, I will outline the most promising directions for future research. In particular I would argue that both improved geochemical constraints on the state of the Archean climate system and numerical experiments with state-of-the-art climate models are required to finally assess what kept the oceans on the Archean Earth from freezing over completely.Comment: 32 pages, 8 figures. Invited review paper accepted for publication in Reviews of Geophysic

Abstract

AppleScript is a scripting language and environment for the Mac OS. Originally conceived in 1989, AppleScript allows end users to automate complex tasks and customize Mac OS applications. To automate tasks, AppleScript provides standard programming language features (control flow, variables, data structures) and sends Apple Events to invoke application behavior. Apple Events are a variation on standard remote procedure calls in which messages can identify their arguments by queries that are interpreted by the remote application. This approach avoids the need for remote object pointers or proxies, and reduces the number of communication round trips, which are expensive in high latency environments like the early Macintosh OS. To customize an application that uses AppleScript’s Open Scripting Architecture, users attach scripts to application objects; these scripts can then intercept and modify application behavior. AppleScript was designed for casual users: AppleScript syntax resembles natural language, and scripts can be created easily by recording manual operations on a graphical interface. AppleScript also supported internationalization in allowing script to be presented in multiple dialects, including English, Japanese, or French. Although the naturalistic syntax is easy to read, it can make scripts much more difficult to write. Early adoption was hindered by the difficulty of modifying applications to support Apple Events and the Open Scripting Architecture. Yet AppleScript is now widely used and is an essential differentiator of the Mac OS. Apple-Script’s communication model is a precursor to web services, and the idea of embedded scripting has been widely adopted