The Evolution of Atmospheric Composition: Why Earth is a Habitable Planet

The long term evolution of Earth’s atmosphere and climate has been an active topic of investigation for at least the last 60 years. My own participation in this investigation goes back more than 45 years, and this monograph relates that story from my personal perspective. One major thread concerns the rise of atmospheric O2 from near-zero levels initially to the 21 percent mixing ratio that we observe today. Photochemical models developed by me and my students, along with some close colleagues, have helped to better constrain the prebiotic O2 concentration and to interpret the constraints imposed by the record of mass independent fractionation of sulfur isotopes. Most geochemists now agree that a so called Great Oxidation Event (GOE) occurred between 2.4 and 2.2 Ga and that the atmosphere has been O2-rich since that time. However, the exact level of O2 during the ensuing Proterozoic Era remains controversial, as do the timing and magnitude of subsequent O2 increases. The corresponding development of the ozone layer is also of interest because of its moderating influence on surface solar UV fluxes and their effect on biological evolution. This can also be studied with photochemical models. A second thread concerns the gradual decline in atmospheric CO2 in response to slowly increasing solar luminosity. The early Earth would have been frozen had the atmosphere not contained high concentrations of greenhouse gases, most importantly CO2. A negative feedback in the carbonate-silicate cycle that controls CO2 over long time scales has ensured that Earth’s surface has remained habitable during most of Earth’s history, despite occasional forays into Snowball Earth conditions. Evidence from palaeosols provides support for this hypothesis. CH4 is an additional greenhouse gas that may have supplemented surface warming prior to the GOE. The increase in O2 at that time may have caused CH4 to decrease, possibly triggering the Huronian glaciations. The same feedback mechanism that controls long term CO2 evolution on Earth could operate on Earth-like planets orbiting other stars, increasing the probability that some of them may harbour life. Large direct imaging space telescopes currently under development may eventually allow us to test this hypothesis and to learn whether we have company in this part of our galaxy.