Exobiology and Evolutionary Biology

The Faint Young Sun Problem in the Early Biotic Atmosphere of the Earth

PI: Owen Toon

We have constructed a three-dimensional climate model designed to better understand the climate and chemistry of the atmosphere of the Archaen Earth. Understanding the climate and atmospheric chemistry of the Earth at the time of the origin of life and its early evolution are essential to understanding why life has prospered on our planet. The climate and atmospheric chemistry are coupled through the faint young sun problem. We have already used a microphysical model within the climate model to show that the formation of fractal haze particles may have provided a significant optical depth gradient between the ultraviolet and the visible. Hence this fractal haze may have provided a shield against ultraviolet light, without cooling the Earth significantly. We propose to expand this model in several ways. We will replace the photochemical model, designed for current Earth, with a model that has been widely used to study the ancient Earth. We will also update the radiative transfer code so that it is able to handle a greater range of greenhouse gases than the present model, which is designed for current Earth. We will then be able to fully link three dimensional dynamics, particle microphysics, atmospheric chemistry, and radiative transfer to better understand the Archaen environment. We will apply the model to better understand the role that the fractal particles play in climate and chemistry. For instance, the chemistry will determine where the particles form, and their abundance. In turn the aerosols will alter the chemistry. For example, their ultraviolet shielding may allow ammonia, a powerful greenhouse gas, to build up. Using a three dimensional model allows us to tackle issues that have not previously been treated extensively. For example, we will explore the role of clouds in the ancient greenhouse. We suspect that clouds will rearrange their distributions in ways that warm the planet. For example, the loss of the ozone layer may allow cirrus clouds to form at higher altitudes than now, warming the planet. In addition, the lack of aerosols in the low oxygen atmosphere will cause clouds to be less bright, also warming the planet. These effects, largely neglected in one-dimensional climate models, may make it much easier for greenhouse gases to warm the planet.