Towards Coupled Modelling of the Biosphere and Atmosphere for the Archean Climate: the importance of Methane

Life has played a key role in shaping the atmosphere since its origin on Earth, but modelling the biosphere’s impact on climate is complicated by the range of time and spatial scales involved. 3D climate models have successfully been used to spatially resolve key processes, but on relatively short time scales compared to those at which the biosphere interacts with the climate system. Whereas, biogeochemical modelling allows us to estimate biosphere driven gas fluxes in and out of the atmosphere over longer time scales [1], but lacks a sophisticated treatment of a spatially resolved atmosphere. Here, we look to bridge these two modelling approaches to better understand the biosphere’s impact on the climate.

We use a biogeochemical model [2] to understand the limits on the potential evolution of the atmosphere, as well as a state-of-the-art 3D climate model [3] to explore potential atmospheric compositions produced by early biospheres. The biogeochemical model, coupled to a 1D photochemical model, has been developed to explore the effects of early biospheres driven by anoxic phototrophs. There is a particular focus on the effect of methane on the early climate, which has predominantly biotic sources. We use the 3D climate model to extend a 1D exploration of methane’s diminished greenhouse potential during the Archean [4] by looking at how methane concentrations affect the cloud distribution, atmospheric dynamics and surface temperature.

We find that global surface temperature peaks for pCH₄ between 30-100 Pa, with the peak shifting to higher pCH₄ as pCO₂ is increased. Equator-to-pole temperature differences also have a peaked response driven by changes in the radiative balance. These changes come about from the balance between the effect of methane and carbon dioxide on atmospheric dynamics due to changes in heating rates vertically and meridionally, which also affects the cloud formation. This work begins to explore how our understanding of early biospheres can be coupled to 3D climate models, to understand the biosphere’s impact on the climate of Earth and terrestrial exoplanets following the origin of life.

References

[1] Kharecha, Kasting & Siefert (2005) Geobiology 3, 53-76.

[2] Lenton & Daines (2017) Ann. Rev. Mar. Sci. 9:1, 31-58.

[3] Mayne et al. (2014) Geosci. Model Dev. 7, 3059–3087.

[4] Byrne & Goldblatt (2015) Clim. Past 11, 559–570.