Task B: The Earth Through Time

This research area combines geological and biological data with ecosystem and climate models to help us determine how Earth’s environment has changed throughout its history. By constraining parameters like surface temperature, atmospheric pressure, and composition, and determining corresponding biogeochemical cycling and greenhouse effects, we can explore a range of ecosystems dominated by many different metabolisms very different from those found on modern-day Earth. Since life co-evolves with its environment, the coupled study of life and its environment throughout history may provide insight into ancient habitability.

VPL has already contributed significantly to our understanding of this long-term co-evolution of the Earth and its biosphere, including the development of tools that we are continually improving. We have developed photochemical models (e.g., Zerkle et al., 2012), atmospheric escape models, weathering models (Bolton et al., 2006), climate models (Roberson et al., 2011), spectral models of the ancient Sun (Claire et al., 2012), paleobarometry measurements (Som et al., 2012), and coupled biology-nutrient cycling models (Claire et al., 2006).

Together, these models allow us to constrain the likely biological productivity and climates of these environments, and simulate planetary spectra that are self-consistent with these environments and metabolisms. We have used these models to interpret various Archean environments, including a haze-rich Mesoarchean atmosphere (Domagal-Goldman et al., 2008; Haqq-Misra et al., 2009), the “faint young Sun” paradox in the Proterozoic (Roberson et al., 2010), and biological productivity in the Archean (Claire et al., 2006; Kharecha, et al., 2005).

The study of Earth’s history provides us with a variety of plausible environments and metabolisms to consider, and by simulating planetary spectra for these unique cases, around stars of various spectral types, we can begin to catalog a range of biosignatures we may potentially detect with exoplanet characterization missions such as JWST and TPF (see also Tasks D and E).

Specific scientific highlights from The Earth Through Time are given below:

The Climatic Effect of N2O in the Earth's Early Atmosphere

In Roberson et al., (2011) we examined the fluxes and climatic impacts of nitrous oxide (N2O) in the Earth’s Proterozoic atmosphere. An anoxic, sulfidic ocean that may have existed during the Proterozoic Eon (0.54-2.4 Ga) would have had limited trace metal abundances because of the low solubility of metal sulfides. A scarcity of copper, in particular could have reduced marine denitrification, as Cu is needed to convert N2O to N2.  Without this denitrification step, the N2O surface flux could have been 15-20 times higher than today, producing atmospheric N2O concentrations of several ppmv. If CH4 concentrations were also high, as has been suggested, the combined greenhouse effect of CH4 and N2O could have provided up to 10 degrees of warming, thereby keeping the surface warm during the Proterozoic without necessitating high CO2 levels. A second oxygenation event near the end of the Proterozoic would have resulted in a reductions of both atmospheric N2O and CH4, perhaps triggering the Neoproterozoic “Snowball Earth” glaciations.

Figure: Global mean surface temperature (K) vs. nitrous oxide abundance. Two curves represent calculations for two solar luminosities: 83% and 94% of present value So. Concentration of CH4 is fixed at 1.6 ppm. (CO2 assumed at preindustrial value of ~320 ppm). From Roberson et al., 2011.

The Role of Clouds in Early Earth Climate

The team published two papers (Goldblatt and Zahnle, 2011a and 2011b) that studied the effects of clouds on the early Earth’s climate. This is relevant to the "faint young sun paradox,” which is the juxtaposition of expectations that the sun was dimmer early in Earth's history with geological evidence for a climate at least as warm as today's. One of the largest uncertainties in models of this time is clouds which could have caused either warming or cooling on early Earth. We conducted a major parametric study, in which we fully explored the phase space of changing clouds, understanding their radiative forcing and allowing objective evaluation of the forcing from various hypothesized changes to clouds (Goldblatt and Zahnle, 2011a). For example, we responded rapidly (Goldblatt and Zahnle, 2011b) to a high profile proposal by Rosing et al. (2010) that less low cloud would resolve the faint young sun paradox, showing that this was not quantitatively plausible. We also showed that a long-standing assumption in early Earth climate models that clouds could be neglected from the atmosphere and replaced with an enhanced surface albedo leads to a systematic over-estimate of the strength of greenhouse gas forcings. A spin-off application of our work on Earth was in astronomy; we were able to show that representing clouds in brown dwarf stars as having partial coverage (cloudy and cloud free sub-columns in a single-column model) gave a much better fit to observed spectra than existing models (Marley et al., 2010).