Warm Climate in the “Boring Billion” Era

billion years ago (Ga) (Holland, 2006; Young, 2013). Especially, the period from 1.6 to 1.0 Ga is known as “the dullest time in Earth’s deep-time history” (Buick et al., 1995). The reason why this period is referred to as the “Boring Billion” is because there were very few ‘special’ or ‘interesting’ events discovered in the geological or geochemical records over nearly one-fourth of Earth’s deep-time history. For example, geological records indicate that glacial deposits were almost absent even in the polar region (Bradley, 2011; Cawood and Hawkesworth, 2014; Eyles, 1993; Young, 2013). This lack trace-leaving events suggests the climate was mild and stable during the Boring Billion. In contrast, severe global-scale glaciations, the so-called Snowball Earth events (Hoffman et al., 1998), occurred both before and after the Boring Billion. Phosphate deposits were well developed during 2.2 1.8 Ga and after 0.75 Ga (Papineau, 2010), but were greatly reduced during the Boring Billion. There were no obvious Sr changes in the ocean (Shields, 2007) or in detrital zircons (Belousova et al., 2010). Banded iron formation (BIF) was abundant between 2.3 and 1.8 Ga, and reappeared right after 0.8 Ga (Condie et al., 2009; Silver and Behn, 2008). However, BIF deposits disappeared during the Boring Billion. Rock-hosted copper deposition was almost absent between 1.8 0.8 Ga (Hitzman et al., 2010). Few orogenic gold deposits have been found during 1.7 0.9 Ga, contributing less than 1% of the present production (Goldfarb et al., 2001). Moreover, O2 levels exhibit no significant fluctuations and probably remained approximately between 0.1% to 10% PAL (Planavsky et al., 2014; Canfield, 2005), and complex multicellular life appeared only towards the Boring Billion’s end. All these patterns suggest that Earth might stay stable in terms of climate, ocean chemistry, and life for very long periods, and that these billion years represent a special period in Earth’s history. These geological and geochemical records raise fundamental questions of climate evolutions in the Boring Billion. Why was the climate warmer in the Boring Billion than at present, so that no glaciers developed? Solar radiation in the Boring Billion was much weaker than that at present. How did the climate remain so stable over such a long period? Why did life evolve so slowly? It has been argued that the stable climate could be due to weak tectonic activity and relatively stable continental configuration (Young, 2013). However, there is evidence showing that the Boring Billion experienced merging and breaking up of the Columbia supercontinent (~1.8 1.2 Ga) and the formation of the Rodinia supercontinent (~1.2 0.9 Ga) (Roberts, 2013; Pisarevsky et al., 2014). Understanding the answers to these questions must involve complex processes of the atmosphere and ocean, surface bio-geochemical weathering reactions and deposits, and the interaction between the Earth’s surface and interior. To date, there have been no complete and convincing answers to these fundamental questions. In the present study, we focus on a specific question: how high levels of greenhouse concentrations are required to maintain a warm climate over the billions of years, given the constraint that no glaciation or only glaciations of small extent occurred. This problem has been studied previously with a one-dimensional energy-balance model (EBM). It was shown that 30 times PAL CO2 is needed to maintain a warmer climate, with no polar ice caps (Kasting, 1987; Reinhard et al., 2016). Here, we study the problem using a fully coupled atmospheric-oceanic general circulation model. The model used in the present study is the Community Climate System Model version 3 (CCSM3), which was developed by the National Center for Atmospheric Research (NCAR), and has been applied to simulate climates of the past, present, and future (Collins et al., 2006). The CCSM3 includes four components, namely the atmosphere, ocean, land, and sea ice. It is designed to simulate varied climate states with a series of spatial resolutions. In this study, we use a low-resolution version that is widely used in the paleoclimate reconstructions. The resolution of the atmospheric component is approximately 3.75°x3.75° and 26 vertical levels. The ocean component has a spatial resolution named “gx3” with 100 and 116 grid points in the zonal and meridional direction, and a vertical partitioning of 25 levels extending to 4.8 km. For the Boring Billion era, we may safely assume that the land surface was unvegetated. The surface is assumed to have been covered by a layer of average soil, consisting of 15% clay, 43% sand and the rest silt. Surface albedo was set to 0.09 0.18 in the visible band and 0.18 0.36 in the near-infrared band, with the lower limit taken if the soil was saturated with water and the higher limit taken if the soil was dry. Two parameters are key in determining the climate of the Boring Billion. One is the level of greenhouse gases (e.g. CO2 concentration) and the other one is solar radiation. The solar luminosity increases with time. It was about 6-13% weaker than present during the Boring Billion (Bahcall et al., 2001). Thus, solar insolation at the top of the atmosphere (TOA) was set to 90% of the present-day value, i.e., 1230 W m. We have Warm Climate in the “Boring Billion” Era