Abstract EANA2025-143

Between about 2.4 and 0.6 Gyr ago, our planet underwent  several episodes of large-scale glaciations, including the last and best-known one, known as “Snowball Earth”, that ended about 635 Myr ago just before the beginning of the Cambrian Period. One of the processes that allowed the long-term persistence of a glaciated state was the positive feedback between ice and albedo. While the ice-albedo feedback is known since long time, the precise causes that can trigger a global glaciation are still debated. Another relevant process is the vegetation-albedo feedback: since vegetation is usually darker than bare rock or sand, its lower albedo tends to warm up the surface (Bisesi+2024). At the inception of the Snowball, 700 Myr ago, the continents were essentially devoid of vegetation and the granite surfaces had a much larger albedo than oceans or forested surfaces, thus favouring the insurgence of a glaciated state. We explored the dynamics of rocky planets that do or do not harbor vegetation on land about 700 Myr ago, for different values of greenhouse gas concentrations in the atmosphere, land albedo and solar output. We explored what parameter values and planetary characteristics are compatible with the insurgence of a Snowball state, and when, instead, such a glaciated state is difficult to trigger. To investigate the potential role of surface albedo in influencing the insurgence of Snowball Earth conditions, we employed the climate model pRT-ESTM (Bisesi+2024), interfaced with radiative transfer computations performed with petitRADTRANS (Mollière+2019). We considered two different geographic setups – modern Earth and Rodinia-like continental distributions (Li+2008), and two different values of the solar luminosity (L = 0.95 Lsun, and L = Lsun). For the solar input typical of 600-700 Myr ago (95% of the current value), the presence of bare continents with mean albedo 0.35 in the position estimated for Rodinia was sufficient to trigger a global Snowball state for CO2 concentrations up to at least 1000 ppm. If continents were instead located in the modern positions, Snowball could be triggered only for values of CO2 concentration up to 400 ppm. At current values of solar input, instead, Snowball states can appear only at or below 100 ppm. For CO2 concentration around 200 ppm and bare continents, extended icy areas can still be present, with ice-covered planet fractions between about 0.7 for Rodinia-like land distribution and 0.3 for the modern position of the continents. The presence of vegetation over the continents, on the other hand, modifies the average land albedo and completely changes the situation. We found that a vegetation cover decreasing the land albedo from 0.35 to 0.15 or lower is sufficient to prevent the onset of a Snowball Earth state for CO2 concentration values of at least 400 ppm, assuming a solar luminosity of 95% the current value and a Rodinia-like distribution of the continents. Modern solar luminosity values do not allow Snowball states, even with equatorial continents, unless the continental albedo is close to that of granite and CO2 concentration is 100 ppm or less.