Tracing the snowball bifurcation of aquaplanets through time reveals a fundamental shift in critical-state dynamics
Authors: Feulner Georg , Bukenberger Mona 
Published: March 2022

&lt;p&gt;&lt;span&gt;The&lt;/span&gt; &lt;span&gt;instability&lt;/span&gt; &lt;span&gt;with&lt;/span&gt; &lt;span&gt;respect&lt;/span&gt; &lt;span&gt;to&lt;/span&gt; &lt;span&gt;global&lt;/span&gt; &lt;span&gt;glaciation&lt;/span&gt; &lt;span&gt;is&lt;/span&gt; &lt;span&gt;a&lt;/span&gt; &lt;span&gt;fundamental &lt;/span&gt;&lt;span&gt;property of the climate system caused by the positive ice-albedo feedback. The &lt;/span&gt;&lt;span&gt;atmospheric concentration of carbon dioxide (CO&lt;/span&gt;&lt;sub&gt;&lt;span&gt;2&lt;/span&gt;&lt;/sub&gt;&lt;span&gt;) at which this Snowball bi&lt;/span&gt;&lt;span&gt;furcation occurs changes through Earth&amp;#8217;s history because of the slowly increas&lt;/span&gt;&lt;span&gt;ing&lt;/span&gt; &lt;span&gt;solar&lt;/span&gt; &lt;span&gt;luminosity.&lt;/span&gt; &lt;span&gt;Quantifying&lt;/span&gt; &lt;span&gt;this&lt;/span&gt; &lt;span&gt;critical&lt;/span&gt; &lt;span&gt;CO&lt;/span&gt;&lt;sub&gt;&lt;span&gt;2&lt;/span&gt;&lt;/sub&gt;&lt;span&gt;concentration&lt;/span&gt; &lt;span&gt;is&lt;/span&gt; &lt;span&gt;not&lt;/span&gt; &lt;span&gt;only &lt;/span&gt;&lt;span&gt;interesting from a climate dynamics perspective, but also an important prereq&lt;/span&gt;&lt;span&gt;uisite for understanding past &quot;snowball Earth&quot; episodes and the conditions for &lt;/span&gt;&lt;span&gt;habitability on Earth and other planets. Earlier studies are limited to investi&lt;/span&gt;&lt;span&gt;gations&lt;/span&gt; &lt;span&gt;with&lt;/span&gt; &lt;span&gt;very&lt;/span&gt; &lt;span&gt;simple&lt;/span&gt; &lt;span&gt;climate&lt;/span&gt; &lt;span&gt;models&lt;/span&gt; &lt;span&gt;for&lt;/span&gt; &lt;span&gt;Earth&amp;#8217;s&lt;/span&gt; &lt;span&gt;entire&lt;/span&gt; &lt;span&gt;history&lt;/span&gt; &lt;span&gt;or&lt;/span&gt; &lt;span&gt;studies &lt;/span&gt;&lt;span&gt;of&lt;/span&gt; &lt;span&gt;individual&lt;/span&gt; &lt;span&gt;time&lt;/span&gt; &lt;span&gt;slices&lt;/span&gt; &lt;span&gt;carried&lt;/span&gt; &lt;span&gt;out&lt;/span&gt; &lt;span&gt;with&lt;/span&gt; &lt;span&gt;a&lt;/span&gt; &lt;span&gt;variety&lt;/span&gt; &lt;span&gt;of&lt;/span&gt; &lt;span&gt;more&lt;/span&gt; &lt;span&gt;complex&lt;/span&gt; &lt;span&gt;models &lt;/span&gt;&lt;span&gt;and for different boundary conditions, making comparisons difficult. Here we use a coupled climate model of &lt;/span&gt;&lt;span&gt;intermediate&lt;/span&gt; &lt;span&gt;complexity&lt;/span&gt; &lt;span&gt;to&lt;/span&gt; &lt;span&gt;trace&lt;/span&gt; &lt;span&gt;the&lt;/span&gt; &lt;span&gt;Snowball&lt;/span&gt; &lt;span&gt;bifurcation&lt;/span&gt; &lt;span&gt;of&lt;/span&gt; &lt;span&gt;an&lt;/span&gt;&lt;span&gt; a&lt;/span&gt;&lt;span&gt;quaplanet &lt;/span&gt;&lt;span&gt;through Earth&amp;#8217;s history in one consistent model framework. We find that the &lt;/span&gt;&lt;span&gt;critical CO&lt;/span&gt;&lt;sub&gt;&lt;span&gt;2&lt;/span&gt;&lt;/sub&gt;&lt;span&gt;concentration decreases more or less logarithmically with increas&lt;/span&gt;&lt;span&gt;ing&lt;/span&gt; &lt;span&gt;solar&lt;/span&gt; &lt;span&gt;luminosity&lt;/span&gt; &lt;span&gt;until&lt;/span&gt; &lt;span&gt;about&lt;/span&gt; &lt;span&gt;1&lt;/span&gt; &lt;span&gt;billion&lt;/span&gt; &lt;span&gt;years&lt;/span&gt; &lt;span&gt;ago,&lt;/span&gt; &lt;span&gt;but&lt;/span&gt; &lt;span&gt;drops&lt;/span&gt; &lt;span&gt;faster&lt;/span&gt; &lt;span&gt;in&lt;/span&gt; &lt;span&gt;more &lt;/span&gt;&lt;span&gt;recent times. Furthermore, there is a fundamental shift in the dynamics of the &lt;/span&gt;&lt;span&gt;critical state about 1.8 billion years ago, driven by the interplay of wind-driven &lt;/span&gt;&lt;span&gt;sea-ice dynamics and the surface energy balance.&lt;/span&gt;&lt;/p&gt;