So, with life struggling so hard and the temperatures continuing to fall, is this planet doomed? Is the ice and snow going to claim the entire planet, turning the living green and blue marble into a dead white one? Perhaps so, as it is not only the dimmer Sun that causes the cold. As temperatures fall, some processes are triggered that make the temperature even colder. This is called positive feedback, and there are plenty of such processes making things worse for life on Earth.
As the years passed since the event, the sudden loss of sunlight triggered a myriad of effects. The cooler air slowed evaporation rates worldwide, depriving the atmosphere of a significant greenhouse gas. The areas once darkened by ocean were replaced with fields of bright ice, reflecting more light and heat back into space. The land also reflected more heat and light as dark green plants and brown soil gave way to bright white ice and snow. In the Northern Hemisphere, the expansive spruce forests, which absorbed heat all year with their dark needles, gave way to larch, which shed their needles in winter, letting the snow beneath reflect heat back to space. In time, many of these larch expansions are replaced with reflective ice sheets. The waters of the globe, now more capable of dissolving carbon dioxide due to their reduced temperatures, steal the gas from the atmosphere, lessening the warmth of the planet’s greenhouse effect. This dissolved carbon dioxide also weakens the shells of many animals, making their outlook more bleak.
With all these factors working together, perhaps it is inevitable that the ice will take the world. The temperature falls, the positive feedback makes it fall more, and so on until the planet is frozen and all life is dead.
But that is only one side of the equation. There are also processes, opposite to the positive feedback processes, that warm the planet in response to cold temperatures. These are called negative feedback processes, and within them may lie the solution.
For example, although the growing sheets of ice reflect more light back into space, they also prevent the rocks under them from chemically weathering. Chemical weathering is the breakdown of rocks and minerals in chemical reactions, most of which consume carbon dioxide from the air. However, with the rocks buried under sheets of ice, they no longer have access to air, so as the ice sheets expand, less and less carbon dioxide disappears into the ground.
Even more influentially, the more the oceans cool, the more carbon dioxide is ultimately pumped into the air. At first, the oceans suck up carbon dioxide, since it is more soluble in water at low temperatures. However, minerals are also more soluble at low temperatures, turning the oceans into a paradise for algae. These convert the carbon dioxide in the water to oxygen, which is less soluble in water than carbon dioxide, so it is more likely to return to the air. Here, terrestrial heterotrophs convert it back to carbon dioxide, but the reduced plant life on land can’t turn it back into oxygen as fast as the algae can underwater. Effectively, a sort of pump has formed, bringing carbon dioxide from the oceans into the air, where it can help hold in heat.
And there are more factors working to stop the freeze. The bicarbonate buffers of the seabed, which help keep the carbon dioxide levels in the biosphere stable, help to keep the gas’s concentration from straying too far. The atmosphere itself, which has contracted around the planet as it cooled, now holds its heat more effectively and makes it less likely for it to lose heat to space. Not long after the world seemed lost, a miracle is underway.
As the growth of the ice and snow slows, less and less of the infrared-absorbing land and ocean is consumed by the whiteness. The negative feedbacks push back at the positive, slowly approaching a new equilibrium. Soon, the seemingly unstoppable advance of the ice shrieks to a halt, as the Earth finds a new balance. Against the odds, the world is saved.