Life on Earth depends on liquid water and the temperature at which it freezes or boils. How much would the values of 0掳C and 100掳C need to change to make life here unsustainable, or hugely different?
鈥� Due to its strong hydrogen bonding, water has a high freezing and boiling point for its molecular weight of 18. In comparison, ethane, with a molecular weight of 30, freezes at -182.8掳C and boils at -88.5掳C. If water had the same level of hydrogen bonding as ethane, it would freeze and boil at much lower temperatures, and Earth as we know it would be dry.
Organisms seem to require a liquid solvent for transporting materials, so Earth might be lifeless if water boiled at around 40掳C. This is because water vapour is a greenhouse gas. Once runaway evaporation of the oceans began, the temperature would rise, causing ever more evaporation.
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Similarly, if water turned to ice at say 30掳C, all the planet鈥檚 oceans would freeze right down to the sea floor. In this scenario, runaway cooling from reflective white ice wouldn鈥檛 be counteracted by any amount of carbon dioxide emissions, leading to a permanent 鈥渟nowball Earth鈥�.
However, one could argue that life maintains its own conditions for survival. The limits for life as we know it are then how much heat or cold organisms can tolerate. We have thermophilic organisms that can live above 120掳C, while psychrophilic bacteria can live at -20掳C. So life could probably exist in some form even if water boiled at 0掳C or froze at 100掳C.
Hillary Shaw, Newport, Shropshire, UK
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