Water from Heaven

Since at least three billion years, liquid water has existed on our planet, making possible the evolution of all forms of life. The Sun has grown brighter over the same period, but the oceans have never boiled. Surrounding our never completely frozen Earth, the atmosphere’s powerful greenhouse kept the oceans liquid when the Sun was weak. As the Sun grew brighter, gradual weakening of the Earth’s atmospheric greenhouse ensured the stability of climate and favorable conditions for the development of life. Ice sometimes covered enormous areas, but the ice always receded, sometimes disappearing almost completely from the planet’s surface. Sea level fell and rose again, and at least once in their long history the Mediterranean and the Black Seas evaporated completely, leaving only deserts of salt behind; but the water always returned.

One day this long and wonderful story will end. The slow steady increase of the Sun’s brightness results from changes in its internal structure, changes made necessary by the inexorable conversion of hydrogen to helium at the center of our star. As hydrogen available for “burning” runs out, energy production in the central core decreases, temperatures and densities remaining too low for burning of helium to form carbon. The Sun keeps shining, its energy production now taking place mostly in an intermediate envelope outside the central core, and as this process accelerates over the next four billion years, the Sun’s outer envelope will have to expand; our star will become brighter and brighter. As it uses up its last hydrogen fuel reserves, it burns faster and faster, brighter and brighter. How can the Earth’s atmosphere and biosphere adapt to the Sun’s evolution? Over the last few decades, human activities have been reinforcing the greenhouse effect and warming slightly the surface and lower layers of the atmosphere of the Earth. But this process, although apparently too slow for many politicians to apprehend, will be but a brief parenthesis in the long history of Sun and Earth. Over the last few billion years, the planet has kept cool by weakening the greenhouse effect as the Sun grew brighter; over the next few billion years, the problem for the planet will grow more acute as the Sun grows still brighter. Can the greenhouse effect be weakened still further? And for how long? According to the “Gaïa” hypothesis of English chemist James Lovelock and Boston University biologist Lynn Margulis, life maintains optimal conditions for its existence and development by modifying the physical and chemical properties of the Earth and, in particular, the composition of the atmosphere. Indeed, the biological process of photosynthesis gradually transformed much of the CO₂ initially present in the atmosphere into organic matter, and the “sequestration” of much of this matter as sediments and as fossil fuel underground explains the long-term reduction of atmospheric CO₂. The resulting weakening of the greenhouse effect seems to have compensated the increase of the Sun’s brightness. For how long can such a process work? Human activities have been putting some of the fossil carbon back into the atmosphere as CO₂, but even if the level is quadrupled for a few centuries, that will not make much CO₂ to be extracted from the atmosphere. Moreover, photosynthesis in its present form could not continue if all the CO₂ were to be removed from the atmosphere. In any event, carbon dioxide is only the second greenhouse gas, the most important being of course water vapor. Climate sensitivity to perturbations depends on many feedback mechanisms, some of which act to amplify change. Positive water vapor feedback plays a central role in determining how much global warming will result from anthropogenic emissions of greenhouse gases, in particular CO₂, over the next few centuries. However, positive feedback will also amplify global cooling in response to a decrease in CO₂. With luck, a beneficent Providence, or geo-engineering by earthlings far more advanced than today’s humans, the Earth system may be able to maintain photosynthesis while at the same time avoiding excessive heating by a brighter Sun, by some appropriate combination of changes in the factors of CO₂, atmospheric humidity, cloud cover, and surface snow and ice cover. For Earth to remain livable a billion or two years from now, clouds or ice must ultimately win out over the greenhouse effect.

Even if life’s lease on Earth can be extended, it will come to an end when the Sun becomes a red giant, its outer envelope approaching the orbit of our planet. When that happens, temperatures will have to rise to the point where the oceans begin to boil. Earth’s waters will evaporate and escape to space. Will those H₂O molecules meet again, some day on some other planet around some other sun? Will the story start all over again? We can always dream . . .

Have a drink of water. If you fill your glass with the excellent quality tap water of Paris, you’ll swallow some drops that fell as rain near the sources of the Seine, perhaps as recently as last winter, perhaps several years or even a decade or two ago.¹ That precipitation came from clouds most probably formed over the Atlantic from water vapor evaporated from the Gulf Stream or from other not-quite-so-warm water drifting northward. At least some of that water has a history of centuries of circulation in the depths of the ocean, between the Norwegian Sea and the Indian Ocean or even the Pacific. If, however, you’d rather quench your thirst with mineral water, the recent history of your drink, before it was bottled, may go back thousands of years, with a long detour in glaciers and underground. In both cases, many of the water molecules have been exchanged billions of times between the oceanic Earth and the skies, ever since the primordial outgassing of the planet’s innards. When you have a drink of water, you are absorbing, for a brief time, a small sample of matter carrying with it part of the history of the Earth and of the universe.

You’d rather have some wine? Fine! To the water evaporated from the Atlantic that fell as rain on the vineyards around Bordeaux, nature and the winegrower have added sunshine, CO₂, loving work, and a dash of other elements, stardust. A votre santé!