by Jacob Haqq-Misra Wednesday, May 14, 2014
About 5 billion years from now, Earth will meet its end in a fiery blaze as it is swallowed by the expanding sun. What happens between now and then, in large part, is up to us and our ability to prepare for the distant future.
Contemporary concerns over climate change present immediate challenges for civilization to adapt to a rapidly warming world over the next few hundred years. Our adaptations now may allow us to prepare to adapt to greater challenges, such as the one presented by our ever-evolving sun.
When Earth formed, the sun was notably fainter than today, and models of solar evolution predict that the sun increases in brightness on billion-year timescales. The sun is powered by nuclear fusion, which releases energy as hydrogen fuel in the core is converted into helium waste. During its lifetime, the sun’s core slowly becomes more abundant in helium, causing the core’s pressure to increase, which in turn drives higher temperatures and increases the rate of fusion reactions. Over time, these gradual changes in the core lead to an increase in the energy output of the sun that will steadily continue throughout our star’s adult life.
One result of this gradual brightening will become apparent about half a billion years from now: Carbon dioxide levels will approach dangerously low levels. (Carbon dioxide is needed for photosynthesis, which, in turn, is needed to sustain an oxygenated atmosphere.) The Earth system slowly recycles carbon dioxide on hundred-million-year timescales, which moderates the planet’s global temperature. The carbonate-silicate cycle draws carbon dioxide out of the air via rainwater that then dissolves minerals on land and carries the resulting ions into the ocean. In the ocean, new sediments form, which are eventually subducted into the mantle and emitted into the atmosphere by volcanoes. The speed at which minerals dissolve by the action of weakly acidic rain depends on global temperature, and this reaction rate will speed up as the sun brightens.
In other words, the Earth system will begin to draw down atmospheric carbon dioxide, thereby reducing the greenhouse effect and compensating for the warming sun. Half a billion years from now, these changes in the sun will cause carbon dioxide levels on Earth to drop so low that a major type of photosynthesis (known as C3 carbon fixation) becomes impossible to sustain. Organisms that rely on C3 carbon fixation comprise about 95 percent of vegetation today, so only a fraction of plant species will survive this first major change to Earth’s biosphere.
Over the next half billion years or so after that, as carbon dioxide continues to deplete, plant and animal life will continue to dwindle. All forms of photosynthesis will cease about 1.1 billion years from now, which will nearly eliminate the oxygen in air and cause a corresponding widespread animal extinction. From this point on, the only likely survivors on planet Earth will be highly adapted unicellular microorganisms that occupy high-altitude, subsurface and other “refuge” environments where small amounts of liquid water remain. However, even these survivors will find Earth to be completely uninhabitable about 2.8 billion years from now, when any refuge environments will dry up completely and leave our planet devoid of life.
Thus, in the absence of human mitigation activities, animal life has less than a billion years of habitable climate left. However, human intervention may allow survivors to persist in climate-controlled habitats for much longer. Geoengineering — such as injecting cooling aerosol particles into the stratosphere — is under discussion today as a possible temporary remedy to offset climate change. In the distant future, geoengineering may be humanity’s only strategy to keep the planet cool enough to support life and maintain civilization. While these distant changes will not concern anyone alive today, perhaps our investment in climate change technology will also someday benefit our descendants — at least for a while.
In the end, the lifetime of Earth is still finite and the planet will still be destroyed by the sun in about 5 billion years. At the end of its lifetime, when hydrogen fuel in the core is mostly spent, the sun will swell up into an elderly red giant and continue fusing hydrogen in its expansive surrounding layers. The actual radius of the sun will increase enough to engulf the orbits of Mercury, Venus and Earth. Of the four terrestrial planets, only Mars has a chance at surviving the sun’s transition into later life, but even Mars will be too hot for life to survive. As Robert Frost wrote about the planet’s ultimate fate, “the world will end in fire.”
Humanity can only survive the full evolution of the sun by leaving our home planet to colonize other worlds — right now the stuff of science fiction. Those other worlds may be in other solar systems, or perhaps even ours, as the outer solar system will provide clement conditions during the sun’s old age. Perhaps future human colonists will occupy the moons of Jupiter or Saturn, or maybe our descendants will live primarily in orbiting space stations. Even more grandiose plans for survival suggest macroengineering feats to either move planet Earth to a farther orbit or disassemble the planet to build smaller habitats at safe distances from the sun. Whether or not these extreme feats are possible, the basic technologies of space exploration being developed today will be needed for humans to reach habitable environments in the future.
Thoughts of this distant future may seem irrelevant in light of present-day issues, and priority should certainly be given to these more pressing matters. But, as a way of maintaining perspective, we should remember that our actions today, and the knowledge we build, will provide a foundation for our descendants to face their own unique challenges.
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