The Sun is midway through its life of fusion. It\u2019s about five billion years old, and though its life is far from over, it will undergo some pronounced changes as it ages. Over the next billion years, the Sun will continue to brighten. <\/p>\n
That means things will change here on Earth. <\/p>\n
<\/p>\n As the Sun goes about its business fusing helium into hydrogen, the ratio of hydrogen to helium in its core changes. Over time, the core slowly becomes more enriched in helium. As helium accumulates in its core, the core\u2019s density increases, meaning protons are more closely packed together. That creates a situation where the Sun can fuse hydrogen more efficiently. After a chain reaction of processes and cause and effect, the end result is that the Sun\u2019s luminosity increases. The Sun\u2019s luminosity has already increased by about 30% since its formation, and the brightening will continue.<\/p>\n Any increase in the Sun\u2019s luminosity can have a pronounced effect on Earth. Environmental cycles like the carbon, nitrogen, and phosphorous cycles sustain Earth\u2019s biosphere. As the Sun becomes brighter, it will affect these cycles, including the carbonate-silicate cycle, which moderates the accumulation of carbon dioxide (CO2<\/sub>) in the planet\u2019s atmosphere. <\/p>\n Scientists think that over the next billion years, the brightening Sun will disrupt this cycle, leading to declining CO2<\/sub> levels. Plants rely on CO2<\/sub> and the levels are expected to plummet, which means that complex land life would end in the next billion years. <\/p>\n It\u2019s a bleak prognosis, but new research suggests it might not happen.<\/p>\n The new research is \u201cSubstantial extension of the lifetime of the terrestrial biosphere,\u201d and it\u2019s been accepted for publication in the Planetary Science Journal. It\u2019s in pre-print now, and the lead author is R.J. Graham, a postdoctoral researcher in the Department of Geophysical Sciences at the University of Chicago.<\/p>\n \u201cApproximately one billion years (Gyr) in the future, as the Sun brightens, Earth\u2019s carbonate-silicate cycle is expected to drive CO2<\/sub> below the minimum level required by vascular land plants, eliminating most macroscopic land life,\u201d the authors write. <\/p>\n As the Sun brightens and warms the Earth\u2019s surface, scientists expect the carbonate-silicate cycle to draw more CO2<\/sub> out of the atmosphere because of carbonate-silicate weathering and carbonate burial. Rainwater is enriched with atmospheric carbon, which reacts with silicate rocks and breaks them down. The products of the chemical reactions that break them down find their way to the ocean floor, where they form carbonate minerals. As these minerals are buried, they effectively remove carbon from the atmosphere. <\/p>\n Normally, the cycle acts as Earth\u2019s natural thermostat. However, higher temperatures make the reactions more efficient, meaning the carbonate-silicate cycle will remove more CO2<\/sub> from the atmosphere. That\u2019s what led scientists to conclude that the CO2<\/sub> will become so low that planet life will perish. However, the authors examined these ideas and found that it may not quite work out that way. <\/p>\n \u201cHere, we couple global-mean models of temperature- and CO2<\/sub>-dependent plant productivity for C3<\/sub> and C4<\/sub> plants, silicate weathering, and climate to re-examine the time remaining for terrestrial plants,\u201d they write. C3<\/sub> and C4<\/sub> plants are two main plant groups that are classified based on how they perform photosynthesis and absorb carbon. They\u2019re relevant because they respond differently to higher temperatures. <\/p>\n The researchers say recent data shows that the carbonate-silicate cycle isn\u2019t as temperature-dependent as previously thought. Instead, it\u2019s only weakly temperature-dependent and more strongly CO2<\/sub>-dependent. In that case, \u201cwe find that the interplay between climate, productivity, and weathering causes the future luminosity-driven CO2<\/sub> decrease to slow and temporarily reverse, averting plant CO2<\/sub> starvation,\u201d they explain. <\/p>\n Instead of a one billion-year outlook for Earth\u2019s plant life, the researchers say atmospheric CO2<\/sub> levels will mean plants have another 1.6-1.86 billion years. When plants can no longer survive, it won\u2019t be because of plummeting CO2<\/sub> levels. Instead of CO2<\/sub> starvation, it\u2019ll be because of what scientists call the moist greenhouse transition. <\/p>\n When that transition happens, a planet\u2019s atmosphere becomes saturated with water vapour as the planet warms. Since water vapour is a potent greenhouse gas, it creates a feedback loop of increased warming. Eventually, it\u2019s simply too hot for plants to survive. The consequences don\u2019t end there. As the Earth\u2019s upper atmosphere becomes more saturated with water vapour, UV energy splits water apart, and the hydrogen drifts off into space. In this situation, there\u2019s a gradual and irreversible loss of water into space. <\/p>\n According to the authors, Earth won\u2019t experience this transition for between about 1.6 and 1.86 billion years. <\/p>\n \u201cWe show that recent data indicating weakly temperature-dependent silicate weathering lead to the prediction that biosphere death results from overheating, not CO2 starvation,\u201d the authors write. \u201cThese findings suggest that the future lifespan of Earth\u2019s complex biosphere may be nearly twice as long as previously thought.\u201d<\/p>\n These results also affect our understanding of exoplanet habitability. It has to do with what are called \u2018hard steps\u2019 in the appearance and evolution of life. The hard steps model says that certain evolutionary transitions were difficult and unlikely to happen twice. Some examples are the appearance of multicellular organisms and the Cambrian explosion. <\/p>\n But if Earth\u2019s biosphere has a much longer lifespan than thought, that affects the hard steps model.<\/p>\n \u201cA longer future lifespan for the complex biosphere may also provide weak statistical evidence that there were fewer \u201chard steps\u201d in the evolution of intelligent life than previously estimated and that the origin of life was not one of those hard steps,\u201d the authors conclude. <\/p>\n If that\u2019s the case, then exoplanet habitability could be less rare than thought. <\/p>\n![\"As](\"https:\/\/www.universetoday.com\/wp-content\/uploads\/2024\/02\/CoRoT-7b1280-1024x524.jpg\")
![\"This](\"https:\/\/www.universetoday.com\/wp-content\/uploads\/2024\/09\/Substantial-Extension-of-the-Lifetime-of-1024x580.jpg\")
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