The final days of life on Earth will come some 2.8 billion years from now, suggests a sobering new study.
Currently at a comfortable temperature for life on Earth, our aging sun will slowly warm over its lifetime. Within about five billion years, the sun will exhaust its nuclear fuel and bloat into a "red giant" star that may even engulf our planet.
Things will get toasty for existing life-forms long before that red giant stage is reached. The question examined by a team led by astrobiologist Jack O'Malley-James, of the University of St. Andrews in Scotland, is: When will things get too hot for life to continue?
Using measures such as temperature and abundance of water and food to examine the future health of Earth's biosphere, the scientists have mapped out how all life may begin to die off.
They also analyzed what Earth's "biosignature" might look like to a distant alien civilization searching for life. The study has been accepted for publication by the International Journal of Astrobiology and released recently on the physics archive maintained by the Cornell University Library.
Plants Go First
The team's long-range weather forecast for the far future shows that as temperatures on Earth begin to slowly rise, more water vapor will form, resulting in the steady removal of carbon dioxide from the atmosphere.
Plants rely on carbon dioxide to generate energy through photosynthesis, so the complete removal of CO2 would be bad news for foliage. The first hints of the death of life on Earth, the study found, will come in 500 million years, when less-hardy species of plants begin to die off as global carbon dioxide levels drop.
As more plant species go extinct, so will the animals that rely on them as a source of food and oxygen.
"When plant numbers decline, these two commodities become increasingly scarce, resulting in the simultaneous end of animals over the next billion years alongside the end of plants," the study says.
Only Microbes Left
By about 2.8 billion years from now, only hardy communities of microbes will be left behind to inherit the Earth.
But as the Earth continues to relentlessly warm, oceans will evaporate, triggering a runaway greenhouse effect, which will lead to rapid further heating of the planet and a very scarce supply of liquid water.
"Only the hardiest microbes will be able to cope with this, until even they can no longer survive when temperatures cross the threshold at which DNA breaks down—around 140°C [284°F]," added O'Malley-James.
Signs of Life
The team hopes that these findings may help our own search for life beyond Earth, by expanding the number of potential signatures of life to look for when we learn to analyze planetary atmospheres in more detail.
"A planet in a later stage of its habitable development may appear uninhabited if we only look for the signs of life as we know it on Earth today," said O'Malley-James.
"Knowing what other potential signatures life could have could help us make a positive detection of life on a planet that may previously have been ignored."
Always Look on the Bright Side
While this model points to a gloomy future for our planet, O'Malley-James and colleagues think that their timeline for future life is probably a conservative one. There are still a lot of unknowns when it comes to predicting what life will do under duress, said O'Malley-James.
"It is difficult, if not impossible, to predict what evolutionary tricks life may have up its sleeve to cope with these future extreme environmental changes," he said.
But the study no doubt shows that life on Earth is naturally quite resilient to change. If the past is any indication, we can take heart: Despite major environmental upheavals and mass extinctions, life has never been entirely snuffed out since it emerged.
"While it is depressing to know that life will end one day, there is still plenty of comfort in the fact that it won't happen for an unimaginably long period of time," O'Malley-James said.
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