Carbon Dioxide is often vilified as a bugaboo greenhouse gas. But its presence here on early earth is likely one of the reasons we are here to talk about it. Carbon dioxide, or CO2, was likely needed in large quantities in earth's early atmosphere to raise temperatures sufficiently to enable the onset of the earliest microbial life. That's because some four billion years ago, our sun had only about 70 percent of its current luminosity.
But there's a catch. CO2 in large quantities can also be deleterious to the evolution of complex life -- - particularly intelligent life like our own. We wouldn't be here at all if our earth hadn't developed a means of ridding its atmosphere of large quantities of CO2 via carbon recycling. That's the process by which carbon in our atmosphere is subducted below earth's surface in a process facilitated by the movement of giant tectonic plates below earth's outermost crust.
Terrestrial mass planets around other sunlike stars may not be so lucky and may have long held onto their CO2-rich atmospheres. That's not too much of a problem for microbial life. But it could be one of the factors that narrows the range of planets that could host intelligent life like ours.
Humans breathe O2 and exhale CO2, which is a waste product, Eddie Schwieterman, an astrobiologist at the University of California, Riverside, told me at the 'Unique Species on a Unique Planet?' astrobiology confererence in Copenhagen. We require abundant oxygen to survive, but CO2 levels which are too high poison us, he says. So, we could not have originated in an environment that would preclude our survival, so we should not be surprised that we find ourselves in a high-O2, low-CO2 world, says Schwieterman.
Earth likely evolved in not just a special place, but also in a special time.
The Sun is getting brighter over very long geological timescales, about one percent brighter every 100 million years, says Schwieterman. This means that if we extrapolate back in time billions of years to when life began on Earth, more greenhouse gases like CO2 would be required for Earth to maintain above-freezing temperatures, he says. Due to geochemical weathering, this CO2 has been drawn down into carbonate rocks as the Sun has brightened, says Schwieterman.
Most planets orbiting other stars in their habitable zones would require substantially more CO2 than is present in Earth's atmosphere to be habitable -- -up to several thousand times as much, says Schwieterman. CO2 is a toxic gas at these levels to large, oxygen-breathing lifeforms like animals, including humans, he says. Therefore, most exoplanets that are technically "habitable" are potentially habitable for microbes, but not for us or life like us, says Schwieterman.
Red dwarf (M-spectral type) stars are the most numerous in the cosmos.
Planets orbiting such stars would have different atmospheric chemistry, which would facilitate the accumulation of toxic gases, Schwieterman noted in his Copenhagen presentation. Thus, the habitable zone real estate where technological civilizations could originate and thrive may be more limited than commonly assumed, he noted.
When looking for extraterrestrial intelligent life, how crucial is finding a planet in a habitable zone?
The habitable zone for microbial life is much larger than the habitable zone, where we could imagine earth like biospheres with animals and possibly intelligent beings, which would be much more restricted, says Schwieterman. This might have consequences for how often we'd anticipate a civilization arising in our galaxy, he says.
As noted in his conference talk, Schwieterman's models suggest the appearance of intelligent life is not necessarily dependent on random evolutionary steps but -- at least in part -- related to the co-evolution of the planet and its host star.
I would anticipate the number of planets harboring intelligent species is much, much smaller than the number with microbial life, says Schwieterman. It's not just a matter of evolutionary probabilities but the number of locations where the emergence of intelligent life is even possible, he says.
As for earth?
Today, Earth is located near the inner edge of the "conservative" habitable zone, says Schwieterman. Due to the amount of warming radiation received from the Sun, our planet requires only modest amounts of CO2 to remain warm, he says. So, our low CO2 phase is actually a special time, says Schwieterman. And it won't last forever, as the ever-brightening Sun will eventually put Earth into a runaway greenhouse like Venus, he says.