Scientists predict that one day, with the Sun slowly evolving into a red giant, the intense heat and light it produces will make life on our planet impossible. This won't occur for millions of years, but when it does, extreme temperatures could theoretically also make the oceans evaporate, creating an irreversible 'moist greenhouse' effect.
But could the same process occur due to other causes unrelated to solar luminosity? Yes, according to a new study, whose authors say that the rise in atmospheric carbon that we're seeing today could, if left unabated, eventually create the same kind of conditions. Carbon dioxide in the atmosphere traps heat – which is why global warming is happening – and if the carbon levels reached a certain point, bye-bye oceans.
To simulate what might happen if the amount of CO2 in our atmosphere kept increasing, the researchers ran computer modelling on an even wetter environment: a conceptual ocean planet, covered entirely in water (whereas Earth's surface is just 71 percent water). Could enough carbon in the air, trapping enough heat, eventually lead all the liquid water on this hypothetical planet to turn into steam and eventually pass out of the planet's atmosphere?
"Basically, yes, a very high CO2 saturation could lead to a state in which water would evaporate and eventually be lost," climatologist Max Popp from the Max Planck Institute for Meteorology in Germany told Brian Merchant at Motherboard. "The process in which water would be lost to space would however take place over a period of millions to hundreds of millions of years (depending on several factors). Even though this is a long time from a human perspective, on geological time-scales this can be considered relatively quick."
The study, published in Nature Communications, found that on the hypothetical water planet, once CO2 levels reach 1,520 parts per million (PPM), average surface temperatures would increase to about 57 degrees Celsius (134.6 degrees Fahrenheit). At this point, the climate would begin to destabilise, resulting in evaporating water eventually being lost to space.
According to the researchers, the same thing could happen here on Earth, although our planet's climate is more complex due to land and ice masses. Our carbon levels are currently about 400 PPM, but they'd need to be much, much higher before our oceans started disappearing.
"So a rough estimate of the CO2 concentrations required to push Earth into a Moist Greenhouse, would be three to four times 1,520 PPM, so of the order of 4,500 to 6,000 PPM," Popp told Motherboard. "Our results are of relevance for the far future of Earth in the order of millions of years, but they are not of relevance for current and near-future climate."
Given the grand timescale of the evaporation, the researchers' findings might not seem particularly relevant to the global discussion on climate change. But they could have far greater significance to humankind's ambitions to one day live on other planets, in terms of helping to determine the long-term habitability of candidate worlds.
"There are several missions looking at other stars for planets and hundreds of planets have already been discovered," said Popp. "In order to learn whether any of these planets may be suitable for life, it is important to understand which of these planets could maintain liquid water at the surface for an extended period of time."
In that vein, NASA is helping getting people get ready for the idea of interplanetary travel with an awesome new collection of retro-futuristic space tourism posters. Not all these worlds would prove truly viable for human habitation, but that doesn't mean we can't dream about visiting them.