In 2013, NASA's former chief climate scientist James Hansen published a short white paper that warned if humans burned all the planet's available fossil fuels, it "could result in the planet being not only ice-free but human-free." He went on to describe "Venus Syndrome"—a situation that could unfold far in the future, in which so much carbon dioxide is loaded into the atmosphere that Earth is rendered a replica of the scorching second planet from the sun. First, though, all of the oceans would evaporate.
It won't happen anytime soon, and it was a fairly radical proposition. Hansen concluded that though it would take millions of years, "Earth can 'achieve' Venus-like conditions, in the sense of ~90 bar surface pressure, only after first getting rid of its ocean via escape of hydrogen to space." We would, in other words, have to heat the Earth up enough to boil away the oceans—a feat that another scientist, Max Popp, of Princeton and NOAA's Geophysical Fluid Dynamics Laboratory, has confirmed is entirely within the realm of possibility. A distant, eventual possibility, but a possibility nonetheless.
Scientists have long recognized that the sun, whose luminosity is steadily increasing, will eventually become hot enough to render the liquid water on Earth unstable—the planet's lakes, oceans, rivers, and streams will simply evaporate into the atmosphere, and, as Hansen noted, be lost into space. (This process is expected to eventually render all water-based planets uninhabitable across the universe.) But it was unclear whether or not we could instigate the process with enough carbon and the greenhouse effect.
"Basically, yes a very high CO2 saturation could lead to a state in which water would evaporate and eventually be lost," Popp told me. "The process in which water would be lost to space would however take place over a period of millions to hundred 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 Sun (and thus, the Earth), he points out, has a life expectancy of 10 billion to 12 billion years.
"A large increase of atmospheric concentrations of greenhouse gases such as CO2 could also destroy the habitability of water-rich planets."
Popp calls this process a transition to Moist Greenhouse, and that's the name of his paper, too, which was just published in Nature Communications. That transition, and the eventual potential Venus Syndrome may not be a concern of you or your children or your great, great X 1000 grandchildren, but it is notable because it shows that there's more than one way to burn the water off a habitable planet. Popp concludes that yes, "a large increase of atmospheric concentrations of greenhouse gases such as CO2 could also destroy the habitability of water-rich planets."
Popp and his team designed a climate model to evaluate a hypothetical water-based planet—one entirely covered in H20—to investigate what might happen to a planet like Earth if it were covered in enough greenhouse gases for a long enough period of time. They discovered that once carbon concentrations rose to 1520 parts per million in the atmosphere of their water-planet, and surface temperatures rose to above 57˚C, interesting things began to happen to its climate.
According to Nature, "Cloud feedback effects destabilise the planet's climate, bringing about moist conditions in the upper atmosphere, where water may be lost to space much faster than could occur in the drier upper atmosphere on Earth today." And, eventually, the oceans are lost to space.
Now, Earth's climate is a lot more complex, and it would be a lot harder to instigate a Moist Greenhouse transition to Venus.
"To obtain the same climate on Earth, CO2 concentrations would already have to be around three to four times the pre-industrial values," Popp said. "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 (parts per million), so of the order of 4,500 to 6,000 PPM." (Right now, we're hovering at about 400 PPM; preindustrial levels were about 280.) Popp isn't worried about humanity's influence in the process—he finds levels that high "unattainable."
"So in summary," he continues, "we are showing in our study that in principle it would be possible to push Earth into a Moist Greenhouse by increasing CO2 values, but the CO2 concentrations required would be much higher than for the water-covered planet in our study. 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."
What gives, then? It turns out that Popp sees the real import in his Moist Greenhouse theory in determining the habitability of planets discovered in other systems.
"There are several missions looking at other stars for planets and hundreds of planets have already been discovered and potentially habitable planets are expected to be discovered in the next years.," he told me. "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."
Historically, the most important factor in determining a planet's habitability has been its distance to a given star—whether or not it resided in the so-called Goldilocks zone; not too far, not too close; but just right.
"The Moist Greenhouse would puts a limit to the time a planet could maintain liquid water, because the water would eventually be lost to space," Popp said. "By showing that greenhouse-gas forcing is as efficient in causing a transition to a Moist Greenhouse as is an increase in solar forcing, we show that the habitability does not only depend on the distance of a planet from a star but also on the atmospheric greenhouse concentrations."
It may be hundreds of millions of years before James Hansen's ultimate climate nightmare comes to pass, and Earth gets Venus Syndrome and turns into an uninhabitable Moist Greenhouse. But knowing the symptoms right now will help us avoid heading out to the planets out there that already have.