A cloudy outlook
Not everyone is convinced Goldblatt’s result is valid, however. James Kasting, a geoscientist at The Pennsylvania State University, suspects that even in theory an anthropogenic runaway remains out of reach of humanity. Kasting performed many of the earlier seminal studies that seemed to rule out a present-day runaway, and with his student Ramses Ramirez is currently polishing a new study that reinforces those conclusions. No matter how much carbon dioxide is pumped into the present-day Earth’s atmosphere in Kasting’s models, the resulting heating is insufficient to cause the planet to rapidly boil off its oceans. “The bottom line,” Kasting says, “is that we do not get a runaway.”
Like Goldblatt’s team, Kasting’s group studies Earth’s climate using a one-dimensional model that simulates the absorption, transmission and reflection of sunlight by a single surface-to-space strip of atmosphere. These models’ sophisticated treatment of light’s interactions with air closely reproduce the observed warming effects of carbon dioxide, water vapor and other greenhouse gases, yet they contain only the crudest approximations of Earth’s changing weather and surface. Such models are particularly poor at accounting for the complex effects of clouds, which, depending on where and how they form, can either cool or heat the planet: Thick, low-lying clouds tend to reduce temperatures by reflecting greater amounts of sunlight back to space, whereas high, thin clouds will warm the planet by letting light pass through then trapping more of the absorbed heat. The differences between Kasting’s and Goldblatt’s conclusions largely boil down to Kasting’s 1-D approximations of clouds providing slightly more cooling whereas Goldblatt’s provide slightly less.
Three-dimensional modeling is the only way around this impasse, yet current 3-D climate models aren’t up to the task of simulating how Earth’s clouds and weather will change within a very steamy or CO2-rich atmosphere. “Using today’s best models to address these extremes is like trying to drive up a mountain in a Honda Civic,” Goldblatt says. “A Civic can take you coast to coast on paved roads, but take it off-road and you run into problems. Today’s models are like that right now—they aren’t designed for extreme atmospheres. If you want to model the runaway greenhouse, you need the equivalent of a Humvee for your climate model that will take you to these wild places.”
Kasting’s group recently received funding from NASA to work with other teams to develop better 3-D models, and a handful of other research groups in Europe are also pursuing similar goals.
Out of the fire, into the frying pan
Outside of better models, other useful constraints on the runaway greenhouse scenario come from the Earth’s long history. Measurements of 56-million-year-old sedimentary rocks have revealed an event during the mid-Cenozoic era called the Paleocene–Eocene Thermal Maximum (PETM) in which a millennia-scale pulse of greenhouse gases warmed the globe. The PETM pulse seems to have been roughly equivalent to what humans could release through burning all recoverable fossil fuels, and may have warmed the planet in excess of 10 degrees Celsius, but clearly no catastrophic runaway occurred, for otherwise we would not now be here. If it didn’t happen then, many researchers suggest, it won’t happen now from a similar, anthropogenic spike of greenhouse gas.
“All these geological records tell us that even with very high levels of atmospheric carbon dioxide in the past, Earth avoided runaway,” Goldblatt acknowledges. “But that doesn’t tell us how much margin of error we have today or how close things came in the past. It’s a bit like walking around on top of a foggy cliff and not knowing whether you’re a meter or a kilometer from the edge. Even simple modeling can let you work out some hard limits to help guide behavior.”