How to cook a planet

The greenhouse effect is possibly one of the scientific topics most frequently covered by the mass media. When reading the news, it is very often difficult to separate ideology from the purely scientific content. However, the greenhouse effect has a profound and direct impact on the planet where we live and, also, in other potentially habitable worlds. And being so, it is essential to approach the topic clear-minded.

Let’s review the basics first. Without an atmosphere, we would be at some -20ºC, not a very friendly temperature for most of us. While a real greenhouse is based on preventing convection, it doesn’t happen this way in the Earth. Our atmosphere, as astronomers do know well, is mostly transparent at visible wavelengths where the Sun emits most of its radiation, but it is largely opaque at the infrared wavelengths where the Earth itself and its atmosphere emit. Most of this absorption, and by far, is caused by water in our atmosphere, but other compounds such as the infamous carbon dioxide, methane and ozone also contribute to a variable though appreciable extent. Those infrared absorbing gases block reflected thermal emission from the surface and re-radiate it again into the atmosphere and back to the surface (see Figure 1). This allows the Earth’s surface to be warm enough to have liquid water on it. So far, so good.

The problem comes when we discover that the greenhouse effect can trigger some positive feedbacks. This means that a little increase in temperature will reinforce greenhouse effect, then temperatures will raise and so on. It seems like the greenhouse effect could be unstable and very sensitive to small perturbations. A little increase in the concentration of, to say, carbon dioxide could produce an unstoppable chain of events leading to what is usually called a “runaway greenhouse effect”. Let’s put aside the origin of such an increase, anthropogenic or not, and focus on the climatic consequences of the runaway greenhouse effect, as recently reviewed by Colin Goldblatt and collaborators in the Nature Geoscience magazine ¹.

The closest place to mirror ourselves is our neighbor Venus. Surface temperatures could melt lead, not a friendly place to be. The runaway greenhouse effect is to be blamed here, as proposed many years ago by Andrew Ingersoll (who in fact coined the term, ²) and later by others. Was it always this way or did it have a habitable period? This question is intriguing not only for our own future but also for our quest of Earth-like planets. Moreover, as the greenhouse effect increases and the atmosphere becomes optically thick at the longer wavelengths, it is expected that the infrared radiation will reach a limit independent of the surface temperature, the so-called Simpson-Nakajima limit. Depending on how much radiation the planet emits at such limit, then the atmosphere will be in equilibrium or not, therefore making a stable or unstable atmospheric regime.

Goldblatt and collaborators have applied the most up to date databases of spectral lines, particularly for water, in order to understand how an atmosphere with a moderate greenhouse effect can evolve into a runaway one. They have used a cloud-free atmosphere, since clouds can both reduce (by reflecting sunlight) and increase (by blocking infrared radiation) the greenhouse effect, so this seems a reasonable first approximation to the problem. The main result of their work is that the thermal radiation limit they find for the Simpson-Nakajima limit is 282 Wm^-2, quite lower than previously estimated. Very interestingly, this value is close to the present insolation (294 Wm^-2) and makes it possible for a runaway greenhouse atmosphere to be stable. Authors have also studied the transition to a runaway greenhouse atmosphere, since this can shed light on whether or not we are doomed to suffer a major warming. Unfortunately, they find that initiating a runaway greenhouse process may be easier than previously thought. Still, more modeling is required but initial results point that, since the Sun is increasing its emission, we can expect the Earth to enter in a runaway greenhouse in 1500 millions of years.

The results are also very interesting for other planets. Venus, for example, may have never had a habitable period. Mars, on the other hand, due to its lower mass, would be more susceptible to a runaway greenhouse. With respect to planets around other stars, the work is particularly interesting for water worlds. A steam atmosphere seems to be potentially stable for an insolation like the one we now have and even an episodic warming could trigger a runaway greenhouse without return. Goldblatt et al. also warns about the concept of habitability around a given star. For our present insolation, there are at least three possible climatic regimes that could be stable: snowball Earth, temperate and steam atmosphere. Given that two of this three possibilities are mostly unhabitable, we cannot simply state the habitability of a planet by its distance to the parent star.

Even though we are apparently on the way to become a steam atmosphere, we should never forget the complex interactions between climate and the biosphere as lately shown in another publication ³. Not only changes in vegetation can lead to more or less carbon absorption, but also climate extremes such as droughts, storms or heat waves leave their footprint in the Earth’s capability to deal with carbon or other species, a feedback which is not easily understood or incorporated into numerical models. The cases analyzed by Reichstein and collaborators fall in the negative side, since most of the extremes are prejudicial to the natural carbon stocks. However, this work beautifully illustrates how complex a system our planet can be.