It Doesn't Take Much to Get a Runaway Greenhouse Effect

During the 1960s, the first robotic explorers began making flybys of Venus, including the Soviet Venera 1 and the Mariner 2 probes. These missions dispelled the popular myth that Venus was shrouded by dense rain clouds and had a tropical environment. Instead, these and subsequent missions revealed an extremely dense atmosphere predominantly composed of carbon dioxide. The few Venera landers that made it to the surface also confirmed that Venus is the hottest planet in the Solar System, with average temperatures of 464 °C (867 °F).

These findings drew attention to anthropogenic climate change and the possibility that something similar could happen on Earth. In a recent study, a team of astronomers from the University of Geneva (UNIGE) created the world’s first simulation of the entire greenhouse process that can turn a temperate planet suitable for Life into a hellish, hostile one. Their findings revealed that on Earth, a global average temperature rise of just a few tens of degrees (coupled with a slight rise in the Sun’s luminosity) would be sufficient to initiate this phenomenon and render our planet uninhabitable.

The study was conducted by Guillaume Chaverot and Emeline Bolmont, a postdoctoral scholar and an Astrophysics Professor with the Observatoire Astronomique de l’Université de Genève (UNIGE) and its Life in the Universe Center (LUC) (respectively). They were joined by Martin Turbet, a research scientist with UNIGE, the Laboratoire de Météorologie Dynamique (LMD), and the Laboratoire d’Astrophysique de Bordeaux (LAB). The paper that describes their simulation and research findings recently appeared in Astronomy & Astrophysics.

Triggering the Effect

Belmont is the director of the LUC, which leads state-of-the-art interdisciplinary research projects regarding the origins of Life on Earth and other planets. According to the team’s simulations, the key to a runaway greenhouse effect is the water content of an atmosphere. Water vapor prevents solar irradiation absorbed by Earth’s surface from being radiated back to space as thermal radiation, effectively trapping heat in our atmosphere. While a limited greenhouse effect is essential for maintaining stable temperatures and habitability, too much will increase ocean evaporation and (therefore) the level of water vapor in the atmosphere.

In previous climatological studies, researchers have focused on either the planet’s conditions before the runaway greenhouse effect or its inhabitable state after the runaway occurred. What Chaverot and his colleagues did was create the first-ever 3D global climate model that examines the transition itself and how the climate and the atmosphere evolve during that process. One of the key points in this transition involves the appearance of a specific cloud pattern that increases the runaway effect and makes the process irreversible.

Based on their new climate models, the team determined that a very small increase in solar irradiation, causing an average global temperature increase of a few tens of degrees, would be enough to trigger this irreversible runaway greenhouse effect on Earth. As Chaverot explained in a UNIGE press release:

“There is a critical threshold for this amount of water vapor, beyond which the planet cannot cool down anymore. From there, everything gets carried away until the oceans end up getting fully evaporated and the temperature reaches several hundred degrees.”

“From the start of the transition, we can observe some very dense clouds developing in the high atmosphere. Actually, the latter does not display anymore the temperature inversion characteristic of the Earth atmosphere and separating its two main layers: the troposphere and the stratosphere. The structure of the atmosphere is deeply altered.”

Implications for Exoplanets

This discovery of this specific cloud pattern could prove very useful for exoplanet researchers. In recent years, the field has transitioned from the discovery process to characterization, where astronomers rely on transit spectra and direct imaging to determine the chemical composition of exoplanet atmospheres – thus allowing them to place tighter constraints on their habitability. By identifying this cloud pattern in exoplanet atmospheres, astronomers could identify those that are about to experience a runaway greenhouse effect.

“By studying the climate on other planets, one of our strongest motivations is to determine their potential to host Life. After the previous studies, we suspected already the existence of a water vapor threshold, but the appearance of this cloud pattern is a real surprise!” Said Blomont. “We have also studied in parallel how this cloud pattern could create a specific signature, or ‘‘fingerprint’’, detectable when observing exoplanet atmospheres. The upcoming generation of instruments should be able to detect it,” added Turbet.

Chaverot and his colleagues recently received a research grant to continue this study at the Institut de Planétologie et d’Astrophysique de Grenoble (IPAG). As per this grant, they will focus on how a runaway greenhouse effect could happen here on Earth.

Implications for Climate Mitigation

One of the main points stressed in the Intergovernmental Panel on Climate Change‘s (IPCC) Sixth Assessment Report is the need to curb greenhouse gas emissions to limit the average global temperature increase to 1.5 °C by 2050. With their new grant, Chaverot and his team will assess whether greenhouse gases can trigger the runaway process in the same way a slight increase in solar luminosity would. If so, then it is absolutely crucial to determine what the threshold is so the IPCC and environmental organizations worldwide can establish a red line that cannot be crossed. As Chaverot concludes:

“Assuming this runaway process would be started on Earth, an evaporation of only 10 meters of the oceans’ surface would lead to a 1 bar increase of the atmospheric pressure at ground level. In just a few hundred years, we would reach a ground temperature of over 500°C. Later, we would even reach 273 bars of surface pressure and over 1,500°C, when all of the oceans would end up totally evaporated.”

So the good news is that our planet will not become a hellish landscape any time soon, at least not by an increase in our Sun’s luminosity. Whether or not we affect that change (assuming it is possible) remains to be seen.

Further Reading: EurekAlert!, Astronomy & Astrophysics