The Runaway Greenhouse: implications for future climate change, geoengineering and planetary atmospheres (1201.1593v1)

Abstract: The ultimate climate emergency is a "runaway greenhouse": a hot and water vapour rich atmosphere limits the emission of thermal radiation to space, causing runaway warming. Warming ceases only once the surface reaches ~1400K and emits radiation in the near-infrared, where water is not a good greenhouse gas. This would evaporate the entire ocean and exterminate all planetary life. Venus experienced a runaway greenhouse in the past, and we expect that Earth will in around 2 billion years as solar luminosity increases. But could we bring on such a catastrophe prematurely, by our current climate-altering activities? Here we review what is known about the runaway greenhouse to answer this question, describing the various limits on outgoing radiation and how climate will evolve between these. The good news is that almost all lines of evidence lead us to believe that is unlikely to be possible, even in principle, to trigger full a runaway greenhouse by addition of non-condensible greenhouse gases such as carbon dioxide to the atmosphere. However, our understanding of the dynamics, thermodynamics, radiative transfer and cloud physics of hot and steamy atmospheres is weak. We cannot therefore completely rule out the possibility that human actions might cause a transition, if not to full runaway, then at least to a much warmer climate state than the present one. High climate sensitivity might provide a warning. If we, or more likely our remote descendants, are threatened with a runaway greenhouse then geoengineering to reflect sunlight might be life's only hope. ...[2 sentences cut to meet arXiv char limit]... The runaway greenhouse also remains relevant in planetary sciences and astrobiology: as extrasolar planets smaller and nearer to their stars are detected, some will be in a runaway greenhouse state.

• The paper clarifies the runaway greenhouse effect by defining critical radiative thresholds through detailed theoretical modeling.
• It demonstrates that increases in non-condensible gases like CO₂ alone cannot breach the radiation limits needed to trigger a runaway state on Earth.
• The findings extend to planetary science, offering insights that enhance our understanding of extreme climates on Earth, Venus, and exoplanets.

An Analytical Exploration of the Runaway Greenhouse Effect

The paper by Goldblatt and Watson provides a meticulous examination of the runaway greenhouse phenomenon, a critical topic in the context of planetary atmospheres and future climate change scenarios. The paper delivers a comprehensive synthesis of the theoretical groundwork and considerations pertaining to runaway greenhouse conditions, with a keen focus on the physicochemical dynamics and implications for both terrestrial and extraterrestrial environments. The authors articulate the nuanced interplay of factors that may contribute to or mitigate the runaway greenhouse scenario, grounding their analysis in a rich tapestry of historical and contemporary scientific insights.

Theoretical Framework and Historical Context

The runaway greenhouse effect describes a process whereby a planet's atmosphere becomes so laden with greenhouse gases, particularly water vapor, that it drastically limits the emission of thermal radiation to space, potentially leading to uncontrollable surface temperature increases. A noteworthy threshold is the point where surface temperatures rise to approximately 1400 K, at which point the thermal emission occurs primarily in the less greenhouse-efficient near-infrared region, thereby stabilizing against further warming.

Goldblatt and Watson meticulously distinguish the runaway greenhouse from conventional water vapor feedback mechanisms, utilizing the lens of radiative transfer and atmospheric thermodynamics. They trace the scholarly discourse back to seminal works by Sagan (1960) and Komabayashi (1967) among others, who provided foundational analytical expressions for the radiation limits that underpin this phenomenon.

Radiative Limits and Atmospheric Dynamics

Central to the paper is the exploration of various radiative limits that influence the runaway process. The authors outline three distinct radiation limits—black body upper limit, moist stratosphere upper limit, and moist troposphere asymptotic limit—each dictating specific atmospheric conditions under which thermal radiation is curtailed.

The paper employs a methodical approach using theoretical models to probe these limits, elucidating the conditions under which a runaway state might be triggered. Critically, the modeling highlights that the addition of non-condensible greenhouse gases like CO₂ alone is unlikely to induce a runaway greenhouse on Earth, given that the radiation limits will not be surpassed through such means.

Implications and Future Considerations

The analysis by Goldblatt and Watson offers significant implications in both planetary science and climate change discourse. Their conclusion that terrestrial activities are unlikely to initiate full runaway conditions—though possibly capable of transitioning to a much warmer and more humid climate state—provides a nuanced understanding of anthropogenic impact potentials.

However, the authors emphasize the gaps in current atmospheric models, particularly regarding cloud dynamics and relative humidity changes in extreme conditions, underscoring the uncertainty that persists. This compilation of limitations necessitates further research, particularly using sophisticated General Circulation Models (GCMs) that can better simulate these complex systems.

On a broader scale, this work bridges terrestrial climate challenges with planetary science, foregrounding the relevance of runaway greenhouse dynamics in the search for habitable exoplanets. As exoplanet discovery advances, characterizing atmospheres with high precision will benefit significantly from the insights presented here.

Conclusion

In their comprehensive treatise, Goldblatt and Watson bolster the understanding of runaway greenhouse conditions, setting a foundation for both theoretical advancement and practical considerations. By clarifying the interplay of atmospheric, radiative, and thermodynamic processes, they offer a critical resource to researchers grappling with the implications of extreme climate states—whether on Earth, Venus, or distant exoplanets. The research stands as a clarion call for continued exploration into the subtleties of atmospheric physics, fostering an informed approach to both planetary stewardship and astrobiological pursuits.