Climate change… on Venus?

The most active searches for otherworldly life in our solar system are centered around Mars, Earth’s cold planetary neighbor, and Europa, an icy moon of Jupiter. In fact, NASA just launched the Europa Clipper on October 14, 2024 with the primary goal of searching for life. And in recent years, many people have floated the idea of sending astronauts to Mars. It was for missions like these that, in 2023, NASA postponed the would-be next exploratory trip to Venus — VERITAS — by allocating its funding to other exploratory programs which were exceeding their budgets.

Often nicknamed Earth’s twin, Venus is roughly the same size and has a similar gravity to Earth, and is very close to our solar system’s habitable zone (a.k.a. “goldilocks zone”), the distance from the sun to allow for potential liquid water on the planet’s surface. So why isn’t NASA prioritizing searching for life on Venus? On average, Earth spends more time closer to Venus than Mars or Europa, and at their closest pass, the planets are a paltry 24 million miles apart. Think of all those fuel savings! But there’s a really good reason we’re not trying to send people to Venus, why any life there would be severely limited _[1]. And that is… well…

Venus is hell.

And not your cold, empty, exposed-to-the-vacuum-of-space hell. On Venus, it rains sulfuric acid (you know, that thing used in car batteries). The atmosphere is crushingly dense, at 92 times what you’d experience at sea level on Earth. This means standing on the surface of Venus would feel the same as being about 3,000 feet deep in the ocean. Compounding all this, temperatures are hot enough to melt lead. Only a few spacecraft have ever survived landing on Venus, and you can find all 6 of the pictures they took here.

Ironically, many works of fiction into the early 20th century depicted Venus as a vast ocean, dense jungle, or paradise (most famously in C.S. Lewis’s Perelandra) _[2]. And why isn’t it? If you were to calculate how warm Venus should be based on its size and distance from the sun, it would sit at a nearly-habitable 165 °F (74 °C). But instead, Venus is far and away the hottest planet in our solar system, at 867 °F (464 °C). Why is this?

The French mathematician Joseph Fourier wondered something similar in the 1820’s. He calculated that the average temperature on Earth should be 0 °F (-18 °C), when really it is a comfortable 59 °F (15 °C). Fourier’s calculations created a puzzle — that extra heat had to come from somewhere. Among other things, he posited that the atmosphere could be acting as an insulator. We credit this idea as the first proposal of the modern “greenhouse effect”: Earth’s atmosphere traps and stores heat, creating a habitable secluded enclosure for life to grow, just as a greenhouse does when the sun shines.

Let’s circle back to Venus. While Earth’s greenhouse effect makes it perfect for life, the situation on Venus is the opposite. Calculations reveal that the greenhouse effect accounts for nearly all of Venus’s high surface temperatures _[3]. How is this? It turns out there is another similarity between Earth and Venus — each planet has roughly the same amount of carbon. The key difference in this carbon is that, on Earth, most of it is stored in rocks (things like limestone and shale as well as coal and petroleum). On Venus, nearly all the carbon is in the atmosphere as CO_{2 [4]}. This is why Venus is an extra 700 °F hotter than expected according to its size and distance from the sun. Turns out having an atmosphere full of greenhouse gases will really get things cooking! 

How does this really work? Certain greenhouse gases absorb infrared radiation from the sun — light we can’t see — and store it as kinetic energy in their atomic bonds; in other words, the gases heat up. Not all gases have this quality. The two most abundant gases in Earth’s atmosphere, nitrogen (N₂) at 78% and oxygen (O₂) at 21%, are transparent to this radiation. Only a small volume of Earth’s dry atmosphere is composed of greenhouse gases — about 0.04%. CO₂ is thought of as the quintessential greenhouse gas due to its abundance, though there are many different gases which can absorb much more radiation and thus contribute more to planetary warming.

For example, check out this graphic from Our World in Data. Methane, CH₄, has a global warming potential of about 30 times more than CO₂, and the even more extreme synthetic sulfur hexafluoride, SF₆ , has a warming potential 24,300 times more. Sulfur hexafluoride and other synthetic fluorinated gases like it are used in industrial processes such as refrigeration and electronics and make up about 2% of global greenhouse gas emissions. Their high global warming potential means they contribute disproportionately to resulting temperature increases. (But as a bonus, when you inhale sulfur hexafluoride, you sound like this!)

Surprisingly, the most important greenhouse gas on Earth is actually our good friend water vapor.  The global warming potential of water vapor is very small, but through its prevalence and absorptive properties, it accounts for half of the greenhouse effect on Earth _[5]. The volume of water vapor in the atmosphere can be up to 4%, varying by location. The actual amount is almost entirely controlled by Earth’s temperature. As Earth’s temperature increases, more water vapor will join the atmosphere. We can’t do anything about the concentration of water vapor directly, but we can control our CO₂ output, which can decrease the greenhouse effect and thus water vapor concentrations.

It’s true that humans are putting greenhouse gases into the atmosphere, including CO₂, and it’s true that those gases are accumulating and contributing to an increased greenhouse effect _[6]. One could imagine a positive feedback loop of the greenhouse effect on Earth where humans overload the atmosphere with CO₂, leading to warming, leading to an increase in water vapor, leading to more warming, resulting in the planet becoming more and more similar to Venus. But Venus is quite an extreme case. For a better view of how Earth’s carbon cycle works, see this figure from the Intergovernmental Panel on Climate Change.

The climate system on Venus is complex, just like on Earth. In fact, one could make the case that Venus could be even hotter than it already is without other factors at play — and we can learn from these as well.

First, the effect of a particular gas depends on how much of that gas you have. More gas means more warming, but the more you add, the less of a difference it makes; it’s a diminishing return _[8]. There’s only so much radiation to go around. In tandem with this, different gases absorb different radiation wavelengths, so a variety of gases with non-overlapping wavelengths contributes to more heating.

Venus’s atmosphere is 97% CO₂, but if you swapped some of that CO₂ for a different gas, it would increase temperatures even more, as currently much radiation not absorbed by CO₂ can escape. This illustrates an important truth: trace greenhouse gases have an outsized effect on the actual heating experienced from emissions. A diversity of greenhouse gases, while a hypothetical on Venus, is reality on Earth. We not only need to monitor our carbon emissions, but others as well.

Additionally, the massive amounts of sulfate particles in the atmosphere contribute to a high albedo (the reflectivity of the planet, a measure of how much radiation bounces off vs being absorbed). Venus reflects about 70% of the sun’s light which hits it, making it the brightest object in our night sky besides the moon — and almost featureless.

 An increase in albedo is known to decrease planetary temperatures because not as much radiation is absorbed. Scientists are actively drawing from this for the development of Stratospheric Aerosol Injection technology — a geoengineering method which involves spraying sulfate particles into the stratosphere to reflect the sun’s radiation, leading to a temperature decrease. Earth already experiences some of this temperature decrease as a result of volcanic eruptions, which often produce sulfate aerosols. As for Venus, you could hypothesize that it would absorb more radiation if it was less reflective. 

Scientists may not be anticipating the discovery of life on Venus, but they still study it closely. Studying Venus allows us to learn from example instead of experience when it comes to a runaway greenhouse effect. These findings could save us if we heed their warming (pun intended). NASA’s next Venus missions are set to launch in 2031 and include a descent probe, which will fall through the atmosphere and land on Venus’s surface. While researchers have gotten pretty good at observing from afar, some things we can only know if we go… or send a probe.

Edited by Robert Howard and Jonah Wirt

References:

1. Cockell, C. S. (1999). Life on venus. Planetary and Space Science, 47(12), 1487-1501. 
2. Westfahl, G. (2022). The stuff of science fiction: hardware, settings, characters (pp. 164-165). McFarland & Company, Inc., Publishers.
3. Pollack, J. B., Toon, O. B., & Boese, R. (1980). Greenhouse models of Venus’ high surface temperature, as constrained by Pioneer Venus measurements. Journal of Geophysical Research: Space Physics, 85(A13), 8223-8231.
4. Svedhem, H., Titov, D. V., Taylor, F. W., & Witasse, O. (2007). Venus as a more Earth-like planet. Nature, 450(7170), 629-632.  
5. Sherwood, S. C., Dixit, V., & Salomez, C. (2018). The global warming potential of near-surface emitted water vapour. Environmental Research Letters, 13(10), 104006.
6. IPCC (Intergovernmental Panel on Climate Change). (2021). Climate change 2021: The physical science basis. Working Group I contribution to the IPCC Sixth Assessment Report. Cambridge, United Kingdom: Cambridge University Press.
7. Prentice, I. C., Farquhar, G. D., Fasham, M. J. R., Goulden, M. L., Heimann, M., Jaramillo, V. J., … & Yool, A. (2001). The carbon cycle and atmospheric carbon dioxide. Climate change 2001: the scientific basis, Intergovernmental panel on climate change.
8. Mitchell, J. F. (1989). The “greenhouse” effect and climate change. Reviews of Geophysics, 27(1), 115-139.