In Mary Robinette Kowal’s The Calculating Stars, a meteor hits the east coast of the United States in the 1950s, obliterating Washington DC and causing global climate catastrophe. Mathematical genius and future Lady Astronaut Elma York calculates that the climate shock will within a couple of generations tip the Earth into a runaway greenhouse state like Venus, destroying all life on the planet. (And note that it is specifically a runaway greenhouse. She explicitly mentions the oceans boiling.)
Later, the scientists back away from this claim and say that with careful control of carbon emissions, this fate may be avoidable, but they won’t know for sure until it’s too late.
Does this make sense? Well…no. While the vast majority of the book is very well researched (as far as I can determine), this part is not good science. To cut a long story short, there’s basically no way for a runaway greenhouse effect to happen on Earth. But this doesn’t necessarily negate the rest of the story. I wanted to investigate how bad this disaster would really be.
This idea falls into a fallacy that is never really said out loud, but I feel like it is implied a lot, even by scientists such as Neil deGrasse Tyson in the second Cosmos series: namely, that Venus is a “cautionary tale” for climate change here on Earth.
Now, I don’t want to create a strawman here. If they’re given more than sixty seconds to talk about it, most scientists will correctly qualify this by saying that a runaway greenhouse is impossible on Earth no matter what humans do. Venus is a very different environment and is more a proof of concept for the greenhouse effect in general than a specific warning. But it is enough for Mary Robinette Kowal (and she’s not the only one I’ve seen) to make this mistake.
The claim here is that by hitting the ocean (well, the Chesapeake Bay), the asteroid would vaporize a large amount of water, and water vapor exerts a stronger greenhouse effect than carbon dioxide. The increased greenhouse effect would warm the planet, causing more water to evaporate, which increases the greenhouse effect, and so on, in a positive feedback loop. This probably is what happened to Venus 4 billion years ago. It’s called a “runaway” because once the oceans start boiling, there’s no way to stop it short of putting a giant mirror in front of the planet, and the temperature shoots up several hundred degrees in a geologically short time.
But this isn’t the way things are on Earth. The hottest recorded temperature in history was only 57°C, and I can’t find a definitive figure, but the hottest sea surface temperatures seem to be 35-40°C. We have a lot of buffer between here and the boiling point of water. And on one level, it should be obvious that an asteroid impact wouldn’t cause a runaway greenhouse because the Chicxulub impact that killed the (non-avian) dinosaurs 66 million years ago didn’t—and that was also a water impact.
“But what about that positive feedback loop?” you may ask. Well, here’s the thing. Even if a feedback is positive, it can still be subject to diminishing returns, and that’s exactly the case with the greenhouse effect. Warmer temperatures cause more water to evaporate, yes, but not enough to cause an equal or greater gain in temperature again. If you iterate the process, you get smaller and smaller changes until it converges to a hot, but still-habitable temperature.
I reached out to Dr. David Catling at the University of Washington, who studies paleoclimate on Earth and Mars, to get a clearer picture of the greenhouse effect as it relates to asteroid impacts, and he pointed out three things that would prevent a runaway greenhouse from occurring.
First the amount of water vapor produced is not that great. It takes a lot of energy to boil water, and even a Chicxulub-sized asteroid only has so much of it. Do the math, and you find that if 25% of the asteroid’s kinetic energy goes into boiling water, it will add about 0.3% water vapor to our atmosphere. This is on top of the typical value of 1% we have now.
Second, water is a strong greenhouse gas, but it’s a very short-lived one. Methane sticks around in the atmosphere for ten years. Carbon dioxide for a hundred. But most of that excess water vapor would rain out in a couple months, while the dust clouds are still blotting out the Sun.
Finally, a large asteroid impact also releases large amounts of sulfur dioxide into the atmosphere, and sulfur dioxide is a cooling gas. It blocks light coming from the Sun so that it never reaches Earth. It would be gone in a couple years, and the CO2 produced by the impact would cause a longer-term warming, but not to catastrophic levels. All of this adds up to a few years of low crop yields and brutally cold winters (which the book describes) followed by some nasty global warming of the kind we’re trying to prevent today (which the book also describes), but not an existential threat.
But let’s go one step further. How much warming would we expect from this impact. Or better yet, since we’re going for maximum drama, what’s the worst case scenario?
First, the water vapor. Water vapor rains out very quickly—in the troposphere (the lowest part of the atmosphere). But an impact of this size will eject at least some of it into the stratosphere. There is a “cold trap” at the troposphere-stratosphere boundary (around where airliners fly)—a minimum in temperature that makes it less efficient for water vapor to cross through it. Water vapor in the stratosphere can stay there for five years or more.
What happens if that 0.3% of water vapor all gets into the stratosphere? Well, you have to follow the thread through a few different equations to do it, but it works out to about 2°C of warming.
The impact would also eject a lot of carbon dioxide into the atmosphere. How much varies a lot depending who you ask, but here there is an important difference from the real Chicxulub impact. We have a lot less CO2 in our atmosphere than there was 66 million years ago, and that means the atmosphere is a lot more sensitive to adding more. The climate sensitivity is a figure used in climate science to estimate the amount of warming expected from a doubling of CO2 over preindustrial levels. There is a very good chance that a Chicxulub-sized impact would double Earth’s modern CO2 level, and that climate sensitivity factor would predict another 3°C of warming.
Now, put those together, and it’s already looking bad, but the worst case scenario in fact or fiction is a catastrophic release of methane from Arctic permafrosts. This area of climate science is still very speculative in terms of how much it contributes to natural feedbacks. It’s one of those wildcards that the IPCC has historically been slow to address. However, some models put the effect of a catastrophic methane release at as much as 6°C.
Well, now, we have the makings of a real disaster story. Climate scientists say that 6°C of warming would be catastrophic and would greatly reduce Earth’s ability to support human civilization. Our total here of 11°C is larger than we have reliable evidence for in the past, and it would render large swaths of the tropical and subtropical parts of the world uninhabitable. The heat would kill anyone who wasn’t in an actively air-conditioned environment, and I’m sure it would do a number on all the other life on Earth too. Nature can adapt to those temperatures, but not overnight. That means collapsing crop yields, and even for people who survived in the temperate zones, famine might get them.
So, no, a Chicxulub-sized impact could not start a runaway greenhouse and destroy all life on Earth. However, it is possible that it would make it categorically impossible to feed the three billion humans who lived on Earth at that time if things went badly.
Is the response to that moving to another planet? Eh…probably not. It would make more sense to develop indoor farms and new agricultural techniques here on Earth, if for no other reason than the sheer logistics of physically moving three billion people off world. However, that’s another story that would need it’s own post.
 And really, this whole thing seems to have stemmed from an attempt to put her earlier novella, “The Lady Astronaut of Mars”, on a solider scientific footing, where it describes Earth still shrouded with dust decades later.
 Without greenhouse gases, Earth’s average temperature would be about -18°C (0°F). Water vapor and clouds increase the temperature by about 24°C, carbon dioxide by 6.5°C, and other greenhouse gases like methane about 1.5°C.