A runaway greenhouse effect is very unlikely on earth (Patreon)

[This is a transcript with links to references.]

When I was just about to log off for the holiday break, I saw these headlines popping up, saying that it doesn’t take muchto turn Earth into Venus with a runaway greenhouse effect which would, quote, literally boil earth alive. I thought it’d be best to ignore this. But then I saw that some people on social media got first worried about it, and then confused because no one was addressing this. So I decided to, well, you know, give you all the boring context because that’s what I do. And here we go.

A runaway greenhouse effect happens when a planet loses its ability to cool. Then, the only way for the surface temperature is up. This might happen for example when the amount of water vapor in the atmosphere increases beyond a critical threshold. That’s because water vapor is an incredibly potent greenhouse gas, and also, the warmer a planet, the more of its water will be in the form of vapor. So, higher temperatures lead to more water vapor which lead to higher temperatures, and the effect can, well run away, hence the name.

Scientists believe that a runaway greenhouse effect is what happened to our neighbour planet Venus. Venus is in size and mass not too different from Earth. When it was young it was probably quite similar our planet, with liquid water on the surface.

But Venus is somewhat closer to the sun than we are. And since it received more sunlight, the amount of water vapor in its atmosphere was higher. At the same time, the sunlight was powerful enough to split the water vapor into oxygen and hydrogen. And, here’s the problem, the hydrogen escaped into space.

This is bad because with the hydrogen gone for good, the water cycle couldn’t stabilize. And water is good to absorb carbon dioxide from the atmosphere. So, as the hydrogen fraction decreased, the carbon dioxide in the atmosphere steadily built up, covering Venus in a thick blanket that retained heat very well.

Today the atmosphere of Venus is mostly carbon dioxide, the water is all gone, the atmospheric pressure is about 90 times that of earth, and the surface temperature is roughly 370 degrees Celsius. If you’ve sometimes dreamed of plucking chickens out of the air fully roasted, then moving to Venus might be the right thing for you, though you’d have to take your chickens with a side of sulphur. The rest of us I guess would find Venus a rather unpleasant place to live.

Now comes this press release from researchers at the University of Geneva which says, quote: “With their new climate models, the scientists have calculated that a very small increase of the solar irradiation ... would be enough to trigger this irreversible runaway process on Earth and make our planet as inhospitable as Venus.”

This sounds hugely alarming. Indeed, an article that was covering this press release warned that we could cause such a runaway greenhouse effect by increasing carbon dioxide levels. “It’s a grim warning of just how stark a future of human-driven climate change can look.”

So watch out, here comes the context. First, let’s have a look at solar irradiation. That’s the radiation from the sun that reaches the surface of earth. It’s basically a measure for the energy we get from the sun. It fluctuates naturally and goes somewhat up and down during the solar cycle.

These fluctuations do have an influence on the temperature on earth of typically a few tens of a degree. That’s noticeable and accounted for in the IPCC projections, but it’s smaller than the effect by human-caused increase of carbon dioxide levels.

Though the brightness of our sun very slowly increases as it gets older and that will eventually cause a runaway greenhouse effect on earth, too. According to estimates we’ve got one or two billion years. So, Elon’s got some time left to get us off this planet.For all I know, if you wanted to substantially increase solar irradiance any sooner than that to levels so high that it’d trigger a runaway greenhouse effect, you’d have to move earth closer to the sun.

So if these scientists say that “small” changes in solar irradiation can cause a runaway greenhouse effect on earth, I’d really like to know what “small” means. Numbers people, show me numbers!

I looked for the numbers in the paper and would you know it, the paper doesn’t so much as contain the word solar irradiation.

What they did in the paper is that they increased a model parameter called the insulation of a planet. Increasing this insulation prevents the simulated planet from cooling, and then they keep track of what happens. The reason they did this is that they want to better understand under which circumstances life might be possible on exoplanets.

The paper doesn’t say anything about a runaway greenhouse effect on Earth. The press release just quotes one of the authors as saying that they’d like to study this in the future.

What do we know about the risk of a runaway greenhouse effect on earth? Well we do know that the past of our planet has had phases that were both hotter and had much higher atmospheric carbon dioxide levels than we have today. Not only this, but it was sometimes also higher than what we are likely to reach even if we keep on happily burning fossil fuels. This shouldn’t be all that surprising because the carbon in those fossil fuels mostly came from the atmosphere in the first place.

And since no runaway greenhouse effect happened back then, it seems unlikely we’ll trigger one now. It also hasn’t happened in any climate models, most of which are much more sophisticated than the ones the guys in that new paper used.

The problem with climate change is really not the temperature or the carbon dioxide level per se, it’s the rapid change. We and the rest of the biosphere must adapt to the changing climate within a matter of decades. That puts a lot of stress on our economies and brings the risk of a strong economic downturn.

It’s bad enough as it is without having to make people afraid of a runaway greenhouse effect. Of course, I can’t say that the risk is indeed zero, because god knows what other stupid ideas humans will come up with to meddle with the climate, but really I think a runaway greenhouse effect is not something we need to worry about. No matter how much they dig in Saudi Arabia, it’s not going to move the earth closer to the sun, I promise.