I read how recently NASA revealed that a large asteroid came close to colliding with Earth, and had gone undetected until it was almost upon us. An article about the ‘volcanic winter’ that could follow such an event had me wondering: “Does the Earth need meteor strikes to avoid greenhouse gas runaway destroying life on Earth?”
Spoiler: No. But, it does provoke thoughts on solutions.
Here are the key points to support this idea:
- It is certain: Global Warming will eventually kill all life on Earth
- Greenhouse Runaway vs Snow Ball Earth: Positive Feedback
- Negative Feedback has its limits
- Did the ‘Dinosaur Meteor’ buy us time?
- The Drake Rare Earth
- Meteors to the Rescue?
Certainty: Global Warming will kill all life on Earth
You have possibly seen explanations on how the Sun will get hotter and eventually, (in around 5 Billion years), expand to a size that would swallow inner planets Mercury and Venus and completely fry the Earth. The detail that may have been missed is that this ‘getting hotter’ is a continual process and back when life on Earth started, the Sun only produced 70% or current heat levels. So way back when life started, we needed far more greenhouse gas to prevent Earth freezing, while now we need almost zero greenhouse gasses or we overheat.
That increase of the temperature of the Sun just keeps continues, from 70% of the current level, to an eventual multiple of the current level 5 Billion years from now. The Earth is already close to the point where the smallest amount of CO2 is enough to trigger thermal runaway. Do we have another Billion years? Half a Billion? Or perhaps a million? Or we at such a precipice even now that human interference could tip us into runaway with a lifetime? Hard to be sure until after the event, but perhaps a ‘volcanic winter’ triggered by a meteor strike is exactly what is needed to establish a new balance.
Thermal Runaway vs Snow Ball Earth: Positive Feedback
History
A look back in time reveals a climate that has seen huge changes. What is not immediately obvious is how things recover from extremes. It appears there have been ‘snowball earth’ episodes with the whole planet covered in Ice, Ice Ages with huge areas covered with Ice, interglacial periods (technically still within an Ice Age) such as now with Polar Ice, and periods with no polar Ice. Interestingly, there has not yet been a Venus style thermal runaway yet, nor is it clear that the Earth could ever recover is there was one. For everything that has happened so far, the Earth has swung back and recovered from extremes. How?
Greenhouse Runaway: Not Yet?
The more greenhouse gas, the hotter it gets. The hotter it gets, the more greenhouse gas in the form of water vapour enters the atmosphere, so it then gets even hotter. The hotter it gets, the less surface ice on the earth, the less reflective the Earth becomes, so the the more heat the Earth absorbs and the hotter it becomes. Clearly, if nothing breaks the cycle, just like audio feedback on a stage, it becomes a problem.
So far on Earth, this ‘keeps getting hotter’ cycle has always been broken, or the Earth would already be like Venus. To limit the cycle going into runaway we have:
- A warmer Earth radiates more energy into space
The increased radiation can limit the increase from the cycle, but cannot send the cannot break the cycle. The candidates to break the ‘hotter’ cycle are:
- a meteor strike to trigger a winter
- a huge volcano to trigger a volcanic winter
- Earth orbital changes to break the cycle
- reduction in greenhouse gasses due to increased plant growth
Only the last two points can act as ‘negative feedback’ with the possibility of being an automatic response to the increase in temperature. . If temperature increases within the correct range, it is possible plant growth will accelerate, and thus reduce atmospheric CO2, which is the only feedback step which could break the cycle. As long as CO2 levels are a significant contributor to temperature, the cycle has the potential to drive plant growth and break itself if conditions for plants are suitable.
Snowball Earth?
The less greenhouse gas, the colder it gets. The colder it gets the less water vapour greenhouse gas in the atmosphere, so it gets even colder. The colder it gets, the more surface ice on the earth, which reflects more light directly back into space, so it gets even colder. It may not stop until there is basically no water vapour in the atmosphere and no more land to be covered with ice. Snowball earth. So what can break this cycle:
- Earth orbital/tilt changes to change the climate
- A colder Earth radiates less energy into space (limiter, not ‘breaker’)
- Increase in atmospheric CO2 due to lower plant growth
Not a big list. But with plants almost dormant under these conditions CO2 previously trapped underground gradually increasing atmospheric greenhouse gas levels could just be virtually inevitable. In any event, if it has ever happened (which does seem to correct at least once) then the cycle was broken.
Negative Feedback has its limits
As the snowball Earth cycle has natural limit when the Earth is covered, the greenhouse gas runaway need only stop once all the water has evaporated and at the resulting temperature evaporated into space. Certainly Venus never recovered. When this happened on Venus there may have been no plants, and although Venus would have had radiation levels at that time more similar to what Earth has today than what Venus has today (due to continuing increase in solar energy from the original 70% of todays levels) it was still most likely getting more Sun that Earth has yet.
As the Sun gets ever hotter, the level of CO2 required to trigger greenhouse runaway get ever smaller. Already Earth is at the point where CO2 levels of 0.04% are considered too high. This is such a low level that it has to limit plant growth, yet as the Sun continues to increase radiation, the acceptable level of CO2 must reduce even further unless something else changes.
The Drake/Fermi Rare Earth
The ‘Drake’ equation provides a formulae for calculating the number of intelligent civilisation we should expect to find in the galaxy, the and Fermi paradox calculates questions “where is everybody?”. The lack of being able to detect aliens so far raises the prospect that something about Earth is very rare. This post contemplates: ‘what if the rare thing about the Earth is similar to the rare attribute of lotter winners’. In other words: not special, just lucky. Given enough planets, even with a periodic risk, one planet will by pure change, avoid the consequences of the risks.
Meteors to the Rescue?
If you look back at those four factors to break the runaway greenhouse gas cycle, clearly the mega-volcano and the meteor strike have happened in the past. Logically, as the Sun continues to warm, there will come a time when a meteor strike would be one way of breaking a greenhouse runaway event that would otherwise reach full thermal runaway.
In fact, for all we know, this has already happened. Without that very event that ‘killed’ (or contributed to a mass extinction of) the dinosaurs, perhaps the earth would be one or two degrees warmer today? Just a thought.
Perhaps we would not be here without certain meteor events to periodically break greenhouse cycles.
