Climate is a high order nonlinear dynamic system and as such:

  • is complex to study
  • is not controllable via linear input as in we pollute less -> temperature drops

I have observed various sources claiming that "assuming greenhouse gasses continue accumulating there will be a point where even if humanity and it's industry magically disappears, global temperatures are going to continue rising significantly". I accept that as a proven fact by now.

The associated big question is "When?". Follow a couple of sources which claim "5 years ago" and a couple of sources saying "perhaps some day".

  • $\begingroup$ YES. Can I get my 10 points now? $\endgroup$ – TheEnvironmentalist Sep 11 at 23:16
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    $\begingroup$ @TheEnvironmentalist - I'm trying to understand what you meant by your comment. Are you also "Vorac"? $\endgroup$ – David Hammen Sep 12 at 3:33
  • $\begingroup$ let me put it this way,during my lifetime(54 years)not a single year has our planet had a lower than average temperature(the sun has had a lower than normal energy output for the last 20+ years)and still earths temperature are going up so yes climate change is real.en.wikipedia.org/wiki/Solar_cycle_24 and en.wikipedia.org/wiki/Solar_cycle_25 $\endgroup$ – trond hansen Sep 12 at 14:39
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    $\begingroup$ With regard to the title, "irreversible" is perhaps a bit strong. Venus underwent irreversible global warming when it went through a runaway greenhouse effect. Mars underwent global climate change when it lost its waters. Note also that "climate change" is now considered a more preferable term than "global warming" for a number of reasons. $\endgroup$ – David Hammen Sep 12 at 15:29
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    $\begingroup$ With regard to is not controllable, we humans certainly have controlled the climate over the course of the last hundred years or so by pumping ever increasing amounts of carbon dioxide into the atmosphere. We have proven that climate is controllable, but not neccessily in a good way. We are perhaps He Who Must Not Be Named: "After all, He Who Must Not Be Named did great things – terrible, yes, but great.” $\endgroup$ – David Hammen Sep 12 at 15:32

Yes and no. From a pedantic point of view, the question is ill-posed: the concept of reversibility does not make sense in a chaotic system.

On the other hand, it is an idea that is widely discussed, and most people have a rough idea of what is meant.

There are three possible interpretations of the question that I can think of:

(1) If the abundance of greenhouse gasses in the atmosphere is returned to pre-industrial levels in the next hundred years, has there been irreversible change which will stop the climate returning to its pre-industrial state at the same time, at least as far as the global mean temperature is concerned?

Answer: no, at least not much. There will be differences because of changes in albedo arising from changes in vegetation and ice cover. Some of the changes in ice cover will be irreversible, at least on the time scale of millenia. This is because the Greenland ice-sheet was formed during the last ice age, and won't recover the mass it is loosing until the next ice age. There will also be a lag in the recovery of the Ocean temperature.

(2) A second possible interpretation of the question is: if we stop burning fossil fuel, and start extracting CO2 from the atmosphere, will the warming trend continue?

Answer: no, probably, if we do it soon enough, but it is possible. The difficulty here is removing enough CO2 to make a difference. If we leave it too late, there is a risk that natural feedbacks (e.g. increased frequency of forest fires, methane emission from permafrost) will release greenhouse gasses faster than we can remove them. I'm not aware of anybody making even a rough quantitative estimate of the risk of a run-away warming feedback, but the risk exists and grows as the temperature rises. If you are of a nervous disposition, events such as the raging fires in Australia last year look very ominous. When it comes to forest fires, however, it is, however, very difficult to disentangle impacts of rising temperature from changes in forest management, but there are a number of studies, e.g. https://nhess.copernicus.org/preprints/nhess-2019-206/, linking climate change to an increase in fire activity.

(3) If the question is about irreversible melting of ice, rather than irreversible temperature change, then the answer is most likely yes -- in many areas ice volume will not recover. There are changes to the surface albedo of Greenland which make it likely that it will continue melting. The Greenland ice sheet is a fossil ice sheet, meaning that it only exists because it was formed in the ice age: there is no mechanism to grow it back in the inter-glacial period.

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  • $\begingroup$ My question was (2). The conclusions are extremely sad. $\endgroup$ – Vorac Oct 13 at 8:47
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    $\begingroup$ Yes ... but I heard a talk by a graduate of 2020 recently which gave some hope. She had submitted her final papers earlier this year and then headed straight into lockdown (in the UK). Her message was that we can change the way we live. $\endgroup$ – M Juckes Oct 13 at 21:59

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