My main question is how do we know that increase in global temperature does not CAUSE an increase in co2 concentration?

I am trying to develop my mental model of what's going on for the climate change issue. There is definitely a correlation between global temperature and co2 concentration, this is not disputed. The general consensus is that co2 increase causes temperature increase, why is the reverse situation not studied (if it is I could not find any studies).

This thought comes from the statement I've seen that water vapor concentration is more correlated to global temperature than co2, but this is intuitive since warmer air holds more water vapor than colder air. Does the same intuition hold for co2?

I am not an expert on the subject; so I understand that I may be way off base with my reasoning. I also openly accept edits to my question if it is not structured or presented properly.

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    $\begingroup$ To be fair: Climate change does cause changes in greenhouse gas concentration. Think methane being freed from the thawing arctic tundra, or decreasing CO2-solubility in warmer ocean water. Unfortunately, that doesn't stop that gas from contributing even more to climate change ... $\endgroup$ – Hagen von Eitzen Nov 15 '17 at 15:36
  • $\begingroup$ "There have been predictions, and some evidence, that global warming might cause loss of carbon from terrestrial ecosystems, leading to an increase of atmospheric CO2 levels." en.wikipedia.org/wiki/Climate_change_feedback#Positive $\endgroup$ – Lehs May 8 '19 at 23:17

One reason is that we know exactly where the current increased CO2 comes from. That is, we know from economic data how much petroleum, natural gas, & coal is extracted and burned. From that simple chemistry lets us compute the amount of CO2 produced, and we find that (after allowing for factors such as e.g. some being absorbed by the oceans) that the amount created by burning fossil fuels matches the observed increase.

(There are also more subtle measurements, such as isotope ratios, which I'm not qualified to expound in any detail...)

Another reason is that for warming to have caused the observed increase in CO2, there'd have to be some other cause for the warming. No such cause has been found. Suggested causes such as variation in solar output have been ruled out by measurements.

  • $\begingroup$ So you're saying since we know what is causing the co2 increase, we can rule out other causes? $\endgroup$ – Acumen Simulator Feb 3 '17 at 19:14
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    $\begingroup$ CO2 has an isotopic signature: "...fossil fuels do not contain 14C. By studying how the concentration of 14C has changed in the atmosphere, scientists have determined that the atmospheric increase in carbon dioxide is dominated by fossil fuel emissions." esrl.noaa.gov/gmd/outreach/isotopes $\endgroup$ – jeffronicus Feb 3 '17 at 21:46
  • $\begingroup$ @user38826: Simplistically, yes. We know where the CO2 comes from, we know from theoretical calculations about how much warming it should cause, and we find that that's just about how much warming is actually happening. That does seem to rule out other causes as major factors, doesn't it? Then add the fact that all the other possible causes we can think of are not actually happening... $\endgroup$ – jamesqf Feb 3 '17 at 23:05
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    $\begingroup$ @user38826: How can "other factors" possibly be the dominant cause, if those other factors aren't present? Also, we'd need two mysterious "other factors": one to produce the observed warming, another to remove the warming that the increased CO2 should have produced - and maybe a third to explain why the observed warming produced by the mysterious "other factor" is exactly what should be produced by CO2 from fossil fuels. You might read up on Occam's Razor. $\endgroup$ – jamesqf Feb 4 '17 at 6:18
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    $\begingroup$ @user38826: Yes, and we know what those factors are. For instance, the Ice Ages are driven by orbital variations - en.wikipedia.org/wiki/Milankovitch_cycles - but those aren't in a warming phase now. Increased solar output could cause warming, but the sun hasn't increased its output. Massive volcanic eruptions could emit lots of CO2 and cause warming (if SO2-driven cooling didn't dominate), but I think we'd have noticed those :-) But we DO know CO2 has increased due to fossil fuel burning. $\endgroup$ – jamesqf Feb 4 '17 at 19:14

Your comparison to water vapor is a bad one.

The amount of water vapor in the atmosphere does increase with atmospheric temperature. This is because more evaporation occurs and can be held as vapor longer, thus offsetting the equilibrium of water vapor. Cold air will cause more water vapor to condense back to liquid. This process of water cycling occurs on the order of hours/days.

This is in stark contrast to carbon dioxide, which does not condense out of the atmosphere when it cools. Nor does carbon dioxide evaporate from the surface of the earth. Instead, it is a byproduct of respiration and combustion. Carbon dioxide stays in the atmosphere for 100s of years before it gets deposited or chemically converted. Furthermore, carbon dioxide becomes part of geological processes in the carbon cycle that takes 1000s of years, not days.

So, to summarize, carbon dioxide does not directly increase in the atmosphere due to temperature rise, because it is not constantly evaporating from the Earth's surface and condensing in the atmosphere. Carbon dioxide does not condense at normal atmospheric temperature... it won't condense until it gets as cold as -78.5 degrees C, which does not occur on the surface of the Earth [in this climate regime].

It should be noted that carbon dioxide will indirectly increase in the atmosphere with temperature rise. This is because of feedback mechanisms. For instance, with increasing temperature, trees will be more dry on average and will thus combust more easily. More wildfires are expected as temperatures rise, and with them come more carbon dioxide emitted during the fire. There is also the possibility that increases in global temperature will cause feedbacks in the geologic cycle that lead to more volcanic activity, which is another source of carbon dioxide emissions. And, as Michael points out, the methane released when permafrost melts will eventually oxidize with the hydroxyl radical into carbon dioxide and water vapor.

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    $\begingroup$ I'd like to add that increasing temperatures cause melting of permafrost, which allows release of trapped methane. This methane then oxidises to carbon dioxide in the atmosphere. $\endgroup$ – Gimelist Feb 4 '17 at 4:16
  • $\begingroup$ Yes Good Point! $\endgroup$ – farrenthorpe Feb 4 '17 at 4:26
  • $\begingroup$ I never believed co2 behaved as h2o, that's just where the idea of the confusion of inverse came from. Good information on it not directly being caused by temperature rise. $\endgroup$ – Acumen Simulator Feb 4 '17 at 4:28
  • $\begingroup$ You used water as your comparison point, which is why I discussed it. $\endgroup$ – farrenthorpe Feb 4 '17 at 4:36
  • $\begingroup$ Actually CO2 would increase further (as a feedback mechanism) with temperature rise, as the solubility of CO2 in the oceans decreases with increasing temperature. $\endgroup$ – jamesqf Feb 4 '17 at 6:22

The general consensus is that co2 increase causes temperature increase, why is the reverse situation not studied (if it is I could not find any studies).

To be brutally honest, it's not studied because what you have posited is nonsense.

That said, the reverse situation has been studied in a broader context. $\mathrm{CO}_2$ is soluble in water, with the solubility decreasing with increasing temperature. This is one of the key reasons the exit from a glaciation is much more rapid than is the onset of a glaciation. Atmospheric $\mathrm{CO}_2$ levels plunge during a glaciation due to absorption by the oceans. Once conditions are right for a glaciation to end, the warming makes that dissolved $\mathrm{CO}_2$ come out of solution, which causes more warming, which causes even more dissolved $\mathrm{CO}_2$ come out of solution, ... This positive feedback loop makes the exit from a glaciation quite rapid.

The last time this happened was about 12000 years ago. It is not happening now. The oceans are currently absorbing most of the excess $\mathrm{CO}_2$ we humans are pumping into the atmosphere.

  • $\begingroup$ Thank you for your on topic answer. Not grateful for the hostility, but good answer. $\endgroup$ – Acumen Simulator Feb 3 '17 at 17:29
  • $\begingroup$ So does the ocean continue to absorb co2 because we are so far from the temperature equilibrium? or is the concentration difference in air and ocean driving this? $\endgroup$ – Acumen Simulator Feb 3 '17 at 18:56
  • $\begingroup$ Oceans absorb about 25% of all the emissions, not "most of the excess CO2". Another quarter is absorbed by plants, and the rest remains in the atmosphere $\endgroup$ – fede_luppi Feb 4 '17 at 8:29
  • $\begingroup$ Check out the Global Carbon Project globalcarbonproject.org/carbonbudget/16/highlights.htm $\endgroup$ – fede_luppi Feb 4 '17 at 8:41
  • $\begingroup$ The amount absorbed by oceans is fairly constant year to year, whereas the proportion absorbed by land varies every year from 20-30%, because for example, years with strong droughts (El Niño) plants struggle and can grow too much, hence absorbing less CO2 from the atmosphere $\endgroup$ – fede_luppi Feb 4 '17 at 8:46

There are processes operating on geological timescales that link CO2 (specifically CO2, not GHGs) to changing temperature, namely chemical weathering (the chemical breakdown of in situ rock at/near the Earth surface). When aluminosilicate minerals react with carbonic acid (rainwater+CO2 formed in the atmosphere), minerals dissolve, releasing their cations (e.g. Si2+, Al2+, Ca2+ etc) and soluble anions (e.g. bicarbonate 2Co3-). These get washed to the ocean where calcium carbonate shelly organisms incorporate them into their shells, eventually die, and fall to the sea floor. This is a principle geological mechanism for removing CO2 from the atmosphere and an important part of the carbon cycle.

The link to climate comes from its control on weathering processes. Generally, chemical weathering processes are more efficient when it is wetter and warmer. So at planetary scale, over long time periods, if it gets warmer, the amount of CO2 in the atmosphere should decrease, which in turn reduces the greenhouse effect.

So from this basic physical understanding, your premise that increasing temperature can CAUSE increased CO2 concentrations has little to no basis in relation to our understanding of the climate and carbon systems, quite the opposite in fact (temperature increases cause draw down of CO2 due to chemical weathering). Alternatively, we have a known mechanism (the greenhouse effect) supported by empirical observations that CO2 influences global temperature.

  • $\begingroup$ So increased temperature actually decreases co2 concentration? If you exclude human interaction... $\endgroup$ – Acumen Simulator Feb 7 '17 at 20:34
  • $\begingroup$ @user38826 on time scales on hundreds and thousands of years, increased temperature will cause this reaction to go faster. This is only one factor out of many controlling carbon dioxide, and it will not decrease concentration. Only slow it down a bit. Just like jumping from a 200 metre bridge instead of 220 metre bridge. You're going to die anyway. $\endgroup$ – Gimelist Feb 7 '17 at 21:36
  • $\begingroup$ This is one of the principle mechanisms by which the Earth natural regulates CO2 in the atmosphere, but it is slow, changing over long timescales (millennia to millions of years). As Michael suggests, the recent increase from 280ppm to 400+ppm is an unprecedented change in natural history and it is highly unlikely that such natural processes will adjust to keep pace. $\endgroup$ – mdh488 Feb 9 '17 at 10:36

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