Does climate change cause increase in CO2?

Question

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.

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

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.

Answer 5

Here's one actual example of a marine ecosystem releasing a huge amount of CO₂ because of a marine heat wave, i.e. where climate change leads to more CO₂ release:

Arias-Ortiz et.a. A marine heatwave drives massive losses from the world’s largest seagrass carbon stocks

On the basis of field studies and satellite imagery, we estimate that 36% of Shark Bay’s seagrass meadows were damaged following a marine heatwave in 2010/2011. Assuming that 10 to 50% of the seagrass sediment C stock was exposed to oxic conditions after disturbance, between 2 and 9 Tg CO₂ could have been released to the atmosphere during the following three years, increasing emissions from land-use change in Australia by 4–21% per annum.

This also is an example of the shrinking ability of the ocean to serve as a carbon sink because of global warming. And it shows that small changes in the ocean's CO₂ concentration can make huge changes in the atmospheric concentration of CO₂.

Answer 6

Warming climate conditions can cause carbon sinks, notably permafrost, to release carbon dioxide and that is in addition to the CO₂ from fossil burning that induced the warming and causes additional warming as well, so yes, climate change causes CO₂ in the atmosphere to increase. This is an example of a carbon feedback or Earth system feedback. Others include methane emissions from wetlands, where warming causes release of a greenhouse gas that makes warming stronger. There is also feed back that offsets warming, such as warming shortening the time for methane to break down.

What we do not have is any evidence of naturally occurring global warming sufficient to cause all or even most of the raised CO₂ levels we are currently experiencing. Most of the CO₂ rise is attributable to fossil fuel burning and land use changes, with an additional component from all forms of carbon feedback.

Answer 7

A rather late answer on this question, but here goes: The idea that "global temperatures cause the increase of CO2." is fairly easily refuted. The usual line of reasoning is that the solubility of CO2 in water decreases as temperature increases, so the rise in global temperature implies that the oceans will have released some of the dissolved CO2 that it contains. The premise is correct, but the conclusion is a non sequitur as it ignores the fact that the solubility of CO2 in water also depends on the difference in the concentrations of CO2 between the surface ocean and the atmosphere. As we have released CO2 from fossil fuels into the atmosphere, the partial pressure of CO2 in the atmosphere has risen, and so the oceans have been taking up more CO2, not less.

How do we know this is true? . The argument is easily refuted by the observation that the rate at which atmospheric CO2 levels are rising is less than the rate at which we are releasing CO2 into the atmosphere from fossil fuel use, which implies that the natural environment must be a net carbon sink, taking in more carbon each year than it emits.

More formally, let Ea represent annual carbon emissions from anthropogenic sources (fossil fuel use and land use change), En represent the carbon emissions from all natural sources (the oceans, soil respiration, volcanos etc.) and Un represent the uptake of carbon by all natural carbon sinks (oceans, photosynthesis, etc.), Ua would be the uptake of carbon due to anthropogenic activities, but this is essentially zero, so we can safely exclude it from the analysis. Then assuming that the carbon cycle obeys the principle of conservation of mass (any carbon emitted into the atmosphere that is not taken up by natural sinks remains in the atmosphere), the annual change in atmospheric CO2 is given by:

C' = Ea + En - Un

This can be rearranged to give an estimate of the difference between annual emissions from all natural sources and annual natural uptake by all natural sinks.

En - Un = C' - Ea

We have accurate, reliable data for the growth of atmospheric CO2 and for anthropogenic emissions (for details, see Cawley, 2011). Both of these are displayed below, along with an estimate of the net natural carbon flux En - Un. The fact that the net natural flux is negative clearly shows that natural uptake has exceeded natural emissions every year for the last fifty years at least, and hence has been opposing, rather than causing the observed rise in atmospheric CO2.

source

Some time back, I wrote a journal paper refuting a related climate myth, which outlines some of the evidence that the rise in atmospheric CO2 is anthropogenic, and is not a natural phenomenon.

Gavin C. Cawley, "On the Atmospheric Residence Time of Anthropogenically Sourced Carbon Dioxide", Energy & Fuels, volume 25, issue 11, pages 5503–5513, 2011.

This answer is adapted from my answer to a similar question on the Skeptics SE.

"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)."

There are actually plenty of studies of this, it is just that the issues were largely resolved over fifty years ago. The main mechanism is the temperature dependent solubility of CO2 in the oceans, there is an excellent textbook on this by Zeebe and Wolf-Gladrow. There is also an effect of temperature on the biosphere, which is a bit more difficult to pin down, but see the references in the relevant section of the IPCC report. The research is there, but we have known for a long time that the rise in CO2 is not the result of natural processes, so there will be no papers explicitly making this claim (at least from people who understand the basics) because there has been no new evidence to support it.