If we look at the carbon cycle, we see that man-made carbon emission only account for a tiny fraction of all $\text{CO}_2$ emissions.

However, we see a steady rise in atmospheric $\text{CO}_2$-levels.

Wouldn't it be logical to assume that earth would somehow adapt to this increase, e.g. through increased plant growth and why isn't this happening?

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    $\begingroup$ The topmost image in the wikipedia-entry you linked shows exactly what you ask, an additional intake of CO2 by earth systems. So how is this not an adaption? $\endgroup$ – Erik Mar 27 at 7:59
  • $\begingroup$ The earth and life will adapt, WE are screwed, human civilization is built around a very specific set of climate conditions that the new equilibrium will be nowhere near. , $\endgroup$ – John Mar 30 at 4:58

That assumption is indeed logical and correct, and increased plant growth is in fact happening (this effect is known as $\ce{CO2}$ fertilization). As you suggests, Earth will adapt to the increased inflow of $\ce{CO2}$, and it will find equilibrium again. What might be missing in your reasoning, is the fact that the atmospheric $\ce{CO2}$ concentration in the new steady state would not be the same as the concentration in the pre-industrial steady state.

The atmosphere is a reservoir of $\ce{CO2}$. Its has sources (volcanoes, plant respiration, decomposition, fossil fuels combustion, etc.) and sinks (photosintesys, ocean uptake, silicate weathering, etc.). To simplify the system let's do an analogy with a bath tub.

Let's imagine a bath tub with a very powerful faucet and a sink. Now let's start opening the faucet until the sink becomes overwhelmed by the incoming water and the level in the bath tube starts to rise. As the level rise, the water pressure at the bottom will rise too, therefore, more water will rush through the sink. Eventually the water going trough the sink will match the water coming in, and the water level will stabilize. At this point we have reached steady state.

If we open the faucet a bit more, the water level will rise again, up to a point where the bottom pressure allows the sink to catch up and a new steady state is reached, but this time at a higher water level. Also, note that after we increase the inflow of water, it takes some time to reach the new steady state, and that time depends on the sizes of sources and sinks relative to the reservoir. In our example, if the same faucet and sink were in an Olympic swimming pool instead of a bath tub, the time would have been much larger.

Going back to Earth, the atmospheric $\ce{CO2}$ concentration was pretty much as steady state before the industrial revolution (if we consider ~1000 years time scales). Then, with the industrial revolution we increased the inflow of $\ce{CO2}$ into the atmospheric reservoir. As a consequence, the ocean and erosion increased their $\ce{CO2}$ intake, and plats increased their photosynthesis rate. In that way Earth, as you correctly assumed is working its way back to a new equilibrium. However, to reach that equilibrium takes time, and once we reach it, the atmospheric $\ce{CO2}$ will be higher than the pre-industrial levels, and that will in turn lead to a stronger greenhouse effect, higher temperatures, and climate change.

It is also important to note, that the extra inflow of $\ce{CO2}$ didn't just increase at one point and stopped (like our faucet). It is constantly increasing to higher and higher levels of $\ce{CO2}$ production, making even harder for natural systems to bring Earth to a new equilibrium.

The first figure in the Wikipedia article you linked shows this neatly:

enter image description here

We can see here in white numbers the most significant pre-industrial sources and sinks (at ~1000 years time scales). We can see that humans produce 9 Gigatons of carbon per year (GtC/yr), due to that extra inflow, photosynthesis is taking 3 GtC/yr more than before, and the ocean is taking an extra 2 GtC/yr as well. However, that is not enough to counteract the 9 GtC/yr we produce, and that is increasing the amount of carbon in the atmosphere at 4 GtC/yr. This means the level in the atmospheric "bath tub" is still rising.

If we were to keep those 9 GtC/yr we produce stable (i.e. not increasing production in the future). The concentration of $\ce{CO2}$ in the atmosphere will rise to a level high enough that the sinks will match the sources, for example with plants taking 5 GtC/yr and the ocean 4 GtC/yr, that would nicely balance the production. But that new equilibrium atmospheric $\ce{CO2}$ concentration would be high enough to rise Earth's temperature several degrees and force a whole reorganization of the Earth's climates.

Finally, we have to say that some of these $\ce{CO2}$ intakes, like the oceanic one, don't come for free, and have their own nasty consequences, like ocean acidification.

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    $\begingroup$ Useful analogy, but I'm not sure I agree with your point near the end. I think all of the CO2 concentration stabilization scenarios I've seen require a reduction in emission rates. There's no guarantee that the sinks can catch up with 9 GtC/yr constant emissions, and it's not unreasonable that the land sink would weaken and, perhaps, even become a source. There may be no new equilibrium atmospheric CO2 concentration at that emission rate. $\endgroup$ – Deditos Mar 28 at 11:17
  • $\begingroup$ @Deditos I understand those stabilization scenarios are those that cap global warming (at +2°C for example). But if temperature go higher, the sinks would eventually catch up. At long timescales silicate weathering is one of the main sinks, and is proportional to temperature. Thus, if temperature rise enough it would catch up even with 9 GtC/yr (also more rock will be exposed due to ice melt). There is evidence that high-$\ce{CO2}$ & high_temperature equilibrium existed in the past. Otherwise, 9 GtC/yr would set us in the rout to runaway greenhouse effect, and I don't think that's the case. $\endgroup$ – Camilo Rada Mar 28 at 16:36
  • $\begingroup$ I was thinking more about CO2 stabilization at any arbitrary level, rather than warming stabilization to some target. But more importantly, I was thinking about $10^3$ to $10^4$ year processes and forgetting about $10^6$ year timescales. It might be worth mentioning timescales in your answer. In practice, wouldn't 9 GtC/yr exhaust the emissions reservoir pretty quickly (e.g., 5000 years) before it had time to reach equilibrium? $\endgroup$ – Deditos Mar 28 at 23:05

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