It takes on average about five years before a CO2 molecule in the atmosphere dissolves in the ocean surface or is taken up by the vegetation through photosynthesis. Many therefore believe in the myth that the CO2 concentration in the atmosphere will return to pre-industrial levels in a few decades if the anthropogenic emissions stopped now. David Archer et al explain the origins of the myth before they smash it [Archer 2009]. But the myth lives on.
Other CO2 molecules return to the atmosphere. Some of them come from the sea, and some of them come from the carbon in the vegetation through respiration, combustion and decay. In pre-industrial time there was balance between these natural transfers to and from the atmosphere.
The anthropogenic CO2 emissions are significantly less than the natural fluxes mentioned above. But they transfer carbon into the atmosphere without opening a corresponding return, and they therefore increase the amount of carbon in the atmosphere. Over time, the increased amount of carbon distributes itself between the atmosphere, the oceans, the vegetation and the soil. After about one thousand years there will be equilibrium in this distribution, but still a significant portion of the increased amount of carbon will remain in the atmosphere. Then it will take in the order of a hundred thousand years before the increased amount of carbon is deposited as permanent sediments on the seabed and thus no longer contributes to elevated CO2 concentration in the atmosphere.
The atmospheric CO2 concentration will rise immediately after a large CO2 release from combustion of fossil fuels. Figure 1 shows how the concentration thereafter will drop, assuming that there is no further CO2 release from combustion of fossil fuels. The concentration will drop asymptotically towards the level it would have had without the release. It is shown as percent of the original increase of the concentration.
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| Figure 1: A large amount of CO2 is released to the atmosphere at time zero. The figure shows how the elevated CO2 concentration in the atmosphere thereafter will decrease with time. Different processes with different timescales will contribute. The figure is based on Figure 10-4 in [Archer 2011], on which also the Nature article Carbon is forever is based. |
The CO2 concentration in the atmosphere will fall rapidly in the first months and years after the CO2 release because much CO2 will be taken up by the vegetation, the soil and the upper layers of the ocean. But the CO2 flux back to the atmosphere from these inventories will increase, so the reduction gradually slows down. The ocean will continue to absorb CO2 when water in the upper layers is replaced with water from the deeper layers. But the ocean is a sluggish system, and it will take in the order of thousand years before the water layers are mixed. The green area in the figure illustrates the CO2 uptake in the vegetation, the soil and the oceans.
After a thousand years, the atmospheric CO2 concentration will still be elevated with approximately 25% of the original increase. Then other processes will take over to reduce the atmospheric CO2 content down to the level it would have had without the release. These processes are slow.
The ocean is basic, but it becomes less basic when the CO2 concentration increases. This is ocean acidification. It causes the calcium carbonate in sediments at certain depths of the ocean floor to dissolve. These reactions reduce the CO2 amount in the ocean, and thus also in the atmosphere. The process is called carbonate compensation. The blue area in Figure 1 illustrates when the carbonate compensation is the dominant process in reducing atmospheric CO2.
The carbonate compensation helps to stabilize the ocean acidity. It is a slow process that takes in the order of ten thousand years. Ocean acidification is now happening much faster than that, and therefore the carbonate compensation will not manage to stabilize the acidity. We will see a spike in ocean acidity in the next thousand years.
Calcium carbonate is an important constituent in the shell of shell-forming algae and plankton. Ocean acidification will make it harder for them to form and maintain their shells. It is not until recently that science has begun to understand the consequences this may have on the marine ecosystem. Many are worried about this, which is reflected in Chapter 3.2.8 in [IPCC AR5 WG1].
After ten thousand years, approximately 10% of the original CO2 increase will still remain in the atmosphere. Then the weathering of igneous rocks becomes the dominant process in reducing the last remnants of the CO2 increase in the atmosphere. The red area in Figure 1 illustrates this process. It needs a lot more time than the 25,000 years that the figure covers. The process is called the Earth silicate thermostat. It has controlled the Earth's temperature through geological times. Times with great volcanic activity cause the atmospheric CO2 content to rise. It gets hotter and wetter, and ice melts. All this causes the weathering to increase, and this drags the atmospheric CO2 content down again causing the Earth to cool. The Earth has previously been in snowball condition with snow and ice down to the equator. Then little igneous rocks are exposed to acid rain, and the weathering is small. But the volcanoes continue as before to release CO2 into the atmosphere. Atmospheric CO2 content rises, it gets warmer, ice melts and reveals igneous rocks, and the weathering increases again [Archer 2011].
The CO2 emissions from our combustion of fossil fuels are much larger than both the CO2 emissions from volcanoes and the amount of carbon that is deposited as permanent sediments on the seabed. This is why the atmospheric CO2 concentration now increases as fast as it does. In the article Volcanic Gases and Climate Change Overview The US Geological Survey estimates that the anthropogenic CO2 emissions in 2010 were 135 times greater than the CO2 emissions from volcanic activity.
Figure 1, and the figure in [Archer 2011] on which it is based, provide information about neither uncertainties nor the exact size of the carbon emissions at time zero. The uncertainties are substantial, as TFE.7 Figure 1 in [IPCC AR5 WG1] shows. It also shows that large emissions are twice harmful. After thousand years, about 15% of a moderate emission will remain in the atmosphere, but 40% of a large emission will remain. The moderate emission in the figure is one hundred PgC, which corresponds to ten years emissions from today's burning of fossil fuels. The large emission is five thousand PgC, which corresponds to the emissions from the burning of the assumed stocks of fossil fuels. One PgC is the same as one GtC, which is one billion tonnes of carbon. The discharge at time zero in Figure 1 is between one thousand and two thousand PgC. This estimate is based on [Archer 2005] and on the aforementioned figure in [IPCC AR5 WG1]. The sum of the anthropogenic emissions will probably be in that order of magnitude before we get control over the emissions.
References
Archer 2005: David Archer, Fate of fossil fuel CO2 in geologic time.
Archer 2009: David Archer et al, Atmospheric Lifetime of Fossil Fuel Carbon Dioxide.
Archer 2011: David Archer, Global Warming: Understanding the Forecast,
IPCC AR5 WG1: IPCC, Climate Change 2013 - The Physical Science Basis
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