I came across an article written by economist Sanjeev Sabhlok on the long term climate future of the earth titled - Limestone proves the impossibility of a runaway greenhouse effect on Earth. Mr Sabhlok has been reading some geology and has found out that the earth can naturally regulate the earth's carbon dioxide levels over geologic time.
The process operates like so: During times of increased volcanism, CO2 levels in the atmosphere increase to a point where the earth starts warming. This in turn enhances rock weathering reactions which pull back CO2 and washes it down into the ocean where it is sequestered as a bicarbonate or carbonate molecule. A fraction of this carbonate gets locked up in limestone precipitating on the sea floor.
Besides this mechanism, photosynthesis also pulls out CO2 from the atmosphere. This CO2 goes into building organic molecules. Some of that organic matter sinks to the ocean floor and is buried, creating another long term carbon sink.
All these natural adjustments to atmospheric CO2 means that a runaway increase where CO2 levels keep rising thousand fold unabated is unlikely to occur. Earth will not turn into a Venus. Mr Sabhlok says that most climate scientists ignore this natural regulator in their panic over a runaway greenhouse effect.
Mr Sabhlok has written quite a nice summary of the geological evolution of the earth's atmosphere. But he entirely misses the point about why scientists ignore geologic sequestration of CO2 in their climate change projections. They do so because it works too slowly to matter to us. Our concern is not a distant future where surface temperatures may or may not reach a Venus like 450 deg C, but one where there is a spike of 3-4 deg C in the next few decades to centuries which nevertheless will result in extreme damage to human society and the ecosystems we depend on.
The geologic thermostat that Mr Sabhlok describes can't prevent these smaller shorter time scale perturbations in atmospheric conditions. Some numbers he shares demonstrates the inadequacy of weathering to neutralize CO2 at short time scales. He quotes from a video put up by a Dr. Johnson Haas; " Typically on an annual basis … about 0.03 gigatonnes of carbon is extracted from the atmosphere and goes into limestone which goes into long-term geologic storage. … [E]ven at that slow rate the drawdown of CO2 from our atmosphere by shell building organisms … would completely exhaust the atmosphere of CO2 in less than a million years”.
What he doesn't add is the impact of human emissions. Our activity is emitting an eye popping 40 billion tons of CO2 to the atmosphere every year. This is 2 orders of magnitude more than what limestone can suck in. About half of this CO2 gets absorbed by the ocean, the vast majority getting locked as a stable bicarbonate molecule (HCO3). The rest remains in the atmosphere, cumulatively increasing its CO2 levels. Over the past 250 odd years, human activity has increased the amount of CO2 in the atmosphere by about 1.5 trillion tons.
When emissions eventually go to zero, absorption by oceans will quickly start reducing atmospheric CO2, putting the brakes on warming. And in the long run, several hundred to a few thousand years after we achieve a net-zero emission scenario, CO2 levels will come down to pre-Industrial amounts. But as long as emissions continue, the earth will keep warming and become a very unpleasant place. The geologic past informs us of the havoc wrecked by increased CO2 levels and a warmer earth. The Late Devonian (372 million years ago), the Late Permian (252 million years ago), and the late Triassic (201 million years ago) mass extinctions were all triggered by increased CO2 levels and warming from sustained volcanism.
The Inter Governmental Panel on Climate Change Synthesis Report outlines many scenarios that might unfold towards the year 2100. No contributing climate scientist on that report is panicking about a runaway greenhouse effect. Instead, they highlight that incremental increases in temperature over the next few decades will place a debilitating burden on our society through myriad impacts on our health, water security, agriculture, and biodiversity. While fixating on an implausible runaway effect, Mr. Sabhlok stays silent on the real impending danger that we are facing.
His sanguine advice that "We should sleep soundly, knowing that no matter how much CO2 mankind emits by burning fossil fuels, our amazing living planet will never go the way of Venus" is utterly irresponsible.
Earth may never go the way of Venus, but if we don't stop burning fossil fuels our amazing planet will turn into a living hell for us and our immediate descendants.
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Fun Facts: I didn't want to quibble about some of the specifics in my post, but I want to share this with you.
1) Biocalcification (limestone formation) results in the emission of CO2! Since most of the carbonate in the ocean is in the form of HCO3, we can write the precipitation equation as-
Ca + 2HCO3 -----> CaCO3 + H2O + CO2 --------- Eq.1.
For every molecule of CO2 that gets locked up in limestone, one molecule is released in the ocean and eventually into the atmosphere. Limestones over time do constitute a CO2 sink, but precipitation of carbonate sediment is not that effective an offset of atmospheric CO2 in the here and now.
2) On the other hand, dissolution of CaCO3 in the deep ocean adds alkalinity, neutralizing the increase in ocean acidity due to CO2 released by the oxidation of organic matter. It is Eq.1. in reverse.
CaCO3 + H2O + CO2 ------> Ca + 2 HCO3 ----- Eq.2.
Carbonate equilibria can be counterintuitive and complex!
3) Mr Sabhlok says that "we are currently close to the lowest levels of CO2 in the Earth’s history". It is true that CO2 levels have steadily decreased over geologic time. But they have sharply increased in the past 150 years from 280 ppm in the late 1800's to more than 400 ppm today and will continue to increase as long as we keep burning fossil fuels. The last time earth saw such CO2 levels was 14 million years ago.