For those who do not know, I am a geologist by training. I have recently been reminding myself what geology and geological history can tell us about Anthropologically Forced Global Warming (AFGW) and how it might inform our perception of risk.
The first thing to understand is that the Earth has experienced periods of warming and cooling. This is due to something referred to as Milankovitch Cyclicity. This is named after a Serbian scientist who hypothesised that variations in eccentricity, axial tilt, and precession resulted in cyclical variation in the solar radiation reaching the Earth, and that this orbital forcing strongly influenced the Earth’s climatic patterns. . This idea is generally accepted as one of the major forces of climate change and is often used by ‘climate change sceptics’ as the basis for the non-existence of AFGW. I call this the ‘climate has always changed’ logical fallacy.
But there is a bit more to this story. It turns out that the Earth does a good job of regulating these climatic changes and the Earth’s average temperature by regulating the amount of CO2 in the atmosphere through the ‘rock weathering cycle’. No one explains this better than the award-winning educator and geologist Richard Alley . Alley also points out that Milankovitch cycles may not cause as a big a variation in average solar energy reaching the earth, ‘it moves the sun around but does not have a big impact on average temperature’. SO you end up with the CO2 rock weathering cycle being the major driver, along with some special events (as I will discuss).
The idea is that the Earth naturally produces CO2 through volcanism. This CO2 in the atmosphere causes a greenhouse effect and warms the Earth, making it conducive to developing life. CO2 is the gas of life, it both warms the Earth and is used by plants and calcareous shelled animals and reef-builders in the oceans. Another climate change septics argument is that ‘CO2 is life and is good for our planet, the more the better’. The answer to this is yes and no. Without volcanism and CO2 there would probably be no life on our planet. But, if CO2 were to keep increasing in concentration in the atmosphere, we would have a runaway greenhouse effect, and the Earth would cook. A runaway greenhouse effect is what happened on Venus . Luckily the Earth moderates CO2 through the ‘rock weathering cycle’. Richard Alley neatly explains this, but here is the general gist of what happens
- Volcanoes produce CO2 and gas. Volcanoes do not respond to climate; they ‘just happen’. But it is the primary way CO2 gets into the atmosphere. How much comes out of volcanoes has nothing to do with climate.
- The CO2 reacts with rocks or is utilised by shell-building animals or carbonate reefs in the ocean or used by plants on land to build biomass.
- The rock weathers and sediment is carried to the ocean where it is deposited as rocks
- The shelly animals and plants die, and are buried by sediment and turned into rocks.
- The rocks are recycled in subduction zones, which causes volcanism and reemits the CO2
This forms a virtuous cycle of CO2 recycling. The neat trick is that as the climate of the Earth changes due to Milankovitch cycles, the Earth can vary this rock weathering cycle to moderate CO2 and regulate the greenhouse effect. Again, Richard Alley explains this better than me. But here is the synopsis.
- When it is warmer at the surface, the climate is generally wetter (due to increased evaporation and increased water vapour). Weathering increases during warmer periods taking more CO2 down into the oceans.
- When it is warmer shelly animals, reefs and plants grow faster and bigger. Thus the carbon sink gets bigger. As these plants die and are buried, that takes more CO2 down into the rock.
- As it gets colder, weathering slows down, productivity drops off, and more CO2 is released into the atmosphere, enhancing the greenhouse effect.
- (More limestones (carbonate rocks) are formed during warm periods and less during cold periods. This can be witnessed in the geological record.
As Richard Alley puts it, CO2 and the Rock Weathering Cycle is the Earth’s Thermostat. Of course, it takes millions of years to respond, so due to natural cyclical variations of the Earth, we do get glacial and interglacial periods. Still, the extremes are moderated by CO2 and the rock weathering cycle.
So are there times where this thermostat is broken, and does it have an impact?
Well it turns out yes, there are times when Earth has broken its own thermostat.
At the end of the Permian, about 250 million years ago a massive eruption event formed the Siberian Traps. The eruptions continued for about two million years and were the major cause of the Permian-Triassic extinction event, the most severe extinction event on record. 90% of marine animals and 75% of land life became extinct . This is the end Permian ‘Great Dying‘.
So what do we think happened?
Massive amounts of CO2 and methane were released during the volcanic event. It is also thought that the volcanism cooked oil and coal deposits which was also emitted as hot gas into the atmosphere. The event converted a large amount of once-living carbon into CO2. This led to two outcomes, an increased greenhouse effect. At the same time, the excess CO2 was absorbed into the oceans, causing the oceans to become more acidic, resulting in the loss of shelly carbonate producing animals and eventually loss of sponges and carbon reefs. “… anomalously high atmospheric pCO2 is a critical driver of both terrestrial and marine biotic crises“, Burgess and Bowring .
Recent studies show that, initially, the CO2 perturbation led to extreme warming and acidification of the ocean that was lethal to many organisms, especially those building calcium carbonate shells and skeletons. The greenhouse effect, however, led to further dramatic changes in chemical weathering rates on land and nutrient input and cycling in the ocean that resulted in vast deoxygenation and probably also sulphide poisoning of the oceans, killing the remaining organism groups. 
We need to be careful invoking the Great Dying of the Serbian Traps, it was an extreme event and an alignment of many factors. The world was different, the ecosystems were different and the CO2 outpouring was massive. Nevertheless, this extreme event gives us some clues about the consequences of current CO2 rises and temperature increases.
The recent study of current trends suggests that ‘if global warming is kept below 2 °C, less than 2% of assemblages globally are projected to undergo abrupt exposure events of more than 20% of their constituent species; however, the risk accelerates with the magnitude of warming, threatening 15% of assemblages at 4 °C, with similar levels of risk in protected and unprotected areas. These results highlight the impending risk of sudden and severe biodiversity losses from climate change and provide a framework for predicting both when and where these events may occur‘ .
Based on the previous discussion I might speculate:
- We are accessing carbon stores to produce energy and cement that interferes with the natural CO2 rock weathering cycle. The Serbian Traps event did similar by cooking coal and oil deposits
- We have reduced the size of the carbon sink through deforestation, destruction of wetlands and peat bogs—Land-use and land-use-change (LULUC) accounts for about 1.6 Gt of carbon emissions.
- Current global warming trends could significantly degrade our ecosystem to the point of harm to the human race.
CO2 is rising faster than at any time in geological history, except for maybe the Siberian Traps event. However, while the magnitude might not be as great, the rate of change is probably faster . To add to this problem, we have lost many of the carbon sinks, similar to how they were lost during the Siberian Traps event. Our oceans are also becoming acidic, risking the death of shelly carbonate building animals .
The obvious answer is clean energy. This clean energy is good in that it reduces CO2 emissions. However, we have already burned our way through much of the carbon that should still be stored in rocks. Also, some choices of clean energy come with a penalty. For example, hydropower is clean, but flooding of land and LULUC plus the amount of concrete and cement means that hydropower can emit nearly as much methane and CO2 on an energy equivalence basis as Coal , which might come as a surprise.
Note to reader:
I have said nothing here that Alley and others do not say in lectures and papers. But I have found people do not watch the lectures and/or read the papers. So this is my Dummies Guide to the teachings of Richard Alley, who is, for me, a Geology Super Hero!
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Richard Alley – 4.6 Billion Years of Earth’s Climate History: The Role of CO2 – YouTube
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