John Mason shows from the fossil record what the knock-on effects will be of elevated CO2 levels, similar to what we’re on target for creating with business as usual. An anoxic extinction event during the Toarcian stage of the Lower Jurassic period, some 183-174 million years ago, shows were we’re headed. He shows a sequence of causal events, like a planet scale rube goldberg device, leading to acidic oceans, widespread anoxia, and flourishing sulfur bacteria. In other words DEAD, DEAD, DEAD - just like Peter D. Ward predicted in Under the Green Sky.
The Jurassic Period in general had a warm global climate, but during the Toarcian stage there was a period of additional global warming in which temperatures are estimated to have increased by 2–3.5oC in subtropical areas and 6–8oC at higher latitudes. The fact that such values are similar to projections for the end of this century under a business as usual scenario will not be missed by readers.
The warmer seawater is, the less dissolved oxygen it can carry.
Localised, short-lived but deadly hypoxic and anoxic events occur periodically around the world, so that it has been possible to study how they evolve.
The Toarcian period of global warming left a geological calling-card behind. Tell-tale rock sequences indicative of deposition under anoxic conditions, consisting of organic-rich black shales (layered mudrocks), occur in many parts of the world where sedimentary rocks of this age are preserved. There is also widespread evidence of a mass-extinction event at this time in western Europe, South America, Siberia and northern Africa.
... there was then a marked and abrupt drop-off in biodiversity...
... the rocks in question are black and rich in organic carbon and iron sulphide is in itself testimony to a low-oxygen environment...
So: we have a carbon-spike, warming and sea level rise all leading to a mass-extinction. But was anoxia the sole cause on a global scale? That, the authors say, remains equivocal: although records of anoxic events are widespread, there were major extinctions at the same time in oxygenated marine environments. Other factors may have been involved: ocean acidification is one candidate worthy of examination.
... anoxia is just one of two or more warming-related environmental stress-mechanisms that led to the Toarcian mass extinction.
some other sources that could have caused the carbon isotope excursion:
1. Degassing of volcanic carbon dioxide from the Karoo-Ferrar Igneous Province;
2. Release of terrestrial and/or continental margin methane clathrates;
3. Biomass burning of terrestrial vegetation;
4. Increased weathering and oxidation of organic-rich sediments and soils.
They suggested a knock-on sequence in which volcanic emission of large quantities of greenhouse gases led to oceanic and atmospheric warming; in turn this led to methane clathrate release accentuating the greenhouse effect; in turn, weather extremes stepped up and terrestrial wildfires increased in frequency, exposing organic-rich soils to both erosion and oxidation. One could also point out that if you want to get a lot of nutrients into the sea quickly, thereby generating phytoplanktonic blooms that lead to hypoxic or anoxic marine events, exposing organic-rich soils via wildfire then eroding them quickly by rainstorms will do the job as well as anything.
The take-home message from all of this? By whatever means an initial atmospheric carbon spike is generated (be it gigantic volcanic episodes or humans burning fossil fuels), the knock-on effects can be substantial, leading to a portfolio of severe environmental stresses that manifest themselves in the fossil record as mass-extinctions. Will Mankind's footprint, already involving severe carbon pollution and overfishing, be likewise visible in Anthropocene strata some 180 million years from now? [emphasis mine]
I think we are safe from that judgement. There won't be anyone on the earth capable of judging us 180 million years from now. We'll have messed everything up too bad for that to happen. ;)