I've written before about the important distinction between a slow rate of CO2 increase and an accelerating rate. Geologists know that the planet's oceans and atmosphere are very resilient to fluctuations in its component parts. We are undergoing a sustained increase in atmospheric and oceanic concentration of CO2, and the past is not consistent with regard to how the planet responds to these changes.
An article at the Skeptical Science website explores this issue in detail. The differential impact of CO2 is determined by the rate of change of its concentration, whether it is fast or slow. As is explained there, there are numerous chemical and biological processes involved, and some of them act quickly and others take millennia. Basically, the absorption of CO2 into the oceans surface, where most CO2 emitted into the atmosphere goes, occurs far more quickly than the deeper processes that deal with the CO2. The consequence is a large buildup of CO2 in the atmosphere and ocean surface while the deeper processes work on it.
The different impacts depending on the rates of CO2 entering the atmosphere then explains the different outcomes for past occurrences. If the CO2 emissions rate is slow, there is not very much accumulation in the atmosphere and ocean surface because the deep processes handle it as it happens. But when the rate is fast, the CO2 accumulated in the atmosphere and ocean surface with dire consequences to global temperatures and loss of sea species to acidification because the deeper process take too long to manage the increase. From the article:
Scientists estimate that the rate of carbon emissions in the Permian crisis, which came close to extinguishing complex life on Earth, was comparable to current emissions, even if the magnitude and probably the duration of the Permian emissions was much larger.
The end-Triassic was a similar time of mass extinction, triggered by colossal eruptions at the Atlantic margins of both Americas, Europe and Africa. This event appears to have involved 5 intense pulses of volcanic CO2 release, each lasting perhaps 1,000 years or possibly as little as 400 years. Each pulse doubled or tripled atmospheric levels, causing about 0.15 units and a drop in reef carbonate production by 15-30%, with levels recovering somewhat in between pulses. The cumulative effect was the loss of some 70% of species on Earth.
Other Large Igneous Provinces had similar effects – the Deccan Traps LIP at the end-Cretaceous, the Karoo-Ferrar LIP during the Toarcian age, and so on. But some LIPs did not trigger environmental catastrophes. Why?
The researchers concluded that the Paraná-Etendeka LIP was a slow-burn phenomenon that emitted CO2 slow enough for the oceans' natural compensation mechanisms to neutralize them harmlessly, and therefore it did not trigger a shift or environmental crisis.
The bad news,
Current human emissions are at a rate comparable to those in Earth’s past that triggered powerful global warming and associated with mass extinctions.