In looking at the potential impacts of human caused climate change over the coming decades and centuries, scientists have often pointed toward more recent times such as the Eemian (the most recent warm interglacial in which global temperatures are similar to what they are now and where they are expected to be over the next 20 years), the Pliocene (2-3 million years ago and the most recent time in which CO2 levels were about equal to those of today), and the PETM (about 55 million years ago and the most recent period during which CO2 levels were above 600 ppm and in which there was very rapid warming, possibly due to methane hydrate release).
The PETM has been a period of very intense study for leading climatologists such as James Hansen who has warned of the potential for a mini-runaway warming event of this kind should humans continue along a business as usual path of fossil fuel burning through the 21rst Century. In particular interest in the PETM corollary scenario is both the amazing velocity of the initial human warming, with CO2 and greenhouse gas releases occurring at rates that are five (CO2) to 27 (methane) times faster than the PETM (Hat tip to Timothy Chase, Source: Skeptical Science). So rapid and powerful a rate of forcing puts at risk of greater release a number of very large global carbon deposits including the massive CO2 and carbon stocks stored in the world’s melting permafrost as well as the even larger stores of carbon locked in methane hydrates scattered across the world’s oceans. Hansen and other scientists have noted a potential for a 4-7 degree Celsius or greater warming by 2100 (at between 700 and 1000 ppm CO2) through a combination of human greenhouse gas emissions and Earth systems carbon emissions. Overall warming by 2300 from Earth Systems feedbacks, even if human emissions were to stop by 2100, is likely to be twice this level.
That such a massive warming would be catastrophic is a given. There is no evidence in the geological record for such a stunning pace of warming over so short a period. And the potential climate change impacts from such high levels of heating, alone, would be extraordinarily difficult for human civilizations and the innocent inhabitants of our living world to manage.
(Image source: Ron Blakey, NAU Geology)
But this scientific scenario is based, in part, on knowledge gleaned by studying past geological periods such as the Eemian, Pliocene, and the PETM hyperthermal (other information is derived from the still-developing climate models of terrestrial, ocean, and Earth systems). And, in looking at each of these paleoclimate periods, we find that a single key factor is missing: they all occurred during periods in which Earth was either ice-free, or in which Earth was settling into its current period of glaciation. In the case of human-caused warming, the exact opposite process is ongoing. As during the great Permian Extinction event of around 250 million years ago, the Earth is rising out of a period of glaciation and into a potential human-caused hot-house.
No More Ice Ages and a Start Down the Path Toward De-glaciation
In the current period of human-caused warming we encounter the novel and relatively uncharted territory of an Earth System that is being forced to arise out of a 40 million year long period of glaciation. This period has been characterized, first, by the freezing of the vast land mass of Antarctica, then by the freezing of Greenland and, later, by long ice ages in which glaciers expanded from the poles to cover large areas of land and water. This latter ice age-interglacial period began about 800,000 years ago and has dominated until today.
(Image source: James Hansen)
With atmospheric CO2 levels now at 400 ppm and with humans continuing to emit high volumes of CO2 for at least the next two decades, we can officially declare the period of ice ages and interglacials at an end (or at least put on extended hold). For retaining even a very small portion of our current greenhouse gas emitting infrastructure or agriculture would be enough to stave off another ice age. Hansen notes:
Forces instigating ice ages, as we shall see, are so small and slow that a single chlorofluorocarbon factory would be more than sufficient to overcome any natural tendency toward an ice age. Ice sheets will not descend over North America and Europe as long as we are around to stop them.
Ice ages are now stopped in their tracks and current human levels of CO2 at 400 ppm are now sufficient to begin melting Greenland and West Antarctica. We can see this melt in yearly losses exceeding 500 gigatons of melt water and ice from Greenland and from Antarctic melt losses in the range of 300 gigatons per year or more. And with the increasing human heat forcing, these melt rates are on a very rapid incline. Greenland is showing a doubling in its melt rate every 5 years.
Yet even this, rapidly expanding, melt pace may seem slow if humans continue along their current path of greenhouse gas emissions growth. Last year, over 32 gigatons of CO2 were emitted into the atmosphere and the net human greenhouse gas emission was equivalent to more than 45 gigatons of CO2. At the current rate of emissions and emissions growth, we are now on track to hit between 500 and 600 parts per million of CO2 by the middle of this century. And this range of CO2 is enough or nearly enough to melt all the world’s ice, setting us on a path toward a place not seen in at least 40 million years. A path toward long-term temperatures in the range of 6 degrees Celsius hotter than the 1880s. If emissions continue until the end of this Century, the path is almost certainly toward that of a hyperthermal and one with unique consequences given the speed at which we approach it and the fact that we will send massive volumes of fresh meltwater into the oceans as we approach it.
The PETM and the Great Dying
And this is where we encounter a bit of a problem. Because the world is rapidly rising up out of a 40 million year long glacial period, it is bound to encounter changes not visible 40 million years ago as the Earth was steadily cooling down toward glaciation or even during the PETM as the Earth emerged from a lesser cool period and entered a hothouse state. In the case of the Permian and the current day, Instigating the loss of glaciers presents its own, rather unique, set of problems and difficulties.
In looking at the geological record, we find that the last major cold period with temperatures close to those of the recent ice ages (aside from a somewhat cool period during the late Jurassic and early Cretaceous) occurred during the late Carboniferous and the early to mid Permian period.
(Image source: Commons)
During the late Permian and early Triassic, however, very rapid and intense warming roughly equivalent to that of the Eocene of 55 million years ago occurred. Both events resulted in extinctions in the oceans and on land. Both events showed major temperature spikes toward the end that are theorized to be linked with large methane pulses and amplifying Earth Systems feedbacks. And both are typical to a mini runaway hyperthermal of the kind James Hansen warns is possible under a regime of human warming.
The primary differences between these two events is that, first, the Permian Triassic extinction event occurred after a long period of glaciation and, second, that the Permian extinction was the greatest mass extinction ever recorded in the geological past. What resulted killed off a devastating 96% of the species in the oceans and 80% of all species on land. It is for this reason that the Permian-Triassic boundary layer extinction is known as the great dying.
By contrast, the PETM resulted in a similar, but far less, extreme event. About 35-50% of the benthic forminifera of the deep ocean went extinct. Many other ocean species, especially those of the deep ocean, exhibited stress and losses. Life on land, especially among mammals, was pushed toward dwarfism to deal with the extreme high temperatures. But, overall, stresses to land and ocean animals was far, far less than that of the Permian extinction.
Putting a Lid on the Ocean — Glacial Melt’s Role in Enhancing Anoxia
At issue here is the likely anoxic ocean states resulting from major warming events. As the oceans are heated, they are able to hold less oxygen in solution. This steady depletion results in growing regions of anoxia and related algae blooms that can be very dangerous to marine and, in extreme cases, terrestrial organisms. Warmer, anoxic oceans are more likely to host blooms of deadly green and purple algae.
(Image source: NASA/Lance-Modis)
These primordial creatures once ruled the seas during the days of ancient Earth, before higher levels of oxygen were present. Now, a mixed, oxygen rich ocean keeps their development in check. But the warmer ocean during the time of the PETM is thought to have brought anoxic states back to the world’s deep oceans.
In short, ocean circulation is thought to have reversed. Heating at the tropics resulted in seas becoming saltier as waters there evaporated. These saltier waters grew dense and sank toward the ocean bottom drawing fresher, cooler water in from the poles. This type of ocean circulation is thought to have dominated for about 40,000 years during the PETM and contributed greatly to anoxic ocean states by concentrating warmer, anoxic water at the bottom of the world’s oceans.
During the Permian, anoxic ocean states were thought to be far, far more intense. Paleontological research conducted by Peter Ward found a massive series of three extinction events ranging over the course of about 165,000 years in which death began at the bottom of the Permian ocean and climbed toward the atmosphere.
It is thought by some scientists that rapid warming during the Permian enhanced both glacial melt even as it amped up the hydrological cycle to increase fresh water runoff from the continental land mass. The result was a much greater freshening of the ocean surface. Enhanced evaporation at the equator is thought to have driven a similar ocean circulation to that of the PETM in which hotter, saltier water sank to the ocean bottom. Glacial melt, in this case, greatly enhanced an ocean circulation change that was already leading to anoxic ocean states. The result was that ocean layers became even more stratified and less mobile further amplifying anoxia. In the case of the Permian, ocean anoxia eventually enveloped a majority of the worlds oceans, permeating all the way to the surface and eventually invading the atmosphere.
The Emergence of the Canfield Ocean
A stratified, anoxic ocean developed which started increasing mortality among deep water life forms first. As anoxia rose through the deep and mid levels of the ocean, death advanced up the water column as green and purple algae found sunlit regions and proliferated, adding hydrogen sulfide gas as a killing mechanism to ocean acidification and low ocean oxygen levels. Eventually, the hydrogen sulfide reached the surface waters at which point it began bubbling into the atmosphere. The anoxic ocean had fully transitioned to a primordial Canfield Ocean.
Hydrogen sulfide gas is directly toxic to both plants and animals alike and this great out-gassing likely resulted in the massive loss of land species. Ironically, high temperatures (on the order of 9-12 degrees C hotter than now) enhance the lethality of hydrogen sulfide gas. When the gas reaches the stratosphere, it depletes the ozone layer, causing even greater harm to land species. Fossil remains show evidence of genetic damage indicative of a depleted ozone layer and related Canfield Ocean state.
Human Warming is Much, Much Faster
It took about 20,000 years for the Earth to warm 6 degrees Celsius during the PETM. During the Permian, the final extinction and related warming events lasted about 165,000 years. In the case of the PETM, it is thought that volcanism in India stoked global warming until a rapid methane release over a 20,000 year spike period occurred. During the Permian, volcanism is thought to have burned through coal patches over a large region of Siberia, possibly eventually setting off similar very large methane pulses to those suspected to have occurred during the PETM.
In both cases, temperatures rose to between 9 and 12 degrees Celsius hotter than today. But, in the case of human warming, we have the potential to warm the Earth by as much as 7 degrees Celsius by the end of this century and, possibly, to Permian/PETM levels over the next 300 years. Such a rapid pace of warming holds no corollary in either the Permian, the PETM or during any other major warming event visible in the geological record of Earth’s past. So while we may look to the Permian for potential enhanced ocean circulation and anoxia impacts due to glacial melt and increasingly intense ocean stratification, we have no rational means by which to determine how far behind increasing temperatures and glacial melt such events may arise. In the case of the Permian, it took about 165,000 years for a Canfield Ocean to arise. But anoxic ocean states emerged and intensified as warming ramped up. So it is likely that ocean anoxia and stratification will become an increasing problem as the Earth rapidly warms due to human forcing. We can also expect glacial melt to amplify the problems caused by anoxia by increasing stratification and by pushing warm, oxygen-poor waters toward the ocean bottom where they have little opportunity to recharge oxygen stores. Lastly, in the worst case, we can look for Canfield Oceans as a potential tail-end risk for human warming, especially if global temperatures approach 9 to 12 degrees Celsius above the 1880s average and if very large fresh water pulses from glaciers shut down and reverse current ocean circulation.