For those who feel secure that consensus climate science assures us all that it is highly improbable that Earth will reach what Steffen et al. (2018) call 'hot house' conditions (see the first attached image), I offer the following considerations:
1. We are rapidly approaching GMSTA conditions similar to point 'B' (for the Eemian) in the first image; and per the IPCC report on the 1.5C aspirational goal, within a few years we are almost certain to reach this value of GMSTA.
2. In my Reply #626, I provide some logic regarding 'Super Interglacial' periods (during the past 2.8 million years), this illustrate how we might reach GMSTA conditions similar to point 'C' (for Mid-Pliocene) in the first image, which might then trigger an abrupt collapse of the WAIS beginning circa 2040 (per DeConto, Pollard & Alley 2018).
3. The first linked article discusses similarities between probable modern climate forcing pathways and Mid-Miocene conditions (between 15 million and 17 million years ago and point 'D' on the first image, and see the second image), and the associated following extract provides a few reasons why we should not be complacent about the possibility of reaching such conditions, and warns that without appropriate precautions we might be heading towards end-Permian conditions.
Title: " Why the Miocene Matters (and doesn’t) Today"
https://www.skepticalscience.com/print.php?n=2845Extract: "In case we’re lulled into complacency by the relatively benign effects of the MMCO, we should remember:-
• Because greenhouse forcing is a log function of CO2 concentration, the Miocene increase in radiative forcing was much smaller than the increase since the industrial era to today, let alone levels projected by the end of the century. The Miocene increase from 400 to 500 ppm is a radiative forcing increase of about 1.2Wm-2, which is much less than the modern jump from 280ppm to 400ppm today, already a radiative forcing increase of 1.9Wm-2 (it’s really 2.9Wm-2 including other greenhouse gasses) let alone 6.0 Wm-2 for the IPCC’s RCP6.0 scenario (CO2 670 ppm), and 8.5 Wm-2 for the IPCC’s RCP8.5 business-as-usual scenario (CO2 936 ppm) by the year 2100 (figures from IPCC AR5).
• Animals of the Miocene had no cities, no agriculture, no power stations, factories, roads or rail networks. They could migrate and spread as their environment altered on a pace very slow compared to their reproduction rate.
• The Miocene saw reforestation of arid lands which may have mitigated some of the CO2 rise and warming, whereas we have been deforesting the planet for millennia, removing an important carbon sink that was available to the Miocene but not to us.
• Our oceans are currently acidifying much faster than in the Miocene or any time in the last 60 million years – a sign of how much more rapid and how far out of equilibrium with short term carbon sinks our modern climate system is.
• The MMCO global warming took place over many millennia, giving life time to adapt and migrate. Our warming is at least a thousand times faster, a rate that, if left unchecked, could well result in a climate that more resembles the end-Cretaceous or end-Permian disasters, rather than the relatively gentle MMCO.
4. The second linked article contrasts possible modern pathways with the hyperthermals (starting about 56 million years ago, and see the third image), which highlights the risks of potential abrupt emissions of methane (including in our case from the permafrost & thermokarst lakes) and carbon emissions from peat (including in our case from Indonesia and the Congo).
Title: "Hyperthermals: What can they tell us about modern global warming?"
https://www.carbonbrief.org/hyperthermals-what-can-they-tell-us-about-modern-global-warmingExtract: "Around 50m years ago, Earth was struck by a series of short-term phases of rapid global warming, known collectively as “hyperthermals”.
Lasting for a few thousand years at a time, each hyperthermal saw the world’s temperatures rise by as much as 5C. This rise in global temperature is believed to have caused widespread changes to some of the world’s habitats and waves of species extinctions.
These hyperthermals were entirely natural climatic events, driven by large releases of gases, such as CO2 and methane, into the atmosphere. The likely causes of these events are still disputed by scientists, with some suggesting waves of volcanic eruptions, releases from natural methane stores and changes to the world’s soils could have been responsible.
The rate of CO2 release during the hyperthermals was much slower than the rate recorded by scientists today. However, climate scientists hope that getting a better understanding of these past events could tell us more about how human-driven emissions might affect the planet in the coming century.
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A number of scientists believe that the hyperthermal that is most comparable to current climate change is the Paleocene–Eocene Thermal Maximum (PETM), which was the first and largest of all the hyperthermals.
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The PETM is comparable to current climate change because it experienced a relatively high rate of global warming, says Prof Appy Sluijs, a researcher of paleoceanography at Utrecht University in the Netherlands. He tells Carbon Brief:
“I think there are important things to learn from all kinds of time periods but certainly the PETM stands out in terms of rate of warming. Although, carbon input during the PETM was likely still 10 times as slow as in the modern [era], this is still the closest we can get in the geological record. Therefore, the PETM is often considered a nice analogue, although we all realise the analogy isn’t perfect.”
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“What we know for sure is that large masses of carbon came from below the ground. A recent heavily debated paper suggested that it was caused by volcanism in the North Atlantic, which at the time was very active in terms of volcanic activity. Most colleagues think that carbon input came from sources such as methane hydrates below the seafloor or buried terrestrial organic matter, which is buried peat essentially.”
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“You can say: ‘Well, it’s definitely going to be worse in the future, because we’re releasing carbon much more rapidly.’ Because of this, the consequences of ocean acidification in the future are almost certainly going to be worse than they were during the PETM.”
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“One thing to keep in mind is the climate sensitivity that we may be dealing with in this century, only includes processes that are active on a timescale of maybe centuries or less. If you look at the PETM, there the timescale is probably order of millennia. Over thousands of years, you have different feedbacks.
“By looking at the PETM, we can look at a lot of feedbacks that were active at this timescale. These could be the feedbacks that we may need to worry about in the future.”"
5. Finally, I provide the fourth image from the third linked article; which shows the risk that if the equatorial SST increases from about 27C to about 32C the atmosphere for the North Hemisphere could be abruptly transitioned from modern to equable climate (or 'hot house' in the first image) conditions. Such a 5C SST increase in the equatorial oceans, could conceivable occur this century from a combination of: a) ice-climate feedbacks from the collapse of the WAIS & bipolar seesaw interaction with the Arctic & Greenland; and b) a cascade of other tipping points:
Title: "The Effects of Ocean Freshening on Marine and Atmospheric Circulation: Impacts and Solutions"
https://www.sciencebuzz.com/the-effects-of-ocean-freshening-on-marine-and-atmospheric-circulation-impacts-and-solutions/Extract: "As studied by Piana (n.d.), should the SST rise by 1°C, the tropopause’s temperature would increase by around 7.5°C. If the equatorial SST were raised from the average 27°C to 32°C then the tropopause would be heated 37°C above average. A 5°C SST increase, combined with other factors, would hypothetically cause the Hadley cells to increase in height allowing them to reach the poles. This effect terminates Ferrel and Polar cell convection and replaces it with a large Hadley cell (Figure 5). This causes global climates to become more equable than previously existed as temperatures are more evenly distributed across the globe (Piana n.d.)."
Caption for the fourth image: "Figure 5: a. This shows the Hadley, Ferrel, and Polar Cells in the troposphere. For a Hadley cell warm air rises near the equator and falls after cooling at 30° latitude, creating a convection cell. b. This illustrates a single large Hadley cell due to increased SST (Hagerman design)."