updated 9/21/17: typos and eliminated some redundancy to other posts.
Geologic History of the Earth with Respect to Climate
The geological history of the Earth places the Greens in a very unflattering perspective. That history is full of facts that the Greens do not want to hear about. Before going into the details, I would like to point out that climate data from the geologic record is fact; it describes how the actual climate changes. IPCC et al run lots of model simulations to describe climate and justify their positions, buts models aren’t hard data.
I won’t spend much time on the first half-billion years when the Earth formed by accreting solar debris. Until 3.8 billion years ago, the Earth was pretty much a volcanic hell hole with an atmosphere so toxic that a couple of deep breaths would be immediately fatal to a human. However, by 3.8 billion years ago the asteroid bombardment had abated, the crust had solidified into continents, and the Earth had cooled enough to condense out oceans from the steam in the atmosphere. At that time primitive life appeared almost immediately, mostly in the shallow coastal seas, despite the fact that the atmospheric temperature was probably well above 50°C. They were very simple, single celled organisms, Prokaryotes, that were not readily identifiable as either plant or animal.
There were vast amounts of CO2 in the atmosphere, probably around 40% by volume, with no O2. So those single cell organisms adapted to the environment and they began to feed on the atmosphere via photosynthesis. For two billion years those organisms diligently converted CO2 to O2. In so doing, they reduced the greenhouse effect by removing most of the CO2. At the same time, large amounts of water vapor condensed out of the atmosphere as rainfall as temperatures dropped. So the Earth continued to cool. (Caveat: these organisms were cyanobacteria that had a different respiration chemistry from most plants and animals today. That was because there was no free oxygen and they obtained electrons from the large quantities of sulfur in the primordial oceans that came from intense volcanic activity. Cyanobacteria are still around today, but many varieties are actually toxic to our present environment, such as the infamous Red Tide.)
In fact, the increase in O2 caused a problem because it was actually toxic to some of these organisms in large quantities because it was a waste product of photosynthesis. So about 2b years ago they evolved into a more complex form of single-celled life — the Eukaryotes, which had a nucleus protected by a membrane that made them somewhat hardier in the changing environment. The Eukaryotes also formed colonies similar to stromatolites we see today in littoral environments, but they were still individually single-celled organisms. (At the same time there was stress in the environment because the atmospheric pressure changed significantly. Planetary scientists are still trying to figure out what happened, but it clearly stressful enough to drive mutations.)
Though the seas had lots of tiny organisms, the Earth itself was still basically just a big rock floating in space. Because space is very chilly, it radiated heat back out into space (the black body radiation discussed in the previous post). There are models for finding a balance between incoming radiation from the Sun and the outgoing black body radiation. Given the lack of land-based flora and fauna back then, the equilibrium temperature for the surface would be around -18°F. Indeed, that’s pretty much where the Earth ended up 640 million years ago after most of the CO2 and H2O was removed from the atmosphere – an ice ball with the oceans frozen solid to a depth of at least several thousand feet and the continents completely covered by ice sheets thousands of feet thick. The surface temperature was about -13°C.
About 640 million years ago there was a cataclysmic event – probably a major asteroid strike that instantly melted at least an entire hemisphere of ice, putting vast amounts of steam into the atmosphere, along with molten rock, ash, and other particulate matter. The energy of the explosion also decomposed carbonate rocks to produce lots of CO2. That was followed by sympathetic earthquakes and volcanism over a long period of time, perhaps as much as 100m years, as the Earth compensated for the impact. (There are isostatic forces that restore a planet to a round shape after a chunk has been gouged out.)
There is another theory involving plateau basalts. However, those that we know about in the geologic record only raised the temperature about 5°C, which was not enough to cause the ice to melt. Thus there would have to coincidentally be multiple plateau basalts, which is rather unlikely because supercontinents take tens of millions of years to break up, which would spread out the plateau basalts in time and it only takes a few millennia to scrub the CO2 from a previous plateau basalt from the atmosphere, thus losing the cumulative effect.
This changed everything. There is a theory in biology that evolution only takes place when there is stress in the organism’s environment. The idea behind the theory is that if an organism is ideally suited to its environment, any mutation will be less suited and will not survive. Stress essentially creates a new environment for which critters are no longer ideally suited and they need to evolve to adapt to it.
What happened 640 million years ago certainly provides anecdotal evidence of stress-driven evolution. For 2.5 billion years life on Earth had not changed significantly; it remained very simple single-celled organisms eating CO2. Yet in the aftermath of that cataclysm, multi-cellular life evolved, and it evolved spectacularly. By 500 million years ago the Earth’s seas were populated by an amazing variety of complex creatures, comparable in size and variety to those we see today. And they were almost all animals that ate O2 and produced CO2! The single celled organisms were still around doing photosynthesis, but they were now on the bottom of the food chain.
As a result, CO2 levels started to climb, the Earth continued to warm up, and the ice sheets went away. As animal respiration brought back CO2, some single-celled organisms evolved in a different direction and managed to make their way onto land in the form of algae-like organisms that grew in colonies and developed their own more plant-like multi-celled variants. Thus multi-cellular life split into a fairly clear distinction between plants and animals roughly 400m years ago. The biomass was still dominated by the single-celled critters doing photosynthesis, but the complex plants and animals were catching up.
The animals reached a tipping point in the Carboniferous Period, about 350 million years ago. The O2 content of the atmosphere peaked at around 40-50%. (If you lit a kitchen match, it would literally explode and burn your fingers.) The Carboniferous Period was the Age of Insects and there were dragon flies with wingspans of 3 feet and beetles the size of Great Danes. Insects generally have very high metabolisms because they are extremely active relative to their size, so large insects require a lot of O2. (No matter how well leveraged its legs are, physics still requires a lot of energy for that ant in your ant farm to carry a food morsel five times its body weight for several feet.) So the animal respiration outstripped the photosynthesis of the plants and bacteria by a large margin, and the O2 content began to drop while the CO2 content rose faster. As the O2 content dropped, those huge, active insects ceased to be viable and insects reverted to the very small sizes we see today. (They were victims of the square-cube law in physics. External area increases as the square while internal volume increases by the cube of the radius as critters grow. Insects breath through their skin rather than lungs, so they are physically limited in how large they can grow for a given level of atmospheric O2. Animals have lungs with tiny fractal-like membranes whose total area vastly exceeds the total area of their skin.)
That took a while, however, and CO2 from respiration was still winning the battle. That continued on until end of the Permian Period, 250m years ago. All that time, the Earth’s atmospheric temperature and CO2 content were growing. Near the end of the Permian Period the climate was very inhospitable at about 20°C warmer than present and CO2 content was well in excess of 8,000 ppm. Coincidentally, a massive plateau basalt formed in what is now Siberia that dumped on the order of 100b tons of CO2 annually into the atmosphere for nearly a million years. That raised the temperature another 5°C. That warmed the surface layer of the oceans enough so that vast quantities of the methane hydrate deposits on continental shelves sublimated. As I indicated in the previous post, methane is much more effective than CO2 as a greenhouse gas. When that methane was released over as little as a few centuries, the Earth’s temperature shot up another 5-8°C. Finally, the plateau basalt also liberated lots of sulfur dioxide (SO2) and hydrogen sulfide (H2S), resulting in deadly acid rain. When absorbed in the oceans, that made them anaerobic. The result was a mass extinction where about 95% of all species on Earth at that time went extinct (depending on whose estimates you use). Paleontologists all agree that it was, by far, the worst mass extinction in the geologic record.
With almost all of the large CO2 producers at the top of the food chain gone, the Earth quickly cooled. Evolution tried another tack that culminated with the heyday of the dinosaurs in the Jurassic Period, 145m years ago. Coinciding with the surge of the dinosaurs, the Earth began to seriously warm up, in large part due to the dinosaurs themselves. In part that was due to plate tectonics as the huge super-continent, Gondwanaland, began to break up. The new continents drifted away from the equator into the regions 30° and 60° north and south of the equator. Due to the way atmospheric Hadley Cells form, those latitudes tend to be very dry. Today those latitudes contain many of the world’s largest deserts. So large areas of continents simply couldn’t support huge dinosaurs any more. When the Earth is warmer than present, those deserts increase in size and intensity.
However, the dinosaurs created environmental problems for themselves. Huge herbivores must consume tons of fiber every day. So they did not eat grass; instead they ate large shrubs and whole trees. If you have ever flown over the African veldt in a light plane, you have seen elephant tracks. A herd of elephants cuts a swath of complete devastation about 10-15 yards wide. They uproot every shrub, trample the grasses into the dust, and knock over every tree with a bole less than six inches thick. At the right time of year, those tracks turn to caliche and it can take decades before they are grown over. Imagine what a bunch of dinosaurs five times as large would do over 150m years to the planet’s forests. The only vegetation that could survive would be fast growing grasses and shrubs, all with higher respiration. At the same time, the dinosaurs were being forced into the few tropical lands remaining. That increased competition for food and the rate of deforestation accelerated.
However, changing plant respiration wasn’t the dinosaurs only problem. When a dinosaur consumes tons of fiber every day, it produces a lot of methane. (Today, the largest single source of atmospheric methane is domestic livestock, which has increased 3-fold in the last two centuries.) Thus the dinosaurs ate and farted themselves into a climate where they were no longer viable and they were already gone by the time the asteroid hit. Plate tectonics and the dinosaurs themselves conspired in their extinction.
The media loves the story of an asteroid killing off the dinosaurs. If you ask paleontologists who specializes in dinosaurs, though, they will tell you the dinos were long gone. They have two compelling proofs. If the asteroid killed off all the dinos in a few weeks, one would expect that strata to be littered with dino bones. The K-P boundary that marks the asteroid strike is the most heavily investigated piece of geologic real estate in the world. Yet not a single dino fossil has been found on the boundary. In fact, only a few small fragments of individual dinosaur bones have been found in the three meters of sedimentary rocks immediately below the K-P boundary and those fragments were very likely dragged by erosion from the site of a much older dino skeleton. (Sedimentary rocks are greatly compacted during consolidation into rock, so three meters of sedimentary rock can <sometimes> represent many hundreds of thousands of years of depositon.) The second compelling evidence is that the continental sediments beneath the K-P boundary around the world are almost always typical of hot, arid climates that could not support large herbivores. Thus the asteroid, at most, was a coup de gras for a few pockets of small dinosaurs that had managed to survive.
Now let’s fast-forward to 3m years ago. Most of the Earth was a tropical paradise about 6°C warmer than present. The Americas were moving in lockstep towards Asia, but they were not connected by land. The volcanism associated with moving plate edges caused a long string of volcanoes to form islands between the Americas, much like today’s Indonesian archipelago. Those islands eventually closed up and formed the Isthmus of Panama, which disconnected the Atlantic and Pacific oceans about 2.6m years ago. That caused a mass extinction, though not as extreme as the one 65m years ago, at the end of the Cretaceous, but it had a much more profound long-term effect on our climate. The Earth was plunged into the Quarternary Ice Age, which continues to this day. The Earth’s average surface temperature dropped 14°C very rapidly. Thus the ensuing mass extinction was due to cooling rather than warming.
When I talked about the oceans in the previous post, I mentioned the importance of the Gulf Stream. To understand why things changed so drastically 2.6m years ago, we need to look at what would happen to that current if the Isthmus of Panama was open, as it was 3m years ago. Then the equatorial current would travel straight through and continue across the equator until it hit Asia and turned North and/or South. In doing so it would collect a lot more heat, because the Pacific is much wider than the Atlantic at the equator. So when it finally got to the polar regions, it would have much more heat to transfer to the polar atmospheric cells and Antarctica would be quite balmy all year around. In fact, there would be no polar ice caps at all and there fossils of tropical life in Antarctica dating from 3m years ago. But when the Isthmus of Panama broke that current, we immediately plunged into the Quaternary Ice Age because the heat transfer contributed by the Gulf Stream ceased.
The Quaternary Ice Age will probably not end for at least another million years until the American plates move another several hundred miles or so from Europe and Africa. So where are the ice sheets? The reason we have no ice sheets at present is because the present THC and gyres transfer enough heat from the equator to the poles to almost, but not quite, prevent an ice age. The result is that the Quaternary Ice Age is meta-stable; relatively small events can trigger a brief warming period. In fact, as I mentioned in the introduction, we are presently in what is technically known as an interglacial hiatus. Basically that is a relatively brief period of warming, just enough so that most of the ice sheets melt. There have been several of these in the past 2.6m years and they all behave almost exactly the same way.
The hiatus probably starts with a coincidence of two things. One is the precession of the Earth’s tilt. The Earth’s axis of rotation wobbles at geologic times scales, known as precession events. That is due to a complex juxtaposition of planetary bodies and their orbits in our solar system. When the axis of rotation changes so that it is closer to vertical, that changes the angle of incidence of sunlight at the poles. That can have an effect on the formation of sea ice, which, in turn, can affect albedo and downwelling at the poles (as discussed in the previous post). There is a very high correlation between precession and the interglacial hiatuses of the past several hundred thousand years. The change in heat delivered to the Earth via precession angle, though, is rather small and is unlikely to trigger a hiatus by itself. Also, precession events occur every 26,000 years while interglacial hiatuses occur every 100,000-300,000 years. However, it is quite likely that such a change is possible in conjunction with some kind of geologic event that tosses large amounts of greenhouse gases into the atmosphere, such as a moderate asteroid hit, a supervolcano eruption, or a plateau basalt eruption. (Human recorded history has been an extremely quiet time in a geologic sense. The Earth is actually a very dangerous place and nothing we have seen in recorded history is remotely as bad as any of these three triggers.) This combines with the precession effect on the poles to cause up/downwelling and a resurgence of the enhanced Gulf Stream. (During the ice phases, the North Atlantic gyre is not supplemented with up/downwelling flows.)
The temperature warms rapidly for 1-2 thousand years and then the warming becomes very gradual or even plateaus. After 1,000 to 3,000 years the hiatus suddenly terminates and we go back in the deep freeze to an average temperature about 6-7°C colder than present and the ice sheets return. The hiatus is terminated by the catastrophic collapse for the THC, discussed in the previous post.
The diagram shows five interglacial hiatuses in the past 450,000 years. Note that they all reach a temperature about the same as our present temperature, and then terminate quickly. That temperature is a tipping point for global warming that triggers the THC shutdown and a return to the depths of the Quaternary Ice Age.
That is largely confirmed by very recent data. In 2014 it was discovered that the thermal layer between 1,500 and 3,000 meters in the North Atlantic has been warming rapidly for the past three decades. What that indicates is that the Gulf Stream no longer carries enough salt when it reaches Scandinavia to produce a density contrast large enough to sink all the way into the deep ocean. That is, it is only sinking 1,500-3,000m and then dissipating into that density layer. This says that the return path of the THC over the abyssal plain to the upwelling off Africa has already been broken. The reason the Gulf Stream continues to flow, albeit at a rapidly declining rate, is the inertia in the overall system. However, without that return path to the upwelling, the Gulf Stream will likely be completely shut down within, at most, two decades.
The last time we were in the deep freeze, 70,000 years ago, human population dropped from a few tens of millions spread over Africa and Eurasia in the last hiatus to about 5,000 individuals. Now you can understand why I think the Greens are worried about the wrong problem.
However, the evidence for the Gulf Stream termination is more direct than computer models. Our present hiatus actually started 12,000 years ago. As usual, temperatures rose rapidly for 1,000 years. Then, suddenly, the hiatus terminated and we went back in the deep freeze for 1,000 years. Nobody understood why until some geologists in Canada figured it out. When things started to warm up 12,000 years ago, an enormous fresh water lake formed in Northern Canada that was vastly larger than the current Great Lakes system. Unfortunately, it was locked in by ice dams that had not melted yet. Eventually, though, 11,000 years ago the lake overflowed the dams. Once the water started to flow, the ice dams disappeared quickly. Within a matter of months the entire lake emptied into the North Atlantic in a flood of biblical proportions (boulders the size of small houses were moved dozens of miles). The result was an instant dilution of the Gulf Stream and a catastrophic shutdown of the THC. That timing and the hard evidence of the flood in Canada’s sediments is highly compelling.
There are several important points here that the Greens don’t like to talk about. I already mentioned that present temperature and CO2 content are far below the norm for the past 500 million years while multi-cellular life thrived, so it is hard to believe a rise of 2°C from the present would be catastrophic. (It would melt all the polar ice caps and sea levels would rise. But Man abandoning a bunch of port cities is not a mass extinction.)
It is also noteworthy that CO2 levels started at well over 30% of the entire atmosphere 4b years ago, but on balance photosynthesis has reduced them to the present 400 ppm and the levels in the Quaternary Ice Age reached the lowest point in 4.5 billion years. The minimum range of atmospheric CO2 that can support plant photosynthesis is 185-225 ppm. Thus we are now dangerously close to the point where plant photosynthesis is impossible. At that point all macroscopic life on the surface of the Earth, including us, will become extinct. (Microbes can do photosynthesis at significantly lower levels of atmospheric CO2, so they will still be around.) Kind of makes you wonder why the Greens keep making statements about current CO2 levels being higher than they have been in the last 800,000 years, implying that is very scary, doesn’t it? Once again, the Greens are worried about the wrong problem.
However, there is likely to be a number of extinctions if the temperature and ocean acidity rise another couple of degrees. That’s because the oceans are inhabited by creatures that survived 2.6m years of ice age. They are adapted to cooler, more alkaline water. The oceans have been warming and critters like corals are now right at the edge of their habitable temperature and acidity range. Thus there are already significant die-offs in the oceans.
Nonetheless, it is highly unlikely that further warming will result in a technical mass extinction (i.e., at least 30% of all species disappear worldwide). The geologic record tells us that the planet has come out of ice ages to higher temperatures and more CO2 content before without major disruptions. Typically the changes are slow enough that organisms mutate on the fly. Today there are already corals that have adapted to higher temperatures and acidity levels. Things like plankton at the bottom of the food chain can mutate on time scales of months. There will certainly be major die-offs as new adaptations take time to spread, but most species will likely survive in slightly modified form.
The biggest threat will likely be to critters far enough up the food chain to be on Man’s menu. We are already severely stressing the populations of larger fish through over-fishing the oceans. (In the past 50 years, Man has eaten 90% of all large fish in the oceans.) That, combined with environmental stress, could kill off a lot of larger species and it would take many centuries for the oceans to restock with new large species. Nonetheless, a true mass extinction is unlikely and a catastrophic mass extinction, as in the Cretaceous or Permian, is simply not going to happen.
The Greens would have you believe that Man burning hydrocarbons is the cause of all the warming. I will have more to say about that in later posts. For now, though, just let me point out that at present rates we will completely run out of hydrocarbons to burn long before temperatures return to anything close to planetary norms – even if burning hydrocarbons is the sole cause of global warming. The reality is there are geologic and biological forces in operation that dwarf anything man can do with present technology, so whatever is going to happen, is going to happen.
One thing that underscores that point is the assumption the Greens make when blaming Man for all warming. As I cited in the previous post, they assume that all other sources of CO2 in the atmosphere are in balance. Therefore, the present surplus is all Man’s fault because Man just started burning hydrocarbons heavily in the Industrial Revolution. My primary point in going through this geologic history is to demonstrate as strongly as possible that things like the rates of photosynthesis and respiration are far from constant over time at a planetary scale. In fact, they are very rarely exactly in balance. Though my examples have been extreme and at geologic time scales to make that point, the variance still applies in the short term, as my references to fern meadows and deforestation indicated. The reality is that the Earth is a complex and dynamic system. When the Greens deliberately oversimplify that by looking only at atmospheric CO2, they do so at the risk of ending up looking very foolish.
Before leaving this topic, let me summarize the individual mechanisms that terminate an interglacial hiatus – in the rough order they occur…
- The North Atlantic cools. With less heat transfer from the tropics, such cooling is inevitable. Currently all the oceans are cooling, especially the North Atlantic. I describe the effects in the Northern Hemisphere here, but similar effects occur in the Southern Hemisphere.
- There is less evaporation in the North Atlantic. Evaporation of sea water is proportional to temperature and it is non-linear. When the sea surface temperature is below about 50°F, evaporation decreases rapidly until it is close to nil near freezing. This drastically reduces water vapor in the atmosphere since oceanic evaporation is the primary source of atmospheric water vapor. This is the positive feedback mechanism that underlies virtually all rapid climate change.
- The warming causes arctic ice sheets to “melt”. (Sea ice is actually fresh water; the salt is expelled during freezing as a brine under the new ice.) This “melting” is actually in the form of massive icebergs that break off the edges of the ice sheets and float south. They are caught by the Gulf Stream and are carried along with it as they melt. This accelerates the dilution of the Gulf Stream.
- With less heat from the Gulf Stream, winters get tougher in Europe. There is more snow cover for a longer period. That removes moisture from the atmosphere as snow and ice for longer periods. Snow cover for longer periods increases the Earth’s albedo, causing more of the Sun’s short-wave heat to be reflected directly back into space without being converted to long wave radiation at the surface. Growing seasons shorten and animal populations decline, both reducing respiration and, consequently, both CO2 and H2O.
- Without the Gulf Stream heat transfer, polar atmospheric cells expand and the polar vortex expands, extending deeper down into North America and Eurasia for longer periods during winter. That shortens growing seasons to further reduce plant respiration and reduce animal forage. It also traps more moisture in snow for longer periods and further increases the Earth’s albedo.
- Eventually the polar vortex intrudes over the North Pacific and cools it, further reducing evaporation.
- As waters cool in the North Atlantic, the gyre begins to move cooler water to the equator. That compensates for the warming of equatorial waters by the Sun and oceanic cooling extends over a wider area.
- Late in the cycle, temperatures in the North Atlantic go low enough for sea ice to return. That raises the albedo of the Earth and reflects more solar energy directly back into space. Sea ice also caps evaporation from the North Atlantic, further reducing atmospheric moisture.
- The largest forests on the planet are in Northern Canada and Siberia. These migrate southward, which increases photosynthesis to scrub CO2 from the atmosphere. The reason that the terminations of hiatuses take longer is because of the buffering of CO2 in the atmosphere for hundreds of years. To get to the bottom of the Ice Age, the CO2 content must be near 200 ppm.
There is another effect that has nothing directly to do with the THC shutdown, but it compliments the process. As mentioned earlier, when the Earth warms, the equatorial temperature tends to remain constant while temperate and polar latitudes warm. Thus the poles tend to warm faster than any other part of the Earth. This is known as the Fulcrum Effect where the gradient in temperature between Equator and poles pivots up or down from the Equator. One can think of the midpoint of the fulcrum representing the mean surface temperature of the Earth. Thus changing heat transfers between Equator and poles tilts the fulcrum up an down and can change the Earth’s average surface temperature significantly without changing atmospheric CO2 at all. When the Greens address the THC shutdown, they ignore the Fulcrum Effect and only describe it in terms of atmospheric precipitation/evaporation models.
There are two effects associated with this. The jet stream is largely powered by the difference in atmospheric heat between poles and equator. As the poles warm, the jet stream loses power and its path becomes more erratic. That is the primary cause of the increasingly erratic weather we see in the Northern Hemisphere. (A similar high altitude current operates in the Southern Hemisphere.)
The second effect has been observed empirically, but the underlying mechanism is not understood. There is a large high-pressure mass of frigid air over each of the poles. As the poles warm relative to the rest of the planet, that mass expands more than normal during winter. In the Northern Hemisphere, it moves down into central North America (known there as the Omega Block because of its shape) and Siberia. This expansion results in harsher and longer winters with the same effects as described above for the region around the North Atlantic.
There are three important things to note here. First, the effects are initially local to the North Atlantic and they slowly spread out. (There are similar effects around Antarctica, but they are less intense.) This is very important because the Earth’s overall average temperature does not change much during the initial Gulf Stream shutdown, even though the Gulf Stream shutdown is triggering an abrupt climate change.
Second, the climatic effects are driven by reducing water vapor, not increasing CO2. At the present time, the reduction in water vapor is in a standoff with the increase in CO2, so the Earth’s overall temperature has not changed for 15 years despite adding 40% of the total CO2 output of the entire Industrial Revolution in just the past 15 years.
Third, the effects form a sort of positive feedback chain reaction. Any reduction in temperature in the North Atlantic reduces atmospheric moisture more and causes lower temperatures – an effect that spreads into Europe and Siberia like a cancer as other effects come into play. This positive feedback mechanism creates a cascade effect that is critical and it continues for decades after the THC is completely shut down – until a new black body radiation steady state is reached where the Earth’s average temperature is about 6-7°C lower than present. The overall reduction in temperature also accelerates as the effects spread out from the North Atlantic, which is why interglacial hiatuses terminate so rapidly.
Before leaving the geologic history, I need to address one peculiarity about our current interglacial hiatus versus the previous ones. The current hiatus has lasted 10,000 years while the previous hiatuses lasted only 1-3,000 years. I will anticipate a Green argument that this proves we are breaking out of the Quaternary Ice Age for good and warming will continue. In fact, the reason this hiatus has lasted so long is a combination of supervolcanoes and Man feeding himself.
Supervolcanoes are volcanoes that erupt with massive explosions, tossing at least 100 Km**3 of material into the atmosphere. When they erupt they can also emit anywhere from 100b to 2t tons of CO2. There are 26 known supervolcanoes on Earth and they erupt about every 600,000 years, on average. Do the math, and you would expect one of them to erupt about every 20,000 years. However, each supervolcano tends to have a unique period between eruptions, from 400,000 to 800,000 years that depends on local geology. Thus it is possible to have periods of 100,000+ years with no eruptions and short periods with several eruptions as different cycles “bunch up”. During the first 6,000 years of the current hiatus, there were six VEI 7 supervolcano eruptions, each hurling vast amounts of CO2 into the atmosphere. There was also an increase in VEI 6 eruptions (the class just below supervolcanoes). The eruptions were roughly 1,000 years apart, or about the length of time CO2 stays in the atmosphere on average.
Those eruptions started just after the temperature reached the tipping point, roughly 10,000 years ago, where Greenland switched over from net accumulation of snow and ice to net melting of snow and ice. In effect, that increase in CO2 compensated for the reduction in water vapor as the THC shut down. That prevented the temperature drop cascade, so temperature waffled around the tipping point, even though the THC had shut down. How that played out can be seen in the following graph of temperature. (The colored lines represent different temperature estimates and the black line is a sort of Delphic mean of those estimates.)
About 4,000 years ago the temperature had dropped below the tipping point for Greenland and the THC started up again to stabilize the climate. (There was also one more supervolcano eruption in 999 CE.) About 5,000 years ago, though, something else happened. Man switched over from a hunter/gatherer civilization to an agrarian civilization. That didn’t have much affect on climate until about 500 years ago when Man’s population approached 500m. At that point, deforestation and farming became significant in raising CO2 levels. That went into high gear around 1800 when Man’s population reach 1b. In the two centuries since, the population reached 7b and in the last century there have been remarkable strides in breeding and engineering food crops. Thus we have reached the Greenland tipping point once again. Alas, we cannot count on a supervolcano bailing us out again.