The Story of Climate Change (Part 1)
"In the beginning God created the heaven and the earth. ... And the earth was without form, and void;" - Genesis 1
To have his path made clear for him is the aspiration of every human being in our beclouded and tempestuous existence. - Joseph Conrad
Paleoclimate History
Back in deep time, about 4.6 billion years ago, our planet and the entire solar system were first assembled from of a rotating cloud of gravitationally collapsing gas and dust left here by the explosive death of the previous stellar occupant in this region of space. It is difficult to imagine a time so long ago, when our newborn sun shone first light by fusing hydrogen to helium in its core - but here we are, illuminated - this ragtag group of planets, asteroids, and comets that make up our solar family. From the beginning the climate has always changed. Our freshly consolidated rocky snowball we now call Earth, was forged along with its first atmosphere. Our primordial atmosphere was far from hospitable though. It was composed mostly of hydrogen with a little helium and a few other trace gases probably reflecting the composition of the original accretion disk from which the solar system emerged. The Earth was large enough as rocky planets go with a significant gravity - but not sufficient to hold these light atmospheric gases to the surface for long. Our first atmosphere was doomed from the start. These gases would soon achieve escape velocity and dissipate back into space. Our new sun’s fierce solar wind, significantly more intense than today, would accelerate this process and blast the atmosphere from our planet’s craggy frozen firmament. You see, the Earth had no protective magnetic field back then. Future prospects for our planet were growing dim.
Another hundred million years would pass with final planetary consolidation. The Earth suffered further bombardment from asteroid and comet collisions but the cataclysm that formed the moon about 4.5 billion years ago melted the planet. The resulting magmic mass cooked and bubbled allowing gravity to sequester a molten iron sphere at the very core of our planet. The iron dynamo then found its own rotational frequencies thus enveloping the Earth in a protective magnetic field that even now deflects the solar wind around our planet like the bow of a ship parting the waters.
As our planet cooled and outgassed, liquid water condensed to fill the low places. The new atmosphere was born from within. As the molten planet disgorged water and long-locked gases from the very rocks themselves, the abundant nitrogen, hydrogen, high levels of methane, carbon dioxide and ammonia were now protected from the erosive effects of the incessant solar wind by our new core magnetism. Things were beginning to improve.
There is geological evidence that the Earth had experienced intermittent glaciation after the moon was formed and before about 2.5 billion years ago. There is no geological evidence however, that the Earth froze in a “Snowball Earth” scenario during that time. The early Archean climate appears to have been quite variable. This might seem at odds with the theoretical physics of our faint infant sun. Stellar evolution theory proposes that our young sun was about 30 percent less luminous than it is today. A changing solar composition driven by the nuclear fusion process is thought to be the cause. The faint sun however, as the theory goes, had higher mass losses back then leading to a more intense solar wind than Sol does now. With a variable climate and a generally warmer world, less solar luminosity combined with possible solar wind effects, and the abundant warming from the global greenhouse, our Earth seems to have solved the dim sun paradox. About 3.7 billion years ago, liquid water oceans and a warm-ish climate nurtured the first microbial life on our planet. Our luck was holding.
Some studies indicate that the ancient climate was probably quite variable. Oxygen isotopes in cherts suggest that between 3.2 and 3.5 billion years ago the Archaean climate might have been very hot. Periodic glaciations are evident from rocks discovered in South Africa dated to around 3.5 billion years ago. The presence of glacial till and related glacial detritus in the Pongola Supergroup and the Witwatersrand Basin of South Africa suggests that by 2.9 billion years ago the climate was glacial. Glacial rocks have also been found in India and Montana dated to around 2.7 billion years ago. The Late Archaean may have been relatively warm but glaciation reappeared in the early Paleoproterozoic. Our climate has always been changing. The geological record of stable isotope proxies reveals a highly variable climate pattern indeed, oscillating between warming and cooling with the regularity of a climatological heartbeat.
As usual, things were continuing to change. Around 2.5 billion years ago the ocean dwelling ancestors of cyanobacteria made an evolutionary leap forward by harnessing the sun’s energy in a photosynthetic enzyme process that enabled the extraction of carbon from its dioxide bonds providing sustenance for the bacteria while releasing molecular oxygen into the environment as a waste product. This process was so efficient that the cyanobacteria flourished and filled the seas much as their relatives, the blue-green algae do today.
About 2.3 billion years ago the Earth experienced its first biocidal war. Opening shots resounded with “The Great Oxygenation Event”. During this period massive quantities of oxygen were excreted into the water and air by the flourishing cyanobacteria changing the atmospheric composition forever. To the original anaerobic organisms however, the new oxygen atmosphere was toxic. To survive, the anaerobes had to hide or die. The ancestors of the early anaerobic creatures are still with us today, secluded away from the poisonous air in dark places like deep sea hydrothermal vents where they eke out a living from salts of iron or noxious sulfur gases.
It is thought that the contribution of greenhouse gases to the early warmer environment had already begun to wane even from the beginning. Volcanism was a sporadic process for CO2 production while chemical weathering of the ancient surface rocks steadily removed carbon dioxide from the air. The CO2 dissolved in the rain produced a flood of mineral carbonates ending up settling quietly to the bottom of the seas. The time of the Great Oxygenation Event also saw another carbon sink come to the fore. The cyanobacteria themselves through their metabolic processes reduced the level of atmospheric CO2 even further. Methane, a very powerful greenhouse gas, was also doomed to quickly oxidize in the new air to form carbon dioxide, a much less potent greenhouse gas.
We really do not know that much about the climate variability during the first billion years of the Earth’s existence. As the sun had grown more luminous, while greenhouse gases played a decreasing role in the climate, and as the atmosphere had been totally transformed by microscopic creatures, around 2.4 to 2.1 billion years ago our planet experienced an expansive glacial period, the Huronian glaciation, where several Ice Ages and a possible “Snowball Earth” episode occurred, freezing the planet from pole to pole. The geological record reveals many climate changes since then. The rocks show us Hothouse climates with life flourishing under globally warmer temperatures. We also see many Icehouse climates with polar ice caps, periodic glaciation during the Ice Ages, and globally cooler temperatures. Over and over again the climate has warmed and then cooled. What could cause the climate to oscillate like this over hundreds of millions of years?
About 100 million years ago (Mya), coming out of the peak heat of the hothouse Cretaceous Period - when dinosaurs ruled the planet - what had caused the long-term global cooling resulting in the initial glaciation of Antarctica 65 million years ago? About 50 million years ago the Earth entered the Icehouse where we currently reside. What caused the steady decline in global temperatures after the highs of the Paleogene culminating in the further extensive glaciation of Antarctica 34 million years ago with CO2 nearly double our current value? After the peak temperatures of the Neogene, 15 Mya, why has our planet continued to cool resulting in our present Ice Age?
About three million years ago our first hominid ancestors emerged on the savannas of Africa. The human lineage was there at the beginning of the current Ice Age, the Pleistocene. Glacial advance and retreat froze and scoured the polar regions and higher latitudes while the progenitors of humanity, nestled in the cradle of Africa, survived their own evolutionary ups and downs. Homo sapiens finally broke out of Africa around 70 thousand years ago expanding their territory out of the mother continent. Around 15 thousand years ago most of the higher latitude glacial ice that had accumulated over the previous 80-thousand years finally receded resulting in this current warm interglacial period providing new opportunities for human migration.
Is climate change good or bad for humans or other species? The changes that come will place some species under stress while others might take advantage of new environmental niches. Even the powerful King Canute, the Viking king who ruled over England, Denmark, Norway, and Sweden in the early 11th century could not hold back the tides nor can we stop the tides of future climate changes. Climate change over short time-scales and small temperature variations has many possible causes but the root explanation must answer some basic questions. Why does the global climate grow warmer or cooler in the long term – over geologic time - say a few million to hundreds of millions to billions of years? Why do we experience Icehouse climates with accompanying Ice Ages and glaciers? Why do we experience extreme Hothouse climates and why does the climate vary the way that it does over long-time scales?
Assembly of the Geological Timeline
Before we attempt to answer the why of global climate change, we must first try to visualize how the paleoclimate changed over deep geologic time. Since it is not possible to go back in time to see what early climates were like, we can use geochemical imprints or proxies created during past climates to interpret paleoclimates. Fossilized organisms such as diatoms and coral can serve as useful climate proxies. Other proxies might include ice cores, tree rings, sediment cores, and stable isotopes within mineral sediments and fossils. Paleoclimate variability might now be revealed by assembling many paleoclimate proxy indicators to build a picture of the ancient Earth’s changing climate landscape.
Naturally occurring atomic isotopes of oxygen and carbon in sediments continuously document the long-term history of shallow ocean temperatures. Oxygen and carbon isotope masses of O16/O18 and C12/C13 are different enough that these isotopes within their respective molecular compounds can be effectively separated by natural processes. Natural fractionation can separate inorganic carbonates by the process of evaporation where minerals are precipitated in equilibrium with sea water. The difference between the isotopic ratio of the carbonate and that of the seawater is strictly a function of temperature. If the temperature dependence of that difference has been calibrated, if the isotopic ratio of the seawater can be estimated, and if the isotope ratio of the carbonate has not been chemically altered since formation, the temperature of carbonate deposition can be calculated. It is this calculated temperature that is referred to as an isotopic temperature. The main driver of the evaporation effect in most geological intervals is the amount of water that has been removed from the oceans and is sequestered in glacial and polar ice.
Living organisms prefer the lighter carbon-12 and oxygen-16 isotopes when forming their shells and other structures in a fractionation process driven by biochemical kinetics. This enzyme catalyzed process takes advantage of the lower energy requirement and faster reaction rate for converting lighter isotopes in ocean carbonates into shells and bodies. Kinetic fractionation depends on the speed of biochemical reactions. These reactions and fractionations proceed even faster if the temperature is higher. Living structures formed in the ocean tend to be enriched in the lighter oxygen-16 during warmer periods and relatively depleted during colder periods. An increase in primary biome productivity during warmer climates can also cause a corresponding rise of inorganic carbon-13 values in the sediments as more of the lighter carbon-12 is locked up in plants and animals. In time, the dead bodies and shells of ancient creatures along with the precipitated inorganic carbonates gently settle to the bottom of the seas with their proxy thermometers safely buried by the sands of time.
In Table 1, we can see the mechanism of formation for inorganic carbon and organically derived oxygen isotope fractionation in fossils along with the temperature associated with isotope excursions. These excursions are graphically represented as isotope ratio divergences - delta oxygen-18 (δ18O) and delta carbon-13 (δ13C) calculated against a reference standard. Combining these two stable isotope responses, we can estimate generalized “Hot” to “Cold” temperature responses in shallow seas on a comprehensive geological time scale.
Graphic Data and the Splice
Before we can even start to fully visualize paleoclimate change, these proxy data must be assembled from many individual sedimentary records. The sediment cores are extracted from many different parts of the world and from different time strata. These sediments contain the minerals, organic matter, fossils and shells laid down over millions of years. Chemical and physical techniques are then applied to derive sediment age, isotope composition and most importantly, chemical suitability. Geological remineralization processes can influence the reliability of the isotope ratios in the sediments so unmodified carbonates are essential. With these constraints in mind: axis baselines are determined, age alignments and correlations are completed, and proxy temperature intensities estimated to produce a plausible sequence of variability in ancient climates.
With the previously published graphical results of Veizer and Halverson derived from oxygen and carbon isotope excursions found in fossil shells and inorganic carbonates we can see the estimated climate responses as ocean temperature changes on separate graphic plots. Here, we want to display both oxygen and carbon isotope temperature estimates and project a paleoclimate splice of these curves onto a common axis with a common response symbology resulting in a 930-million-year proxy climate record (Fig. 4).
In 1999, Canadian geochemist Jan Veizer assembled and published over 500 million years of oxygen isotope data derived from fossilized calcitic and phosphatic shells (Fig. 2a). These brachiopods, conodonts and belemnites lived in our early oceans. Millions of years of organic shells collected on the floors of ancient seas produced a Phanerozoic sedimentary record of stable oxygen isotope ratio variability related to the temperature of those seas.
In 2005, geologist, Galen Halverson and associates published carbon isotope data derived from nearly 400 million years of inorganic carbonate sediments dating back in Earth’s climate history to over 900 million years ago (Fig. 3a). These Neoproterozoic sedimentary isotope records are also proxies for temperatures of shallow inland seas.
To graphically splice these two timelines, we will use the Veizer time axis as the standard 500-million-year graphical reference. Veizer and Halverson peaks, valleys, and transitions are graphically determined and transferred to a 930-million-year timeline splice (fig. 4).
“Geology is an interpretation-based science” (Frodeman, 1995)
Isotope ratio value axes of the two graphics sets are aligned for congruent response orientation. “Hot” climates will be projected toward the top of the page with red triangles and “Cold” climates will be projected toward the bottom of the page with blue triangles. The Veizer data will be projected from a median climate line set to 0 part per thousand δ18O. For Halverson, the median climate line will be set to the δ18O median climate line. I have visually estimated the conversion intensities and median of the Halverson carbon isotope data to the Veizer organic oxygen isotope variations. The Tonian peaks of 780-720 Mya represent a hothouse period and will be normalized to the “Hot” Cretaceous from 140-65 Mya. The Tonian climate from 925-800 Mya is projected with the Triassic-Jurassic climate of 250-125 Mya as a model. The intensity conversion and median location of the “Hot” excursions during the Cryogenian and Ediacaran (725-550 Mya) do not exceed the δ18O median climate line as represented by the “Rosetta stone” region in Veizer’s Phanerozoic plot from 50 to 0 Mya: from the peak of the Paleogene 50 Mya, we see the cooling trend descend below the median climate line then warm again to the peak of the Neogene still well below the median. Finally, we continue to cool into our current climate, falling even further below the median. This phanerozoic plot is the δ18O equivalent of the Neoproterozoic δ13C values from 675-725 Mya. Vertically adjusting the Halverson splice segments, we can now project the Cryogenian/Ediacaran “Hot” excursions down into the cold Icehouse climate zone that defines this place in time.
The Splice
The result of this graphical splice and projection is a smoothed proxy representation of ocean temperatures over more than 900 million years. The finer detail in the Veizer and Halverson reconstructions have been ignored with attention paid to the larger climate responses in hopes of revealing the big picture. Of course, we must assess the overall relevance to global climate conditions as the geologic record indicates specific climate changes in specific shallow inland seas at specific geographical locations.
Replay
From “The Splice” (Fig. 4), starting at 930 million years ago and moving toward the present, the pattern of climate responses shows a unique sequence of warming and cooling until we reach about 200 million years ago. At that point the climate pattern begins a repeat of the previous response sequence. The warmer period from 900-825 Mya is replayed as the Jurassic climate of 200-150 Mya. The Cretaceous climate of 140-65 Mya is the hot replay of the hot Tonian of 800-720 Mya. Our present climate is the cold repeat of the cold Cryogenian from 725-675 Mya. The full period of this unique cycle of climate change is about 675 million years as represented by the vertical gray line in “The Splice” reconstruction.
Anomaly
Halverson’s plot depicting the 675 Mya location (Fig. 3a, Namibia) indicates a cold climate trough but no glaciation and with no accompanying large negative δ13C excursion. In our current space/time location (0 Mya) however we are in the middle of the Pleistocene Ice Age. If it is true that the global climate is cyclic and that we are repeating the same global climate conditions as 675 million years ago, this place in Cryogenian time should indicate an Ice age - the Namibia plot does not. The Marin and Gaskiers glacial periods have been dated to around 640 and 580 Mya respectively but some uncertainty exists in the timing and number of glacial periods during Sturtian time (720 – 660 Mya). There were likely multiple phases within the Sturtian glacial period with a single glaciation (Kundelungu, Grand Conglomerat, Zambia) thought to be as old as 770 to 735 Mya (Key, 2001). A south China glaciation was dated to 715 Mya (Liu, 2019). Another glacial period between 685 and 665 Mya was dated from the Windermere supergroup and Pocatello formations in Idaho (Lund, 2003, Fanning, 2004). We must keep in mind that during the Cryogenian Period the supercontinent Rodinia was in the process of slowly breaking apart. After the first Ice age of the Sturtian glacial period around 710 Mya, the landmass containing N. Namibia may have moved to a more hospitable geographical location but then migrated to a higher latitude 60 million years later where the Marinoan glacial period occurred around 640 Mya.
Paleoclimate change is not linear, it is circular.
Cyclization
The continuous periodicity of paleoclimate change can now be illustrated. From “The Splice”, we will snip out the time period of 675 Mya to the present - keep the cold pointing in and hot pointing out - now close a loop with the ends. “0 Mya” and “675 Mya” are now coincident. With the cyclization of paleoclimate change (Fig. 5) we can now project reconstructed paleoclimate hot peaks, cold troughs, and transitions onto the circular timeline (Fig. 6).
The full period of unique climate responses is now fixed in position. This model can provide estimates for past and future climates. As time progresses, we move into future climates and representations of future climates in a clockwise motion on the model timeline. Counter clockwise movement along the timeline moves us into representations of the past climates. “0 Mya” represents the present. Clockwise movement is the direction of the actual flow of time. The time for a full climate change period is 675 million years. This cyclized proxy climate model displays one complete climate change period. Our future global climate will be a repeat of the past.
“Many possible explanations have been advanced for one or more of the abrupt excursions in the δ13C of marine carbonates and δ18O of organic matter that have been documented in the geologic record. Most of these hypotheses have focused on factors that change the global rate of burial of organic carbon, which is isotopically light: changes in upwelling and primary productivity; fluctuations of sea level; changes in ocean dynamics, including ones affecting the extent of anoxic conditions; changes in carbonate weathering rates; release of methane from sediments; changes in nutrient input from the land to the oceans; volcanic degassing; and release of isotopically light CO2 from the deep sea. Given the strong positive correlation between δ13C and δ18O excursions in shallow marine sediments, however, parsimony suggests that one or more unifying explanations should be sought to explain all of these phenomena.”
― Steven M. Stanley, 2010
The continuously periodic nature of global climate change will guide our search for climate variability loops driving the changes.
end part 1