Trends, Rhythms & Aberrations: The Mechanisms of Climate Change


In a classic Science review article, “Trends, Rhythms, and Aberrations in Global Climate 65 Ma to Present,” James Zachos, Mark Pagani, Lisa Sloan, Ellen Thomas, and Katharina Billups described science's best understanding of Earth's climate at the start of the new millennium in the year 2001. They recounted progress made in defining the evolution of global climate over the Cenozoic Era. Now, ten years into that new millennium it is reasonable to take a look at what, if anything, science has learned since. They start out their discussion with a description of the periodic changes in Earth's orbital parameters, also known as the Milankovitch Cycles:

Through study of sedimentary archives, it has become increasingly apparent that during much of the last 65 million years and beyond, Earth’s climate system has experienced continuous change, drifting from extremes of expansive warmth with ice-free poles, to extremes of cold with massive continental ice-sheets and polar ice caps. Such change is not unexpected, because the primary forces that drive long-term climate, Earth’s orbital geometry and plate tectonics, are also in perpetual motion. Much of the higher frequency change in climate (104 to 105 years) is generated by periodic and quasi-periodic oscillations in Earth’s orbital parameters of eccentricity, obliquity, and precession that affect the distribution and amount of incident solar energy. Whereas eccentricity affects climate by modulating the amplitude of precession and thus influencing the total annual/seasonal solar energy budget, obliquity changes the latitudinal distribution of insolation. Because the orbital parameters vary with distinct tempos that remain stable for tens of millions of years, they provide a steady and, hence, predictable pacing of climate.

No surprises here, knowledge of Earth's orbital cycles and their link to climate change has been built up over two centuries of scientific inquiry. It is a theory that has been in and fallen out of favor a number of times as the uncovering of new data altered science's view of the cycle-climate relationship. For a more complete telling of the tale see Chapter 9, Variations In Earth's Orbit, in The Resilient Earth. There are three major components of Earth's orbit about the Sun that contribute to changes in our climate. These are, in order of longest to shortest cycle, Orbital Eccentricity, Axial Obliquity, and Precession of the Equinoxes. Together, these provide the rhythms mentioned in the review article's title.

Primary orbital components are displayed on the left, and Cenozoic paleogeography on the right.

The rhythms overlay longer term trends that provide change on a scale of millions of years. These trends result in a climatic mean that is constantly drifting and are a response to slow changes in Earth’s major boundary conditions. These boundary conditions are controlled largely by plate tectonics, and thus tend to change gradually. They include the arrangement of the continents, as well as their topography—the location of mountain ranges, prairies and deserts. Also important is the form of things we cannot see: the location of oceanic gateways and the shape of the sea floor both help determine where currents flow and how heat is distributed around the globe.

As the configuration of ocean and continents change slowly over time, so can the natural level of carbon dioxide in the planet's atmosphere. This in turn affects greenhouse warming and the growth of terrestrial vegetation and marine plankton. The authors list some of the major events of the Cenozoic:

Some of the more consequential changes in boundary conditions over the last 65 My include: North Atlantic rift volcanism, opening and widening of the two Antarctic gateways, Tasmanian and Drake Passages; collision of India with Asia and subsequent uplift of the Himalayas and Tibetan Plateau; uplift of Panama and closure of the Central American Seaway; and a sharp decline in pCO2. Each of these tectonically driven events triggered a major shift in the dynamics of the global climate system. Moreover, in altering the primary boundary conditions and/or mean climate state, some or all of these events have altered system sensitivity to orbital forcing, thereby increasing the potential complexity and diversity of the climate spectrum. This would include the potential for unusually rapid or extreme changes in climate.

How these changes unfolded and their impact on Earth's living things is studied by examining the fossils preserved in strata, the rock record. From the strata proxy data are extracted and estimations of conditions are derived: temperature, humidity, the amount of CO2 and oxygen in the atmosphere. Then as now, our view of Earth's ancient past is pieced together from a surprisingly limited set of data samples. Below is a graph showing a summary of such data circa 2001:

Global deep-sea oxygen and carbon isotope records based on data compiled from more than 40 sites.

The figure as described by the authors: “The sedimentary sections from which these data were generated are classified as pelagic (e.g., from depths >1000 m) with lithologies that are predominantly fine-grained, carbonate-rich (>50%) oozes or chalks. Most of the data are derived from analyses of two common and long-lived benthic taxa, Cibicidoides and Nuttallides. To correct for genus-specific isotope vital effects, the δ18O values were adjusted by +0.64 and +0.4‰, respectively. The absolute ages are relative to the standard GPTS. The raw data were smoothed using a five-point running mean, and curve-fitted with a locally weighted mean. With the carbon isotope record, separate curve ﬁts were derived for the Atlantic (blue) and Pacific above the middle Miocene to illustrate the increase in basin-to-basin fractionation that exceeds ~1.0‰ in some intervals. Prior to 15 Ma, interbasin gradients are insignificant or nonexistent. The δ18O temperature scale was computed for an ice-free ocean [~1.2‰ Standard Mean Ocean Water (SMOW )], and thus only applies to the time preceding the onset of large-scale glaciation on Antarctica (~35 Ma). From the early Oligocene to present, much of the variability (~70%) in the δ18O record reflects changes in Antarctica and Northern Hemisphere ice volume. The vertical bars provide a rough qualitative representation of ice volume in each hemisphere relative to the LGM [last glacial maximum], with the dashed bar representing periods of minimal ice coverage (<50%), and the full bar representing close to maximum ice coverage (>50% of present). Some key tectonic and biotic events are listed as well.”

The review examined factors that led to the slow decline in Cenozoic temperatures, a change from hot-house to ice-house conditions. The onset of the late Cenozoic ice ages may have been triggered by the isolation of Antarctica and the formation of the circumpolar current, implying that the proximate cause was tectonic in nature. The formation of continental ice sheets on Antarctica would have accelerated the global reduction in temperature. Some have pointed out that atmospheric CO2 levels also declined steeply during this cooling phase. But a strange thing happened toward the end of the Oligocene.

During the Oligocene there was a prolonged period of glaciation—an ice age—but those conditions did not last. Earth warmed back up during the late Oligocene. Proxy data show that “this termination occurred at a time when greenhouse gas levels were declining or already relatively low.” The implication being that, if declining CO2 was driving the global cooling, the Late Oligocene Warming should not have happened. This is why knowledgeable climate scientists have stated that carbon dioxide does not drive climate. Many times in the past the climate did the opposite of what CO2 levels would imply.

In the short-term, on yearly and decadal time scales, rhythms apply as well. Scientists are finding out more about natural oscillations involving ocean and air: the El Niño Southern Oscillation (ENSO), the Pacific Decadal Oscillation (PDO) and the North Atlantic Oscillation (NAO) to name just a few of these persistent climate patterns. Understanding decadal scale climate patterns, such as the NAO, is necessary for predicting regional effects. A positive NAO index means a relatively strong high-pressure center in the subtropics, leading to warm, wet winters for northern Europe and cold, dry winters for northern Canada. It can be seen by looking at the graph below that the current trend may be about to reverse.

The NAO varies with a 30 year pattern.

Though the trends and rhythms of climate change are thought to be well established, science is still finding out new things about the distant past. It is in studying aberrations that scientists hope to gain understanding of how human activity might impact climate change.


This brings us to the final category: aberrations. Zachos et al. define aberrations as relatively short deviations from the expected trend. Such occurrences become harder and harder to identify the farther in the past they lie. This explains why their discovery was considered recent news in 2001:

Perhaps the most interesting and unexpected discoveries of the last decade are the aberrations. These are loosely defined as brief (~103 to 105 y) anomalies that stand out well above “normal” background variability in terms of rate and/or amplitude, and are usually accompanied by a major perturbation in the global carbon cycle as inferred from carbon isotope data. The three largest occurred at ~55, 34, and 23 Ma, all near or at epoch boundaries. This last distinction is significant in that it implies that each of these climate events may have also had widespread and long-lasting impacts on the biosphere.

At the time, climate scientists were very excited by the uncovering of these sharp, transient events. The oldest was a short period of rapid global warming while the other two were bouts of global cooling. The three aberrations cited are:

  • The Late Paleocene Thermal Maximum (LPTM), nowadays most commonly called the Paleocene–Eocene Thermal Maximum (PETM). This event, which occurred at 55 mya near the Paleocene/Eocene (P/E) boundary, is characterized by a rapid 5° to 6°C rise in ocean temperatures.

  • Oi-1 lies just above the Eocene/Oligocene boundary (34.0 mya). It was a 400,000 year long glacial period that started with the sudden appearance of large continental ice sheets on Antarctica.

  • The Mi-1 event coincided with the Oligocene/Miocene boundary (~23 mya) and consisted of a brief but deep (~200 ky) glacial maximum. This event was followed by a series of intermittent but smaller glaciations.

Although records indicate a number of lesser events in the Oligocene and Miocene, none appear to approach Oi-1 and Mi-1 events in terms of magnitude. The PETM has been of particular interest to those who believe humans are capable of causing a sudden increase in global temperatures by the release of greenhouse gases and land use changes (ie. cutting down forests). Recent studies confirm that the PETM may have caused the dispersal and subsequent radiation of Northern Hemisphere land plants and mammals. In fact, contrary to warnings about the catastrophic impact of global warming on species diversity, it now appears that the rapid Paleocene global warming caused a diversity explosion.

How could the PETM aberration been caused? Certainly there were no SUVs or coal-fired power plants spewing CO2 into the atmosphere so humans are off the hook. The authors speculate that the slow process of continental drift altered the configuration of Earth's surface enough to cause a shift in ocean circulation. This caused a rapid warming of deep ocean waters that, in turn, destabilized seafloor clathrate (methane ice) deposits. The sudden release of thousands of gigatons of CH4 into the environment caused a sudden, but temporary spike in temperatures. Of course, this is but one theory, though many still think this a likely scenario today. The sudden, disproportionate response illustrates the non-linearity of the climate system while the rapid return to “normal” conditions shows its underlying stability.

While our view of climate past is still fuzzy at best, more accurate measurements have been obtained during the past decade, revealing events and variations we previously did not suspect. An example of a new discovery is that of the previously unsuspected Middle Eocene Climatic Optimum (MECO). This half million year long period of significant warming was discovered in 2003 and verified using data from a new core taken off the coast of Tasmania (see “CO2 & Temperature During The Middle Eocene Climatic Optimum”).

Note that the 4°C MECO bump thought to have taken place 41.5 million years ago is missing from the timeline from the paper above, but shown on the simplified timeline below. That such a major departure from the previously accepted climate history has surfaced gives pause—what other aberrations await discovery? How could something as significant as the MECO have been missed? The fact is paleoclimatologists have constructed a seemingly complete climate history for our planet based on shockingly sparse data.

The Cenozoic Era geologic time scale with aberrations.

As stated, it is the mechanisms behind the aberrations that make climate so unpredictable. It has often been repeated that Earth's climate system is very complex and highly non-linear, meaning that small changes to part of the system can cause large fluctuations elsewhere. In one sense this would seem to bolster claims by climate change alarmists that man is capable of catastrophically altering the climate by small changes to the trace gas CO2.

This is not borne out by the known major aberrant events, however. They show that the Earth system is quite tolerant to large swings in atmospheric CO2 concentrations. Furthermore, even when perturbed by sizable injections of greenhouse gases, be it CO2 or the more potent CH4, the climate recovers fairly rapidly (at least in geological terms). History also indicates that equatorial temperatures do not rise significantly when the globe warms, rather the majority of the warming is seen at higher latitudes and the poles. This makes sense—life has gotten on marvelously with much higher temperatures than today and no ice sheets at the poles.

Still, those who back the IPCC's predictions of future climate mayhem continue to warn against CO2 emissions. They hunt for feedback mechanisms that could turn rising carbon dioxide levels into a huge swing in global temperature, though no sufficiently powerful mechanisms have been found to date. New linkages and factors are being discovered all the time, however, and our view of Earth's climate system is clearer today than ten years ago.

New Discoveries

What new things have science discovered since 2001, since the knowledge base that was used to concoct that last IPCC report (AR4)? As it turns out, quite a number of surprising discoveries have been made. Here are a few that have been reported on the Resilient Earth website:

That is only a partial list of the research highlights since I started writing this blog: and people ask why I am a global warming skeptic! As I have said before, science never rests and any consensus idea is guarantied to be proven wrong over time. Here is how Zachos et al. summed up their review of climate science in 2001:

In sum, it now appears that extreme aberrations in global climate can arise through a number of mechanisms. This would explain both the random distribution and frequency of such events over time. Some, such as rare anomalies in Earth’s orbit, are predictable, at least to the extent that the orbital computations are correct. Others, like catastrophic methane release, are less so, although closer scrutiny of earlier time intervals when boundary conditions were similar to those preceding the LPTM might reveal the existence of similar aberrations. Correlation does not necessarily prove causation, but in the case of aberrations, the short time scales involved significantly reduces the number of potential variables, thereby rendering the task of identifying and testing mechanisms a more tractable proposition.

In simple terms, climate science needs to know much more about Earth's climate system before it can claim to truly understand how climate works. The IPCC just knew that CO2 was the cause of global warming, they knew that the ocean would falter, forests would be destroyed and fauna decimated. Now we know that none of this has happened. Now we know that the IPCC theory is wrong. To steal a line from Men In Black, “just imagine what we will know tomorrow.” We have learned a lot in 10 years, but there is much, much more to be discovered. For now, the Sun and orbital variation trumps CO2 levels—as they always have.

Be safe, enjoy the interglacial and stay skeptical.

Right On

But my deterministic, numeric model says, but my deterministic, numeric model says, but my deterministic, numeric model says... Hum must be a broken record. Perhaps it is ignorance of the other earth sciences. Dennis Nikols, P. Geol.

T's R's and A's

I am a constant visitor.
Your analysis and writing are absolutely excellent.
Ihave always been a skeptic and wondered why more emphasis was not put on the motions of the earth and plate tectonics etc.,
Thanks for an easy to read and understand article
Al Shelton, Edmonton

Plate tectonics

Probably because tectonic changes take place on geological time scales and the global warming flap is focused on human scale, short term changes. There is no doubt that the shifting continents are a major cause of long term climate change. The northward movement of South America, Australia and India during the Cenozoic are credited with the general cooling trend that has been going on for the past 50 million years or so. See my longer comment here. But continents move slowly and there is nothing humans can do about their meandering. Much easier for activists to blame CO2 and demand sweeping socio-economic change they desire anyway.

Well done!

What a concise review of the mechanisms behind climate change! Plate tectonics, orbital mechanics and the odd nonlinear response. Throw in a variable sun and the occasional asteroid collision and there you have it. Great stuff as always.