This is the first in a series of posts that will be discussing anthropogenic climate change. But before I begin talking about our current bout of global warming, I think it would be good to start off by looking at past episodes of natural rapid warming in order to better understand what happens when the planet undergoes a sudden transition into a greenhouse state. The most well-known of these events occurred around 55.5 million years ago in the Paleogene period, between the Paleocene and Eocene epochs, called the Paleocene-Eocene Thermal Maximum (or PETM).
There were actually a number of smaller temperature spikes during the Eocene, but none were as dramatic as the PETM.
And it’s quite a strange event at that. The PETM is marked by an absolutely massive drop in the ratio of carbon-13 to carbon-12 isotopes in marine sediments (isotopes, for those unfamiliar, are variations of elements whose nuclei have a different number of neutrons than the “primary” atom; your typical garden-variety carbon has 12 protons and 12 neutrons. Carbon-13 has 12 protons and 13 neutrons, hence the “13”). This is important for a number of reasons. First, most organisms, especially plants, tend to prefer the lighter carbon-12 over the heavier carbon-13. Because the carbon that was released into the atmosphere during the PETM is so depleted in 13C, that implies it was biogenic, since life is very good at discriminating elements by isotope. Additionally, the oceans would have absorbed a large amount of this CO2 , reducing the pH (resulting in ocean acidification as the dissolved CO2 forms carbonic acid) which then also increases the amount of carbonate ions (since a more acidic ocean is better able to dissolve carbonates from rocks and shells).
Even today, it’s not clear what the actual source of all this carbon was. Some of the most popular ideas include pockets of methane trapped in deep sea ice called methane clathrates. While these clathrates are normally stable, when deep-ocean temperatures rise above a critical value, they can very rapidly destabilize and release a huge amount of methane in a short amount of time. Methane is a potent but short-lived greenhouse gas, and is 34 times as powerful as CO2 over a 100-year period. But methane in the upper atmosphere is short-lived due to reactions with free hydroxyl ions which break it down into CO2 and water vapor. Biogenic methane trapped in clathrates would also have a reduced 13C/12C ratio. Other ideas include widespread peat and coal fires, but a lack of soots and other combustion byproducts in sediment samples tends to rule this out. A lack of glaciation in Antarctica could have allowed for the release of large amounts of CO2 from thawing permafrost deposits as they warmed each summer. Whatever was the trigger, the end result was that a tremendous amount of CO2 was injected into the atmosphere and kicked off an unprecedented spurt of planetary warming.
The Paleogene itself emerged from a world which had just recovered from the aftermath of a catastrophic asteroid impact. This infamous extinction event marks the boundary between the Cretaceous and Paleogene periods, and is most popularly known for the death of nearly every species of non-avian dinosaurs. The general climate trend of this time was one of cooling (although the climate was still much warmer than that of today’s). The prevailing warmer and wetter conditions of the preceding Mesozoic era gave way to a cooler and drier phase which continues into the present day. Pangaea had long ago fractured and split apart, and the continents were already taking on familiar shapes and orientations.
Despite this cooling trend, it should be noted that the PETM was happening in the context of an already-warmer climate (estimates place the mean global temperature during the Paleogene at around 18° C; 4° higher than the modern average). During much of the Paleogene, there wasn’t very much ice coverage at either pole. Indeed, evidence suggests that for much of the period the poles may have been entirely ice-free. In fact, the Paleogene opened on a warming trend that peaked in the mid-Eocene during the “Eocene Optimum”, where average temperatures were near where they were at the height of the PETM, before the cooling trend that marked the rest of the Paleocene took dominance. The difference between the PETM and the Eocene Optimum was that it took nearly 5 million years for temperatures to ramp up to their max during the Optimum (and then about another 5 million years to fall back down), while the PETM was a brief, intense spike that lasted at most 200,000 years. A geological eyeblink.
What Causes Changes in Global Temperature Over Geologic Time?
Before continuing on, let’s take a step back and talk about how and why climate changes generally. Earth’s climate, or indeed, the climate of any planet, is a vastly complex system. Therefore, it’s difficult to ascribe changes in climate to a single variable. In fact, most changes in Earth’s climate have been a result of several factors all working together, forming feedback loops that either re-enforce each-other, or sometimes work against each-other. For example, while one of the major leading causes of modern anthropogenic climate change is the release of the greenhouse gas Carbon Dioxide, other factors such as deforestation, agriculture, over-hunting/fishing, and urbanization all affect the climate. Deforestation, in particular, brightens the surface of the Earth by removing dark, light-absorbing vegetation and exposing the brighter surface beneath. This increases the albedo of the planet, reflecting more sunlight back to space and cooling the surface. However, forests help lock away a significant portion of carbon from the atmosphere, playing a vital role in the carbon cycle. While their removal results in a slight cooling effect, it also destroys an important carbon sink and thus also contributes to warming the planet as we increase the amount of CO2 in the atmosphere.
These are only some of the things that can influence climate. It’s by no means an exhaustive list, and I am not a climatologist. Real climate is very messy, chaotic, and sometimes hard to explain. A whole blog could be dedicated to the subject and still not manage to encompass them all to the detail they deserve. What I present here is just a brief, simple overview.
There are several factors, both internal and external to the Earth, that can influence climate. Some of the more obvious ones are giant volcanic eruptions, which can inject huge amounts of sulfur dioxide and volcanic ash into the stratosphere. The SO2 forms aerosols which, in combination with the volcanic ash, block out sunlight and cool the planet. Asteroid impacts, such as the aforementioned end-Cretaceous impact that famously killed most non-avian dinosaurs 66 million years ago, also act similarly to volcanic eruptions as they can inject vast quantities of rock and soot into the atmosphere. The resultant fireball upon impact can also ignite forest fires many thousands of kilometers away from the point of impact, releasing huge quantities of CO2. But for all their violence, volcanoes and asteroids typically only result in rather short-term changes in climate.
Longer-term climate changes often result from roughly cyclical variations in the orientation of the Earth relative to the Sun, the shape of the Earth’s orbit, and even the intensity of the Sun itself. The Earth’s orbit is neither circular nor stable. It’s an ellipse, and it wobbles around, or precesses, with a regular period. This is known as apsidal precession, and when traced out over a full period, results in pretty spirograph patterns. For Earth, the average period of apsidal precession is around 112,000 years. However, the Earth is also tilted on its axis. Currently, Earth’s axis is tilted about 23.44° relative to the plane of its orbit; but this too is subject to precession. Like a gargantuan spinning top, the Earth’s axis takes around 26,000 years to complete one full rotation. That is, if you tracked the movement of the Earth’s axis through time, it would trace out a cone. But that’s not all. The axial tilt itself fluctuates between 22.1° and 24.5° on a 41,000-year cycle. And all of these cycles of precession and wobbling stack on top of each-other, making for a very complicated overall motion. These changes are what are collectively referred to as Milankovitch cycles, and generally speaking, most long-term climate variations map well to them.
Other variables include variation in the intensity of sunlight hitting the top of the atmosphere. Over very long periods of time, changes in average solar luminosity can also cause changes in climate. (This is unrelated to sunspot activity, such as the famous “Maunder Minimum”, a period between 1645 and 1715 where very few sunspots were observed. This happened to coincide with the “Little Ice Age” around 1650 when Europe and North America experienced colder-than-average temperatures. While the relationship between sunspot cycles and climate is poorly understood, it is somewhat contentious in the scientific community as to whether a causal relationship exists between the Maunder minimum and the Little Ice Age.) Even the arrangement of the continents can affect climate, since as continents move they can change ocean circulation patterns which deliver warm and cold waters to shorelines. The surfaces of continents are also generally brighter than the dark oceans, and so more continents near the equator tends to cool the planet by reflecting more sunlight back into space, whereas more open equatorial ocean would warm it.
All of these factor into climate change in some way or another. But the major driver for both the PETM and our modern warming has to do with atmospheric chemistry, specifically the balance of greenhouse gasses and what happens when you suddenly add a boatload of it into the atmosphere in a geological instant.
How does the PETM Relate to Anthropogenic Climate Change?
We’ve seen that the massive injection of CO2 can cause an equally rapid change in climate and global temperatures in both the paleoclimate data from the PETM, and unfolding in real time as we continue to pump gigatons of CO2 into the atmosphere every year. The PETM demonstrates clearly enough the relationship between greenhouses gasses and temperature shifts, as a spike in one lead to a spike in the other. And as the closest analog to our current situation, it may hold key insights into what we might expect to see if we don’t act on climate change right now.
To help put things into perspective, the average carbon loading rate during the PETM (which is thought to have occurred in multiple pulses, with the main pulse lasting around 2,000 years) was around 1 Pg (a petagram, or 1015 grams) per year during the main 2,000-year pulse. In 2014, a revised estimate for the amount of CO2 released annually due to burning fossil fuels and other human activities was measured to be around 40 Pg/year, a staggering 40 times higher than the average flux during the PETM. And while that rate hasn’t been constant over the last 150 years or so (it’s been steadily increasing since the start of the industrial revolution), when compared to the 2,000 years it took for CO2to accumulate during the PETM, it’s enough to make one pause. For comparison, volcanoes “only” emit around 200 million tonnes, or 0.2 Pg, of CO2 annually.
As an aside, anyone that claims that volcanoes put out more CO2 than human activity is either misinformed or outright lying. Besides that fact that the ash and soot thrown up during eruptions create a cooling effect much stronger than any greenhouse warming from volcanic CO2.
Global average temperatures have already risen by a full degree Celsius in the last 100 years, with much of that warming happening in the last 30 years or so. Nearly every month in 2016 has been the hottest month on record. The average CO2 concentration in the atmosphere is officially over 400 ppm, over twice the pre-industrial level, and sea levels continue to rise steadily at a rate of 2.9 mm/year. If the PETM is anything to go by, if we as a global civilization continue to burn fossil fuels and release ever-increasing amounts of CO2 into the atmosphere, we might create the same kind of intense warm period in only a matter of centuries. The PETM shows just how sensitive the climate system is to changes in CO2 concentration. And if much of the warming was in fact due to a release of methane from deep-sea clathrates and polar permafrost deposits, a rapid warming event due to high CO2 levels like the kind we’re continuing to produce could very well trigger such a runaway destabilization in the future. This is especially disturbing since there are signs that this process may have already begun.
A change in global temperature of just 2° by the end of the century would be significant enough. We would be extremely lucky if we could cap warming at that level. But, despite international efforts such as the Paris Agreement, I don’t think that many of the industrialized nations, especially the United States given the recent President-elect’s statements on climate change policy, will do much beyond token efforts at seriously decarbonizing their energy infrastructures. If worst comes to worst, can our civilization adapt to a world that is 8° warmer? Even though such a change would likely play out over centuries, it would be a change that is, as far as we know, unprecedented in the geologic record, happening many times faster than the PETM. What kinds of challenges would that present? It would certainly have impacts on agriculture, trade, water and food security, weather, and countless other facets of life.
This problem isn’t going to go away. The longer we continue to drag our feet, the worse it will be for all of us. We’re already feeling the early signs of climate change. This will be a centuries-long process, but that is not an excuse to hesitate to act. We know what the climate is capable of during natural warming events. What we are doing is far quicker and has the potential to be far more devastating. That, in part, is why we study the past. History, both natural and cultural, is full of lessons that can help steer us towards a better future. We just have to listen.
References and Additional Reading:
McInerney, F. A., & Wing, S. L. (2011). The Paleocene-Eocene Thermal Maximum: A Perturbation of Carbon Cycle, Climate, and Biosphere with Implications for the Future. Annual Review of Earth and Planetary Sciences. Retrieved from http://www.whoi.edu/fileserver.do?id=136084&pt=2&p=148709
Archer, D., & Buffett, B. (2005, March 3). Time-dependent response of the global ocean clathrate reservoir to climatic and anthropogenic forcing. Geochemistry Geophysics Geosystems. Retrieved from http://geosci.uchicago.edu/~archer/reprints/archer.2005.clathrates.pdf
Le Quéré, C., Peters, G. P., & Andres, R. J. (2014). Global carbon budget 2013. Earth System Science Data. Retrieved from http://www.earth-syst-sci-data.net/6/235/2014/essd-6-235-2014.pdf