I did a double take when I saw the Scientific American headline “CO2 levels for February eclipse prehistoric highs”. I knew we were past 380 ppm, but I didn’t quite think we had pushed our planet to above the Ordovician 5,600 ppm record. But no, we can breathe a sigh of relief; they meant that our current 400 ppm level is only greater than any peak our species has seen in the 200,000 years of our existence. Once I think about it, that’s still stunning.
We’ve all seen it – that iconic graph in “An Inconvenient Truth” (originally from Mann, Bradley, and Hughes’ 1999 paper) that shows temperatures and CO2 spiking dramatically in this past 1000 years. The graph of climate proxy data such as tree rings and glacial ice is so controversial (verdict = the data is correct) that it merits an entire Wikipedia page to itself. I know it’s not a scientific source, but Wikipedia is a good starting place…
Graphs are wonderful. They give weight and substance to numbers, and make data accessible to a much broader audience. However, that easy-on-the-eyes property also makes them prey to misunderstanding. The “hockey stick” is often one of the first ideas that people bring up when I mention that I study climate change and we get into a discussion about it. Some people say that it motivated them with a new degree of panic – “Temperatures are higher than they’ve ever been, right?” Some actually use the graph as an excuse to disregard climate data – “I remember from high school that palm trees once grew in Antarctica and that Al Gore graph shows that temperatures have never been this high, so it must be wrong”. The rest have the same casual “yay, data!” reaction that I do.
Moral of the story – check out the X and Y axis ranges on your graphs, y’all!
I could go on a whole tangent about the controversy and how the open-ended discourse of the scientific process is taken for weakness by non-scientist decision makers. However, I’m going to take a deep breath and dive into the fascinating geologic record of temperature changes instead.
Let’s put it in some context:
Here, the “hockey stick” effect of the first graph becomes so dwarfed by the scale that it doesn’t show at all. However, we know that while CO2 once was almost 14 times greater than today’s level, the average surface temperature of the Earth has never been 14 times higher (57.2°F x 14 = 800°F!). What gives? The CO2’s greenhouse gas effect isn’t the only factor that affects climate – solar irradiance is another huge piece of the puzzle. It turns out that solar activity falls as we look further back through the geologic record. 550 million years ago, the planet received 4% less solar energy. Corrected for that new factor, the graph of climate forcing agents looks like this (from Royer 2006):
OK, but why did the stromatolites of the Precambrian receive less solar energy than we do today? The maturation process of our star has caused solar radiation to increase between 25% and 30% over the 4.6 billion year lifespan of our planet. The Sun grows and becomes brighter as its fusion metabolism gradually eats through the star’s initial supply of hydrogen, creating helium. That sky-high CO2 level in the early part of the Phanerozoic, and the thicker blanket of greenhouse gasses that it implies, was likely the reason that the surface of the Earth wasn’t completely frozen over. (Check out the “faint young sun paradox” for how scientists debate exactly how that works, and what factors beside CO2 could be involved)
We’ve established that CO2 levels were much higher in the Paleozoic, but why did they fall? The answer lies in the leafy greens that we now take for granted. The steady spread of photosynthetic life across the oceans and continents started the carbon cycle as we know it, where plants absorb CO2, produce oxygen, and take the CO2 out of the system temporarily when they’re buried in anoxic conditions. CO2 reached a minimum comparable to the pre-industrial Holocene during the Carboniferous – the aptly named age of plenteous vegetation that got buried, compressed, and turned into the coal that we now burn. The Carboniferous had two things going for it that led to a cold period – fully developed tropical forests and swamps, and the presence of a landmass covering the South Pole that allowed for glaciations. The burial of carbon and riotous growth of plants allowed for proportionally more oxygen in the atmosphere, leading to disconcertingly large insects. (Six-foot-long centipedes, anyone?)
When trying to predict where our gas-guzzling ways might take us, scientists look back to comparable warming events at the Paleocene-Eocene Thermal Maximum (PETM) around 55 million years ago and the end of the Permian, 250 million years ago. In both cases there appears to have been some kind of “runaway” climate change created by positive feedback systems, and in both cases extinctions and anoxic seas resulted. Several different causes are proposed for both, but they have one set of hypotheses in common: that CO2 released from extensive volcanism (Siberian Traps for Permian, Deccan Traps for PETM) bumped up global heat to the point where the methane hydrates on the seafloor release their gases, creating a positive warming feedback.
The end-Permian extinction doesn’t have such a neat cause as the meteorite that ended the Cretaceous 190 million years later. Nonetheless it managed to wipe out 90% of life on earth, including some awesome mammal-like reptiles such as gorgonopsions. Scientists lean towards the “Siberian Traps belch massive quantities of CO2 leading to climate change” hypothesis, but several other sets of evidence for unrelated catastrophes are also in play. The main obstacle is dating – while good dates for the duration of the extinction have been gleaned from the Meishan deposits in China, scientists haven’t yet been able to pin down exact causation relationships for all of these different puzzle pieces. One of the lines of thought for a contributing cause of the extinction goes something like this:
- We have evidence that suggests that the earth warmed by 10 degrees Celsius over the course of 50,000 years. (see Joachimski 2012 source, at bottom)
- We also know that by the end of the Permian the earth no longer had any ice caps: this “flattening” of the temperature gradient between poles may have weakened oceanic and atmospheric circulation.
- We also have evidence that the oceans became anoxic on a huge scale at the end of the Permian, which is caused by a lack of circulation and increased heat, and creates a positive feedback loop of algal overgrowth
- That algal overgrowth created massive quantities of hydrogen sulfide (H2S) as a metabolic byproduct.
- #4 could be backed up by the fact that cold-blooded animals (such as reptiles) are much less effected by the poisonous properties of H2S that warm-blooded animals (such as mammals), and:
- We have fossil evidence that while many reptilian species survived the extinction, only one species of mammal made it through.
- The warming also kicked of the positive feedback loop of melting methane hydrates on the ocean floor, which thickened the greenhouse blanket even further.
In the more recent temperature excursion of the PETM, temperatures rose at around 1° Celsius per 4,000 years, compared to the 1° per 5,000 year rate during the Permian. We’re already warming our current atmosphere by 1° per century – a hundred-fold increase in rate. CO2 also drove the PETM warming; in that case scientists suspect the sources were a surge in volcanism, burned coal seams, and release of methane hydrates, but in a much smaller quantity than our fossil fuel combustion. Besides a similar CO2 environment, there’s another reason why PETM is interesting to researchers trying to predict current climate shifts – the locations of continents 55 million years ago weren’t too different from how they are currently. This adds in another semi-controlled variable – if the continents were in roughly the same places, ocean currents and atmospheric circulation would be similar as well. In contrast, during the Permian temperature excursion/extinction the massive continent of Pangaea blocked ocean circulation between hemispheres completely. That configuration affected the heat and moisture transfer around the globe.
What we do definitively know about the PETM is that something caused a large release of 2000-7000 gigatons of carbon unusually poor in the isotope carbon-13. Somewhere, somehow, a large buried carbon source of biological origin got exhumed and spewed into the atmosphere. We know that it was a biological source, such as coal, methane, or peat, because plants preferentially take carbon-12 out of the atmosphere. Therefore, they end up carbon-13 deficient in comparison with the air around them. (Cui et al. 2011)
Unlike the Permian temperature excursion, the PETM did not cause terrestrial extinctions, although it did manage to do in 35%-50% of benthic foraminifera. That tiny single-celled creature creates calcium carbonate shells and is also commonly used as a temperature indicator.
These little guys are not quite as glamorous as the dinosaurs or synapsids, but still geologically important – their demise raises a few questions. First of all, were the seas made more acidic (which would stop the forams from building shells)? Did ocean stratification and stagnation lead to scarcer food supplies? And why did their planktonic (surface-floating) relatives thrive instead?
We humans don’t tend to think along 50,000 year time spans, let alone millions of years, so all of these effects seem pretty improbable to us. However, these positive feedback loops and extreme periods in the earth’s history might serve to remind us of how inconvenient the planet’s cycles of self-regulation can be. Yes, if we drive up CO2 life will eventually settle into new patterns and thrive, but will we be able to weather the planet in the meantime?
Selected sources and resources:
The article that started my ramblings: http://www.scientificamerican.com/article/co2-levels-for-february-eclipsed-prehistoric-highs/
A clear overview of the geological history of CO2 in the atmosphere: http://www.skepticalscience.com/co2-higher-in-past-intermediate.htm
A brief introduction to the different hypotheses for what could have caused the Permian extinction: http://science.nationalgeographic.com/science/prehistoric-world/permian-extinction/#page=1
Peter Ward, of the University of Washington, explains how extinctions, climate change, and anoxia could be linked at the end of the Permian: https://www.youtube.com/watch?v=HtHlsUDVVy0
Selected publications from MIT’s project on the Siberian Traps: http://siberia.mit.edu/publications-talks-press/articles
A comprehensive paper on the timeline of the Permian Extinction: http://www.pnas.org/content/111/9/3316.short
Joachimski, MM, et al. (2012) Climate warming in the latest Permian and the Permian-Triassic mass extinction. Geology 40:195–198. Abstract/FREE Full Text
Wignall PB, Twitchett RJ (1996) Oceanic anoxia and the end Permian mass extinction. Science 272(5265):1155–1158.
Helpful page with links to PETM resources: http://tugpullpushstop.blogspot.com/2009/07/resources-on-palaeocene-eocene-thermal.html
Excellent basic resource for PETM: http://www.wunderground.com/climate/PETM.asp_1?MR=1
Carbon isotope excursions, as found in the PETM: http://www.science-com.concatenation.org/archive/cie_symposium_2010.html
Cui, Y.; Kump, L.R.; Ridgwell, A.J.; Charles, A.J.; Junium, C.K.; Diefendorf, A.F.; Freeman, K.H.; Urban, N.M.; Harding, I.C. (2011). “Slow release of fossil carbon during the Palaeocene-Eocene thermal maximum”. Nature Geoscience 4: 481–485. doi:10.1038/ngeo1179.