La figura muestra una comparación entre el posible aumento de temperaturas derivada del actual proceso de calentamiento global (en rojo) con aquel registrado durante el Máximo Térmico del Paleoceno Tardío (más conocido como PETM, su acrónimo inglés), hace unos 55 millones de años (línea azul). Eje vertical, aumento de las temperaturas; eje horizontal, escala temporal. Nótese la tremenda diferencia entre la temporalidad inferida para ambos procesos.
Bajo el título “Something Wicked This Way Comes”, el sitio web Dosbat (http://dosbat.blogspot.com.ar/2012/12/something-wicked-this-way-comes.html) publica un post sumamente interesante esta semana: compara el actual proceso de cambio climático con el Máximo Térmico del Paleoceno Tardío (PETM, acrónimo en inglés). Lo copio en forma sustancial (eliminé párrafos demasiado técnicos, incluyendo las citas bibliográficas al final. No tiene desperdicio. Tengan en cuenta estas abreviaturas: Petagramo / Gigatonelada (Gt): un billón de kilos; CO2, dióxido de Carbono; CDIAC, acrónimo del sitio web Carbon Dioxide Information Analysis Center (http://cdiac.ornl.gov); ppm, partes por millón; k, mil (por ejemplo, 40k= 40.000).
“Probably the best analogue for the current Anthropogenic Global Warming (AGW) is the Paleocene Eocene Thermal Maximum (PETM). How good an analogue is it, and are we really at risk of re-running it? I think we are going to re-run the PETM, not in the sense that we are likely to achieve the same absolute temperatures, although that may be possible, but in the sense that a temperature increase of at least 3ºC is achievable from our fossil fuel emissions alone and this will be amplified by methane and carbon dioxide emissions from the Arctic region causing a d13C drop as seen in the PETM. Talk of lowering to 350pm or keeping temperature below 2ºC is idle fantasy.
"The Paleocene, before the PETM, was warmer than now, it's been hard to pin down by how much. Robert Rhode has produced this useful graphic, which suggests temperatures were of the order of 4ºC higher than preindustrial, however the PETM itself seems to have involved a temperature increase of about 6ºC, the temperature rise in that graphic is much less. The PETM was a minor extinction event, there were no large losses of species on land, and the ocean extinction was restricted to bottom dwellers in the deeps of the oceans in certain regions, probably due to anoxia, which was likely due to changes in ocean overturning circulation. However with regards land species, during the PETM, mammals radiated profusely which indicates environmental stress, changes in the environment driving evolution to suit new niches and niches made available by the decline or movement of other species.
"At the time of the PETM the Arctic was temperate, with temperatures not falling below zero, even during the winter. This presents the equable climates problem, in that models are not able to reproduce conditions; they produce tropics that are too hot, or poles that are too cold. Abbot & Tzipperman propose a cloud radiative feedback, with carbon dioxide levels and ocean/atmospheric heat flux keeping a blanket of cloud over the Arctic throughout the winter. The cloud backradiates infra-red radiation to the surface, keeping the region warm. In their 2007 paper they find that atmospheric heat transport of 140W/m^2 plus doubled CO2 could keep the Arctic ice free during the winter, for comparison, current atmospheric heat transport is of the order of 100W/m^2 at 80ºN. Given what appears to be the unfolding of a rapid transition of the Arctic to a seasonally sea ice free state, finding of possible bifurcations (tipping points) in the transition to a perennially sea ice free state (Eisenman 2011), and the increasing evidence that methane hydrates in the Arctic are thawing, plus the near certainty that they will more actively do so in the decades to come: The prospect of substantial emissions of methane from the Arctic is a very real prospect. So the question posed is can we avoid a re-run of the PETM?
"The following graphic is taken from this article. It's worth noting that it states 25 petagrams are being emitted each year, that's 25 gigatons (Gt the unit I'll use in this post). However from the CDIAC data used in the following paragraph, actual emissions from fossil fuels in 2008 were 8.75 Gt carbon, or 32.1 Gt CO2. This shows visually what Skeptical Science have noted, based on Cui et al, 2011, that current emissions dwarf the PETM's rate of emission.
"Sexton et al examine warming events, hyperthemals, during the Eocene. The PETM was a hyperthermal but was substantially different from the Eocene hyperthermals that they examined; the recovery from the PETM took over 100k years, whereas the other Eocene hyperthemals were up to around 40k years in duration. This suggests that whereas the PETM involved an external source of carbon, such as methane hydrates, these shorter events did not involve such a source. Sexton et al propose that the hyperthermals involved movement of carbon between the atmosphere/land/ocean reservoirs, hence whereas an external perturbation would need to be removed by geological timescale processes such as weathering, the carbon in the hyperthermals could move back from atmosphere to ocean/land, a much quicker process. Crucially this process resulted in substantial changes in the CCD, with dissolution intensity being found to be greatest in the South Atlantic. Sexton et al propose that changes in meridional overturning were probably responsible.
"I think we are seeing the start of a process that, in terms of human timescales, will continue indefinitely. The millennia to come will see vast stores of frozen carbon in the Arctic melting and being release as carbon dioxide and methane.
"But will what we face be as bad as the PETM? We're starting from lower baseline temperatures, and we probably haven't enough fossil fuels to get atmospheric CO2 up to near 2000ppm, so on the face of it, it seems it won't be as bad. The oceans are certainly colder, for example in the tropics at Demerara ridge temperatures were around 13ºC at 3000m depth, equivalent temperatures are today around 5ºC. However contrary to this, I think a re-run of the PETM in terms of species loss, and equivalent human impacts is the best scenario we can hope for. The reality will probably be much worse.
"In opening this post I mentioned that the PETM was a minor extinction event, around 40% of benthic (deep ocean floor) foraminifera were lost. It's worth comparing that with the current situation, since the 1950s about 40% of phytoplankton have been lost from the non-coastal regions of almost all the oceans (Boyce et al, 2010), as all readers of this blog will know, phytoplankton are a major base of the ocean food chain. Then we have the effect of overfishing.
"Then on land we're already seeing Hansen's Climate Dice come into play, I've blogged on it here, some 10% of the globe are now covered by 3 standard deviation warming events. We're seeing mid latitude effects from Arctic amplification that may be involved in floods, persistent weather patterns, and extreme winters. Tamino has done an excellent job of cataloguing weather disaster increases, wildfires, heatwaves, that's not to mention other bloggers and scientists doing sterling work cataloguing what's going on. These changes are happening after only around 0.8ºC of AGW, with the last decade seeing surface warming abate due to the effects of the El Niño, sulphate pollution, and low solar output, e.g. Foster & Rahmstorf 2011 & Kaufman et al 2011. What will happen by the middle of this century? How much acceleration of warming, and further global weirding will the coming decades bring?
"As the graphic of this post shows, the pace of change we are forcing is far greater than in the PETM. With the impacts above intensifying, human agriculture covering 40% of land, and increasingly impinging on what wilderness that is still out there, we will probably see a greater mass extinction then during the PETM. We're giving natural systems neither the space nor time to adapt. Furthermore the super-interglacial warming we are prompting is driving the planet from a glacial/interglacial ecosystem, evolved over the last several million years, to a much warmer state, whereas the PETM was a warm event from an already warm baseline.
"I still remain unconvinced by the claims of some that within decades we'll see a catastrophic methane blow out, it's un-necessary to reiterate my reasoning which is provided here, and here. The issue is not the amount of free gaseous methane that is available for a Hollywood disaster movie style blow out, but the amount of fossil clathrates stored during the Quarternary, this has the potential as it melts in the super-interglacial warmth of the anthropocene to massively amplify our emissions. And as Sexton et al find, if we warm the oceans enough they may add to the effective emissions as the overturning circulation reduces, causing abyssal anoxia and reducing sequestration of carbon into the abyssal deeps. But the PETM shows that this is a slow process when measured in terms of a human lifetime. Both the clathrate containing sediments and the oceans have large thermal mass and take a long time to respond to warming.
"The current events in the Arctic region presage the awakening of a slow, cumbrous, but inexorable beast that will be with us for longer than human history, gradually adding to the damage we have done, warming the climate well beyond 3 degrees. The idea of stopping at 350ppm, or 2 degrees, of staying in safe levels of AGW will be totally out of our hands within a matter of decades. Indeed in view of what is practically achievable, given the time needed to shift from fossil fuels, time to develop and roll out alternatives, the problem that as the rich move off fossil fuels the price will drop, and the poor will take up the slack in consumption: It is already too late in terms of the "art of the possible". The notion of reducing to 350ppm, or limiting to 2 degrees is as dead as the broader aim of avoiding dangerous climate change.
"How much further we drive the process depends on how much of the available fossil fuels we burn. But we are already at the stage where, barring a miracle, we are committed to dangerous climate change. As I fully expect us to burn all the fossil fuels economically and technically available, I expect that the best outcome is a re-run of the PETM.
"The good news? We haven't anywhere near enough fossil fuels to re run the End Permian extinction event. That we cannot do so due to circumstance rather than due to intelligent choice is not something humanity can feel justly proud of."
Bibliografía (en el sitio web de este artículo están los links a las publicaciones originales):
- Abbot & Tziperman, 2007, "Sea ice, high-latitude convection, and equable climates."
- Archer, 2006, "Destabilisation of Methane Hydrates: A Risk Assessment."
- Boyce et al, 2010, "Global phytoplankton decline over the past century."
- Cui et al, 2011, "Slow release of fossil carbon during the Palaeocene–Eocene Thermal Maximum"
- Eisenman, 2012, "Factors controlling the bifurcation structure of sea ice retreat".
- Foster & Rahmstorf, 2011, "Global temperature evolution 1979–2010"
- Gu et al, 2011, "Abundant Early Palaeogene marine gas hydrates despite warm deep-ocean temperatures."
- Higgins & Schrag, 2006, "Beyond methane: Towards a theory for the Palaeocene–Eocene Thermal Maximum"
- Kaufman et al, 2011, "Reconciling anthropogenic climate change with observed temperature 1998–2008"
- Lawrence et al, 2008, "Accelerated Arctic land warming and permafrost degradation during rapid sea ice loss."
- Sexton et al, 2011, "Eocene global warming events driven by ventilation of oceanic dissolved organic carbon."