Taking the C out of CAGW
The following is not intended to “prove” that AGW does not lead to CAGW (defined as extinction events and/or collapse of civilization), but rather outlines why I don’t currently believe it. I do not consider myself well-informed regarding palaeobiology or palaeoclimate, so I welcome corrections to what I have outlined.
Extinction level events
Argument 1A: A 2-6C rise in temps will trigger an extinction event.
Reply: There have been many step changes of similar magnitude (6-8C Antarctic, 2-3C global?) over the last million years, and again with the melt of Antartica 10 million years ago (~4C polar ocean), and again with the PETM 56 million years ago (6C global). Such abrupt changes are anomalous, but they are not unprecedented. None of the upward step changes are associated with named extinction events. Although the *cooling* in the midMiocene has been described as a disruption with notable extinctions.
Argument 1B: The rate of change over the next several hundred years is too fast for the biosphere to handle.
Reply: The rate of change during the PETM may be the best natural analogy. It might have occured over a period of 20,000 years or so, about 1.5 orders of magnitude faster than the rise due to the release of CO2 from fossil fuels. (about 500 years). Wiki says of the PETM: The PETM is accompanied by a mass extinction of 35-50% of benthic foraminifera (especially in deeper waters) over the course of ~1,000 years – the group suffering more than during the dinosaur-slaying K-T extinction. Contrarily, planktonic foraminifera diversified, and dinoflagellates bloomed. Success was also enjoyed by the mammals, who radiated profusely around this time. … There is no evidence of any increased extinction rate among the terrestrial biota. So while impactful, the PETM event does not support a theory that AGW induce a widespread biotic collapse.
Counterpoint: Bad analogy. 1.5 orders of magnitude is too great a difference.
Argument 1C: The equilibrium temperature post fossil fuels will be too high for the biosphere to handle.
Reply: The middle Miocene Climatic Optimum, ~17 to 15 Ma, is representative of climate under x2 preindustrial CO2 and about 3-6C warmer in mid-latitudes. While I haven’t seen actual NPP studies for the Miocene, most descriptions are ‘warmer and wetter’, suggesting higher NPP. I am intrigued by the apparent lack of arid deserts in the few Miocene papers I have read.
Counterpoint: http://www.pnas.org/content/107/21/9552.full.pdf Sherwood, Huber (2010)
2. Collapse of civilization
Argument 2A: Climate change will severely impact agricultural production leading to collapse.
Reply: Climate models show a strong increase in “Net Primary Production”, much of which is directly attributable to the increase in CO2. Models are much less certain if the changes in “climate” sans CO2 will have a net positive or negative effect, although they lean towards the negative. It does seem likely that there will be shift in C3/C4 plant ratios and distribution. In addition, barring collapse for other reasons, it seems likely that technological innovation in both modified crops and improved farming technologies will occur faster than climate stress will decrease production. Caveat: the net effect of climate change with other environmental degradations and economic stresses may outstrip our innovation speed. Which is not to say that some regions won’t be net losers in the shifting climate. I believe that the US South West and most of Mexico, North West China, the Mediterranian, and South African(?) regions will be counted among the losers.
Fig 5 in Terrestrial plant production and climate change Friend (2010)
Fig 3b from Climate–Carbon Cycle Feedback Analysis: Results from the C4MIP Model Intercomparison Friedlingstein et al (2006)
(models cover wide range of results for T-driven NPP; but all model large increase in net NPP)
Dustbowlifaction Romm (2011)
Argument 2B: Coastal flooding will lead to collapse.
Reply: While sea level rise is inevitable in AGW world, total melting of the Antarctic is unlikely. The Pliocene world at 2-3C warmer, sea levels were about 6 meters higher. Even if we had a rise of tens of meters, this is likley to occur over a period of centuries. Port infrastructure is likely to be replaced several times over that time scale, so it is hard to see how this would lead to collapse of civilization, although the loss of coastal lands is not insignificant.
3. Ocean Acidification
Argument 3a. Ocean acidification will trigger an extinction event
See the following for both the point and the counterpoint …
The most salient paleo-analog to the current atmospheric CO2 increase is the strong ocean
acidification event at the Paleocene-Eocene thermal maximum (PETM) 55 Mya. The PETM is
marked by the sudden and massive carbon input to the ocean/atmosphere system, a shoaling of
the deep ocean’s calcite saturation horizon by at least 2 km in less than 2000 years that did not
recover for tens of thousands of years, global warming of at least 5?C in less than 10,000 years,
and major shifts in marine planktonic communities (Kennett & Stott 1991; Zachos et al. 1993,
2003, 2005). The only major extinctions occurred within the benthic foraminifera, though it is
unclear whether ocean acidification was the main factor or whether changes in ocean circulation
led to anoxia in bottom waters (Zachos et al. 2008).
However, the similarity of the PETM and several comparable, but smaller, Eocene events to
modern conditions is incomplete. First, whether the carbon excursion at the PETM was as rapid
as the present-day excursion remains unclear. Second, the PETM and smaller events occurred
within a background of already high CO2 and global temperature. Third, the Mg:Ca ratio, an
important factor that affects the carbonate mineralogy of many organisms, was also significantly
different from that of today (Stanley & Hardie 2001). Finally, the marine biota during the PETM
were also different. Corals and coral reefs had not yet re-established following the Cretaceous-
Tertiary extinction (Wood 2001); modern coccolithophores are very different from those of the
early Tertiary (Young 1994); and modern thecosomatous pteropod families appeared after the
PETM, in the Eocene and Miocene (Lalli & Gilmer 1989).
Ocean Acidification: The Other CO2 Problem Doney et al (2009)
There are many serious consequences aside from extinction events and collapse of civilizations.
Argument 4A: You are neglecting severe consequences by concentrating on extreme crisis.
Reply: I am not neglecting severe consequences. I am trying to define catastrophic consequences and (informally) assess their likeliehood. I am totally open to the possibility that my assessment could be off and welcome references that could be inform it.
Edits for typos. I am also likely to edit at will to include additional cites or details.