The DOE Report on Plant Life During Low CO2

One of the frequently repeated tropes in climate contrarian circles is the supposed "line of death" at 150 ppm below which, we're told, all photosynthesis stops, so all plant life dies and there's a mass extinction of life on Earth. CO2 Coalition loves to trot out this trope to claim that, since CO2 became as low as 170 ppm during the glacial cycles of the Quaternary, we were dangerously close to a mass extinction. They claim we're still "CO2 starved." They also tell us that Berner's 2001 CO2 models shows "an alarming downward trend toward CO2 starvation."  We're then apparently supposed to be thankful for fossil fuel emissions that have saved us from perhaps the extinction of all life on earth.

The DOE report, authored by the "climate working group" (CWG), tones down this rhetoric a bit, but it's essentially the same claim with slightly different numbers: "At the end of the last glaciation CO2 levels had fallen to about 180 ppm. As discussed in Chapter 2, C3 plants begin dying at CO2 levels below about 140 ppm and C4 plants at levels below 100 ppm, so if CO2 levels had continued falling plant life would have been imperiled" (p. 13). On the next page, CWG made sure we could see these two supposed lines of peril for plant life.

DOE Report Graph (p. 14)

On p. 4, the DOE report attributes this bit of alarmism to Gerhart and Ward 2010 (GHW10).[1] Here's the text of CWG's claims, apparently developed exclusively from G10:

In response to low-CO2 conditions some plants evolved another photosynthetic pathway called C4, in which CO2 is concentrated in the vicinity of Rubisco, allowing for the C3 process to function more efficiently. For agricultural purposes the plant categories are:

• C3: rice, wheat, soybeans and most other crops
• C4: maize (corn), sugar cane, millet, sorghum

Had atmospheric CO2 levels continued declining, plant growth would have declined and eventually ceased. Below 180 ppm, the growth rates of many C3 species are reduced 40-60 percent relative to 350ppm (Gerhart and Ward 2010) and growth has stopped altogether under experimental conditions of 60—140 ppm CO2. Some C4 plants are still able to grow at levels even as low as 10 ppm, albeit very slowly (Gerhart and Ward 2010).

Given my experience to date with this report, I think it might be valuable to check up on the CWG to make sure they got this right. In fact, Gerhart and Ward do not seem to share the alarmism of the CWG. GW10 indicate that even though modern plants become severely stressed by low CO2, we lack the evidence of plant extinctions when CO2 was low.

Modern plants grown at low [CO2] (150–200 ppm) exhibit highly compromised survival (Ward & Kelly, 2004) and reproduction (Dippery et al., 1995) at conditions that occurred only 18,000–20,000 yr ago. Such findings beg the question of how glacial plants survived during low [CO2] periods, especially considering the lack of evidence for plant extinctions during these times. Furthermore, past work has demonstrated that low [CO2] has the potential to act as a strong selective agent on plants, and therefore evolutionary responses may have ameliorated some of the negative effects of low [CO2] in the past (Ward et al., 2000).

GW10 also understand that there is documented evidence that even C3 plants can adapt to low CO2 levels. 

It is clear, however, that modern C3 plants are products of an ancestry that had undergone at least 2 million yr of glacial–interglacial cycles, with corresponding changes in climate and [CO2] (Ward & Kelly, 2004). Thus, glacial plants must have had the genetic capacity to adapt to changing [CO2].

In other words, according to the single study cited in the DOE report, even though modern plants experience "highly compromised survival" rates at 150-200 ppm CO2, we don't see evidence of C3 plant extinctions during glacial maxima when CO2 was 170-180 ppm. C3 plants adapt to lower CO2 conditions given the benefit of generations of natural selection. In fact, Ward et al 2000[2] conducted experiments over multiple generations that showed when plants were selected at low CO2, they survived better than those selected at high CO2, suggesting that C3 plants are able to adapt to low CO2 conditions:

Selection at low CO2 resulted in an average 35% increase in biomass production, due to an increase in the length of the life cycle that resulted in a longer period for biomass accumulation before senescence. From the Arabidopsis model system we conclude that some C3 annuals may have produced greater biomass in response to low pCO2 during the Pleistocene relative to what has been predicted from studies exposing a single generation of C3 plants to low pCO2.

While single generation studies show stunted growth and reproduction that is reduced or even prevented, after many generations natural selection allows for C3 plants to become more tolerant of low CO2 conditions. GW10 does not support the notion of a line of peril below 140 ppm. If I had to guess, I'd think CWG just read in GW10 that CO2 "possibly dropped as low as 150 ppm during the late Pliocene," subtracted 10 ppm, and made that the point below which C3 plants become imperiled.

Perhaps more importantly, GW10 is not optimistic about the ability of plants to continue to benefit from increasing CO2. They write,

Furthermore, most studies focusing on the full range of plant responses to past through future [CO2] report much greater physiological and growth enhancements in response to increases in [CO2] below modern concentrations than to increases above modern concentrations (Sage & Reid, 1992; Polley et al., 1993b; Dippery et al., 1995; Ward & Strain, 1997). Thus, plants may have already exhausted much of their potential to respond to rising [CO2], unless, for example, major evolutionary changes occur in the future. From these findings, it is clear that assessing the full continuum of plant response to changes in atmospheric [CO2] through geologic time is essential for making accurate predictions regarding the functioning of both past and future ecosystems.

This clearly undermines the CWG claim on p. 104 of the DOE report that "There is reason to conclude that on balance climate change has been and will continue to be neutral or beneficial for most U.S. agriculture." That conclusion involves a non sequitur, and GW10 suggests that we cannot assume that the benefits have experienced from CO2 fertilization will continue in the future. So predictably, GW10 is not discussed in that context.

But things get worse for the DOE report, because paleoclimate studies of the Carboniferous and early Permian indicate that CO2 levels likely have been reduced to levels even lower than 140 ppm. At the end of the Devonian, trees began to proliferate throughout the continents. These were strange looking trees, some exceeding 160 feet tall. These forests grew tall, but they lacked deep root systems, so they fell over fairly easily. These trees took great quantities of carbon out of the air and pumped oxygen into it. Over ~100 million years, carbon was sequestered in fossil beds from which we now are our sources of fossil fuels. With the long-term sequestration of CO2, Oxygen levels also rose. The excess oxygen allowed for insects to get large—dragonflies with a wing span of 2.5 feet lived at this time. The reduction of CO2 levels cooled the planet to what was probably the coldest temperatures of the entire Phanerozoic.

By the beginning of the Permian, CO2 levels decreased to 100 ± 80 ppm. Orbital cycles operates then as they do now, so there were almost certainly swings in CO2 above and below 100 ppm (the proxy evidence had sufficient resolution to show orbital variations between 308 and 301 million years ago, but it was not sufficient for this between 298 and 297 million years ago). Glaciation likely reached its maximum during the early Permian.[5][6][7][8] At this time there were no C4 plants (the first evidence for the C4 pathway comes from about 35 million years ago). While we can't say by how much these C3 plants were stressed, they obviously did not go extinct. Certainly adaptations occurred to allow plants to survive low CO2 levels, and photosynthesis did not stop. We know this because C3 plant life continues to exist today.

This is clear evidence that the Earth spent about 1 million years at least periodically below the so-called C3 plant "line of peril" (140 ppm CO2), and the CWG has again not done their homework. To be clear, I'm not suggesting that 100 ppm CO2 would be a good thing, but the suggestion that 140 ppm CO2 would cause C3 plant life "imperiled" is not supported by evidence. Instead, this appears to be an alarmist rhetorical tactic to feed into the contrarian notion that increasing CO2 is a net benefit for humanity. 

Reference:

[1] Gerhart, L. M., and Ward, J. K. (2010). Plant responses to low [CO₂] of the past. New Phytologist, 188,674–695. https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-8137.2010.03441.x

[2] Ward, J., Antonovics, J., Thomas, R. et al. Is atmospheric CO2 a selective agent on model C3 annuals?. Oecologia 123, 330–341 (2000). https://doi.org/10.1007/s004420051019

[3] Ward, Joy K., and John K. Kelly. "Scaling up evolutionary responses to elevated CO2: lessons from Arabidopsis." Ecology letters 7.5 (2004): 427-440. https://doi.org/10.1111/j.1461-0248.2004.00589.x

[4] Dippery, J. K., Tissue, D. T., Thomas, R. B., & Strain, B. R. (1995). Effects of low and elevated CO2 on C3 and C4 annuals. Oecologia, 101(1), 13–20. doi:10.1007/bf00328894.
https://link.springer.com/article/10.1007/BF00328894

[5] Georg Feulner. Coal formation and global glaciation. Proceedings of the National Academy of Sciences Oct 2017, 114 (43) 11333-11337; DOI: 10.1073/pnas.1712062114. https://www.pnas.org/content/114/43/11333

[6] Montañez et al 2016 finds that the low CO2 values correlate with a global eustatic fall and the apex of glaciation during the time period. Montañez, I., McElwain, J., Poulsen, C. et al. Climate, pCO2 and terrestrial carbon cycle linkages during late Palaeozoic glacial–interglacial cycles. Nature Geosci 9, 824–828 (2016). https://doi.org/10.1038/ngeo2822

[7] The estimates in the 2016 study are revised from his earlier estimates, which can be found in Montañez et al 2007. Montanez, I. P., Tabor, N. J., Niemeier, D., DiMichele, W. A., Frank, T. D., Fielding, C. R., … Rygel, M. C. (2007). CO2-Forced Climate and Vegetation Instability During Late Paleozoic Deglaciation. Science, 315(5808), 87–91. doi:10.1126/science.1134207

[8] Breeker wrote a computer program that allowed for more accurate estimates of atmospheric CO2 levels from the proxy data that Montañez used in his 2007 paper. Breecker, D. O. (2013), Quantifying and understanding the uncertainty of atmospheric CO2 concentrations determined from calcic paleosols, Geochem. Geophys. Geosyst., 14, 3210–3220, doi:10.1002/ggge.20189.