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We have all heard what too much CO2 can lead to, but so far the biosphere have never been exposed to too low levels of CO2. But what if it did? And please, this topic has nothing to do with the ongoing discussion about global warming. We are talking about the last ice age and a hypothetical scenario where the terrestrial flora and fauna would have suffered because of too little CO2.

Apparently the agricultural revolution happened 11,000 year ago all over the world because of increased levels. Pre-industrial levels are said to have been 280 ppm.

Aerobic plants have three types of photosynthesis. C3, C4 and CAM. The original and most common type is C3. C4 and CAM have evolved to deal better with heat, drought and lower levels of CO2. C4 is a little better than CAM in that regard.

Some quotes found on the net:

"Studies have shown that the average biomass production of modern C3 plants is reduced by approximately 50% when grown at low (180–220 ppm) CO2, when other conditions are optimal … (The abortion of all flower buds) suggested that 150 ppm CO2 may be near the threshold for successful completion of the life cycle in some C3 species." "Although some C3 plants like palm trees can cope with a combination of high oxygen levels and warmer, sunnier, and dryer conditions, most C3 plants can lose efficiency in productivity of up to 40% in warm, sunny, and dry conditions." (But less CO2 means more stomata, and the more stomata, the more water loss)

"About 85% of all plant species are C3. All trees, fruits, vegetables, and most food crops are C3. The only C4 food crop exceptions are maise, millet, sorghum, and sugarcane. On average, at 150ppm the primary C3 plant productivity was reduced an average 92% as measured by dry weight biomass. During glacial periods land ecosystems have much lower productivity."

And from this page:

https://environmentcounts.org/evidence-of-biological-feedback-driving-warming-at-ice-age-terminations/

"There is evidence that atmospheric CO2 concentration has never dropped below 190 parts per million (ppm). Analyses of ice cores have revealed that a minimum of 190 ppm was reached at the end of eight ice ages over the past 800,000 years. Although atmospheric CO2 concentration has varied widely, even reaching more than 1,000 parts per million (ppm), there is little evidence for values lower than 190 ppm. Studies have shown that within a single generation of exposure to low CO2, modern plants that rely on photosynthesis show an average reduction in photosynthesis of 50% when grown at low (180–220 ppm) vs current (350–380 ppm) CO2 concentrations. When CO2 is reduced even lower to 150 ppm, research has shown that biomass production may be reduced by over 90%. When applied to the entire Earth ecosystem, these physiological responses imply large reductions in Net Primary Productivity or NPP (the net carbon uptake by plants after accounting for plant respiration) and carbon storage during glacial periods. During the initial glacial period, the high reflectivity of the northern ice sheets reflects most of the solar radiation resulting in cooling. As the oceans and atmosphere cool, more atmospheric CO2 is absorbed by the oceans. Atmospheric CO2 concentrations eventually reach a critical minimum of about 190 ppm, which combined with cool arid conditions, cause a die-back of temperate and boreal forests and grasslands, especially at high latitudes. The ensuing soil erosion generates dust storms, resulting in increased dust deposition on the northern ice sheets and greater absorption of solar radiation. As northern hemisphere solar radiation increases during the next Milankovitch cycle, the dust-laden ice-sheets absorb more solar radiation and undergo rapid melting, which forces the climate into an interglacial period. In support of this mechanism, Antarctic ice cores provide evidence of increasing atmospheric dust at the end of all ice ages over the past 800,000 years."

(Also, plants themselves exhales a lot of CO2 at night, but with fewer plants, less CO2 will return to the atmosphere.)

So, if a combination of several factors had occurred, which had resulted CO2 levels so low that most or even all C3 plants had gone instinct (with the possible exception of cave flora, where C4 plants have adapted to low light and the CO2 levels are sometimes higher than outside the caves, and/or frozen seed and plant matter in permafrost was discovered), but high enough to allow the C4 and possibly CAM plants to survive, more than 90% of all plant species could still disappear. Practically all trees, in addition to most ferns, gymnosperms and moss. There is a risk that peat moss would disappear. Lichen and terrestrial algae would probably be hit hard as well. Which in turn would lead to a mass extinction of animals and fungus.

It's a good thing it never happened, but from a fictional point of view, it could be interesting to imagine what world humans (assuming they survived) would find themselves in once the ice age ended and the production of plant matter returned to previous levels, but with far fewer species.

There are woody C4 plants in the deserts, even if they are usually of modest size. It would take time for them to evolve into trees. C4 and CAM plants are usually not found in cold areas of the world. There would barely be any plant species at all in places like Canada and northern Europe, Russia and China. Would hoofed animals still be around? Some cave dwelling bats would have been able to survive, but bats and birds that lives in trees would be gone.

It would have been an extinction even of a kind never experienced before in the history of earth.

Would a technologically advanced civilization have been possible at all in such a world? One could still mine for coal, and perhaps use other plants than trees for fuel, or build houses and furniture with other materials, but it would have been far more difficult. And with fewer trees, little or no paper, and without paper, not many books that one could use to pass on knowledge from one generation to the next.

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  • $\begingroup$ Yes, we live in a world with little CO2 in the air, well into the area of dimished photosynthetic efficiency for many plant species. It is well known. I also get it that the concentration of CO2 in the air could have decreased a little more during the last glacial maximum. But why didn't it bounce back? With no plants to consume it, geologically produced CO2 would accumulate, wouldn't it? Then plant life would recover etc. $\endgroup$ – AlexP Sep 13 at 6:38
  • $\begingroup$ Isn't this exactly what happened during the Cryogenian? $\endgroup$ – TheDyingOfLight Sep 13 at 12:46
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    $\begingroup$ Your question at the end does not match the title. Are you asking about extinction or tech level? $\endgroup$ – SRM - Reinstate Monica Sep 13 at 13:24
  • $\begingroup$ The CO2 didn't bounce back because colder oceans meant an increased ability to absorb CO2 from the atmosphere. Also, plants themselves release huge amounts of CO2 at night: "Researchers have estimated that plant respiration releases five to 10 times as much carbon dioxide as fossil fuel burning". With less plant activity, less CO2 would return to the atmosphere. Also geologically processes absorbs CO2. In a time with little volcanic activity, it would take time for the gas to accumulate in the atmosphere again. $\endgroup$ – Tim Hansen Sep 13 at 13:26
  • $\begingroup$ The title and the question at the end are connected. Would humans be able to not only survive but also develop an advanced civilization in a world that has just been exposed to a mass extinction due to massive plant deaths cased by too little CO2 in the atmosphere? $\endgroup$ – Tim Hansen Sep 13 at 13:28
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Let's get to the basics of plant physiology first, and then get into the hypothetical question of whether or not mankind could survive or thrive in such an environment.

First of all, yes; plants actually NEED CO2 to survive. No, they don't 'breathe' it as you rightly point out, but their roots are embedded in a medium called soil from which they primarily draw water and nutrients. There are no meaningful carbohydrates in normal plant soil and so a plant without CO2 available to it can starve, not because of the lack of oxygen, but because of the lack of sugars produced by photosynthesis.

So, yes, a lack of CO2 is bad for plants; no question. But, the real question is whether or not CO2 levels can ever get down that far. Let's discuss that next.

CO2 and water are lower energy states for hydrogen, carbon and oxygen to be in than (say) O2 and sugars. That means that all things being equal, this is the more likely state of these elements in a natural environment. This is especially the case with oxygen, which is more likely to bind to exposed iron ores than remain in the atmosphere as molecular oxygen. Also, because we animals don't produce our own oxygen and sugars, we are constantly releasing the highly toxic (to us) CO2 back into the atmosphere, meaning that as long as there are animals alive, there is enough CO2 in the atmosphere to support plant life to at least some degree, meaning a mass extinction because of a lack of CO2 is highly unlikely.

Another way of looking at this is that in order for the plant extinction to occur as a result of this, you would pretty much have had to wipe out all animal life first. Remember too that the reason why plants evolved to do what they do in the first place is that it is assumed that the early Earth's atmosphere contained massive quantities of CO2 and that the Great Oxygenation Event, which is believed to have taken around a billion years to occur, was only possible because plants had so much CO2 to process in the first place. Once they were well underway freeing up all that oxygen, it was possible for animals to evolve to consume that oxygen directly rather than free up their own via a costly endothermic reaction like photosynthesis. All of a sudden the oxygen could be reacted directly with the carbohydrates harvested directly from the plants that are eaten by these animals, those plants also providing essential nutrients, and our oxygen cycle is born.

Put simply, the likelihood of a critical shortage of CO2 at any point in the evolution of life on Earth after the first animals appear is highly unlikely, not to mention that volcanoes alone are likely to provide enough CO2 to the atmosphere at critical points so as to make this scenario unrealistic.

Is it possible? Yes, mathematically at least. but it's so improbable that it doesn't surprise me in the least that it's never happened.

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    $\begingroup$ The amount of CO2 on the atmosphere has been in decline ever since land plants evolved. Sometimes it has increased again, but due to a combination of events, like the creation of Himalayas and lower temperatures, it has been very low lately (geologically speaking). We are talking about 40-50 ppm lower than it was during the last ice age, which is actually not that much. Respiration from terrestrial animals alone would not have been able to prevent this scenario from happening if geological and biological processes continued to strip the atmosphere of CO2. $\endgroup$ – Tim Hansen Sep 13 at 13:26
  • $\begingroup$ Re oxygen binding to iron ores, I think you have that backwards. Most Iron ores are already oxides: the binding to oxygen took place during the Great Oxygenation Event, when newly-produced oxygen combined with dissolved iron in the oceans to produce the banded iron formations: en.wikipedia.org/wiki/Banded_iron_formation $\endgroup$ – jamesqf Sep 13 at 18:02
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Yes, it is possible, and it's one of the scenario for Earth future:

The luminosity of the Sun will steadily increase, resulting in a rise in the solar radiation reaching the Earth. This will result in a higher rate of weathering of silicate minerals, which will cause a decrease in the level of carbon dioxide in the atmosphere. In about 600 million years from now, the level of carbon dioxide will fall below the level needed to sustain C3 carbon fixation photosynthesis used by trees. Some plants use the C4 carbon fixation method, allowing them to persist at carbon dioxide concentrations as low as 10 parts per million. However, the long-term trend is for plant life to die off altogether. The extinction of plants will be the demise of almost all animal life, since plants are the base of the food chain on Earth.

It can be that life will develop new adaptations to overcome this obstacle. However, life as we know it is based on carbon. Further down the road we have the end of carbon cycle:

In about one billion years, the solar luminosity will be 10% higher than at present. This will cause the atmosphere to become a "moist greenhouse", resulting in a runaway evaporation of the oceans. As a likely consequence, plate tectonics will come to an end, and with them the entire carbon cycle.

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  • $\begingroup$ And it almost happened last time, which was just some thousand years ago. What could happen 600 million years from now is depressing to think about, but that's far longer than multicellular life (as far as we know) has existed on land. In fiction we have heard about ecological disasters, like "The Death of Grass" or rising sea levels, but I have never heard about a disaster caused by too low CO2 levels during the last ice age, or the next one that will arrive more or less some thousands years into the future. (Hopefully our descendants will be able to live under domes or something by then) $\endgroup$ – Tim Hansen Sep 13 at 13:32
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Many landplants produce equal or more CO2 than they consume. O2 production is sort of energy waste and more advanced plants produce less net O2 and more CO2. It's a reason why air is more fresh in pine forests.

So all landplants has a very low impact on O2/CO2 ballance. Almoust all O2 produced and CO2 consumed by water-based plants (most of them unicellular) and by water itself. And there are huge amount of CO2 in ocean - more than in atmosphere.

The main reason why during glacial period CO2 levels were so small it the fact that solubility of CO2 greatly rises at low temperatures ("keep it cool" - you know), and ocean "takes" more of this gas from atmosphere.

All it means that your scenario is possible only when Earth losses almoust all of it's (surface) carbon. And if this event somehow happen - it imply dissapearence of any carbon-based life.

In all other cases CO2 level just can't drop below of what it was in glacial period - it a question of global chemical ocean/atmosphere ballance.

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  • $\begingroup$ It is true that marine organisms absorbs huge amounts of oxygen, but don't underestimate the role of terrestrial in that regard either. Coal deposits all over the world is a reminder about their impact. All of the atmospheric CO2 wouldn't need to disappear, just enough of it. And only long enough to kill the C3 plants. Yes, more CO2 is absorbed by the oceans when they are colder, which I mention in the main post. But could the levels have become too low for terrestrial C3 plants to survive? They were low enough to slow down the production to just 50% of what it is today. $\endgroup$ – Tim Hansen Sep 13 at 13:41

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