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:
"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.