Answer: Mass starvation
Through photosynthesis, plants convert CO2 and water into the food that supports all life on earth. The benefits to plants from increased atmospheric CO2 include: increased growth, more flowers and fruit, better water-use efficiency, and better resistance to environmental stressors such as plant pathogens, macro and micronutrient excesses or deficiencies, high soil salinity, high/low air temperature, and high/low air temperature.
Increasing CO2 content in the air by 300 ppm will raise the productivity of most herbaceous plants by about one third. The results of thousands of studies have been catalogued.
Over the past 33 years, our planet has grown visibly greener as seen from space, from the tropics to the Arctic, because of greater productivity of croplands, grasslands, and forests.
Responses to CO2 deprivation are even more pronounced than the effects of CO2 enhancement. On average, a 50% reduction in CO2 cuts photosynthesis by 38%, water-use efficiency by 48%, and total plant dry biomass by 47%.  During Ice Ages, plants were likely at their lower carbon limit, around 90 ppm.  Below that, they die.
Would lowering the CO2 level decrease the average global temperature? That is not precisely known, but probably not by much. CO2 is a minor greenhouse gas. However, it is certain that lower temperature will lower atmospheric CO2 because of indisputable principles of thermodynamics. CO2 is more soluble in the ocean at colder temperatures, so more of the CO2 in the atmosphere/ocean system will be in the liquid phase and less in the gas phase at colder temperatures. During the great Ice Ages, CO2 levels plunged, after the temperature did, as shown by Antarctic ice cores. CO2 is a lagging indicator of temperature, not a leading one as some try to claim.
The graph [3,4] shows the economic benefit to agriculture from the fertilizing effect of CO2 from 1961-2011 ($3.2 trillion), and from the projected increase from 2012-2050 ($9.8 trillion), in constant 2004-2006 U.S. dollars. If we stop the increase, we would forgo approximately $9.8 trillion in economic benefits, and the earth could support far fewer people and animals.
- CO2 is a requirement for life, not a pollutant.
- Carbon chemistry (organic chemistry) is the chemistry of life. Without carbon, there is no life.
- While the magnitude of the effect of increased atmospheric CO2 on climate is disputed, and may be negligible, the effect on life is indisputable and enormously beneficial.
- A CO2-enriched earth is a greener earth.
- CO2 restrictions decrease food production and are thereby harmful to plants, animals, and humans.
- Temmee AA, et al. Meta-analysis reveals profound responses of plant traits to glacial CO2 levels. Ecology and Evolution 2013;3:4525-4535.
- Gerhart LM, et al. Glacial trees from the La Brea tar pits show physiological constraints of low CO2. New Phytologist 2012;194:63-69.
- Idso C. Expected biospheric impacts of rising atmospheric CO2. 35th annual meeting, Doctors for Disaster Preparedness, Aug 12, 2017.
- Idso C. The positive externalities of carbon dioxide. Center for the Study of Carbon Dioxide and Global Change; Oct 21, 2013. Available at: http://www.co2science.org/education/reports/co2benefits/MonetaryBenefitsofRisingCO2onGlobalFoodProduction.pdf.
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