OPTIMUM CO2 FOR PLANTS
CO2 Is Not Pollution. It's a Vital Nutrient For All Life.
Solving the CO2 Debate
The optimum concentration of CO₂ in the atmosphere for maximum plant growth—assuming all other factors like light, water, nutrients, and temperature are ideal—is generally between 1,000 and 1,300 parts per million (ppm) for most plant species1,2,3,4,7.
Photosynthesis and Growth: At this range, plants can photosynthesize at or near their maximum rate, resulting in faster growth and higher yields1,2,3,4,7.
Beyond 1,300–1,500 ppm: Increasing CO₂ above about 1,500 ppm does not further improve growth and can even become toxic to many plants3,7.
Species Variation: Some specific crops, like winter wheat, reach their optimal growth at slightly lower concentrations (around 890–970 ppm), but the general optimum for the biosphere is in the 1,000–1,300 ppm range5.
Current Atmospheric CO₂: The present global average is about 420 ppm, well below the optimum for maximum plant growth2,7.
Note: These optimum levels are based on controlled environments (like greenhouses or growth chambers) and assume no other limiting factors. In natural ecosystems, other constraints (nutrients, water, temperature) may prevent plants from fully realizing the benefits of elevated CO₂1,2,3,4,7.
Effects on the Animal Kingdom
At 1,500 ppm, CO₂ is still well below levels considered immediately toxic to humans and most terrestrial animals. However, studies show that even short-term exposure to CO₂ at or above this level can cause mild physiological effects in humans, such as increased blood pressure, heart rate, headaches, and drowsiness2,4,6. For most land animals, these effects are minor at 1,500 ppm, but chronic exposure could cause subtle stress or behavioral changes6.
Mass Extinctions
{This shows that high levels of CO2 were seldom if ever corellated with mass extinction.}
It is correct that the Cretaceous–Paleogene (K–Pg) mass extinction, which wiped out the non-avian dinosaurs and about 75% of all species 66 million years ago, was triggered by a massive asteroid impact—the Chicxulub event in present-day Mexico. This is the most well-documented example of a meteorite causing a mass extinction, supported by layers of iridium-rich clay, shocked quartz, and the discovery of the Chicxulub crater1,2,5,6.
However, not all mass extinctions in Earth's history have been linked to meteorite impacts. While there is some evidence suggesting possible impact involvement in other events, such as the end-Permian "Great Dying," the scientific consensus for many other extinctions points to causes like massive volcanic eruptions, climate change, ocean anoxia, and rapid shifts in atmospheric composition. For example, the end-Permian extinction is still debated, with some studies suggesting volcanic activity (Siberian Traps) as the primary driver, while others investigate possible impact evidence3,4,7.
In summary:
The K–Pg extinction was indeed caused by a meteorite impact1,2,5,6.
Other mass extinctions have more complex or debated causes, often involving volcanic activity, climate shifts, and changes in ocean chemistry—not just meteorite impacts3,4,7.
The relationship between CO₂, climate, and extinction is multifaceted and not solely determined by impacts or atmospheric composition.
Toxic Levels for the Animal Kingdom
CO₂ becomes immediately toxic to humans at concentrations of 40,000 ppm (4%) or higher. This level is classified as "Immediately Dangerous to Life or Health" (IDLH) by both the CDC and OSHA1,2,3,8.
5,000 ppm (0.5%): Maximum recommended 8-hour workplace exposure; higher levels can cause discomfort over time.
10,000–15,000 ppm (1–1.5%): Possible mild symptoms like drowsiness or mild respiratory stimulation1.
30,000 ppm (3%): Moderate respiratory stimulation, increased heart rate and blood pressure; short-term exposure limit for workplaces1,2.
40,000 ppm (4%): IDLH—immediately dangerous to life and health; risk of asphyxiation and severe symptoms1,2,3,8.
50,000–100,000 ppm (5–10%): Strong respiratory stimulation, dizziness, confusion, headache, loss of consciousness, and risk of death after a few minutes1,2,5.
Toxic Levels for the Plant Kingdom
Optimal CO₂ for plant growth: 1,000–1,200 ppm
Toxicity threshold: ~10,000 ppm
In summary:
CO₂ levels become toxic to most plants at about 10,000 ppm, causing physiological stress, reduced growth, and potentially plant death if exposure is prolonged1,2,6.
All Time CO2 Levels
{This shows that life thrived in the past at high levels of CO2.}
Time Period --- CO₂ Level (ppm) --- Notes
Archean --------- 40,000–400,000+ --- Early volcanic atmosphere, up to ~70% CO₂
Neoprot’zoic --- ~120,000 --- Volcanic outgassing ended global glaciation
Cambrian -------- 3,000–9,000 --- Early animal life flourished; warm, high sea levels
Ordo–Silurian --- 4,000–7,000 --- High CO₂, fluctuating climate
Devonian -------- 2,000–4,000 --- CO₂ drops as land plants expand
Carboniferous --- 150–700 --- Major coal formation, CO₂ at historic low
Mesozoic ------- 2,000–3,000+ --- Dinosaurs, warm climate
Eocene ---------- 1,000–2,000 --- Hot, ice-free poles
Oligocene ------- ~760 --- Antarctic glaciation begins
Pleistocene ------ 180–300 --- Ice ages and interglacials, ice core data
Pre-industrial --- ~280 --- Stable before fossil fuel era
Current (2024) --- ~420 --- Modern anthropogenic increase