Tom, I appreciate your high school biology knowledge, but the relationship between CO2 and climate is more complex than "CO2 is plant food." **On CO2 as plant food**: While CO2 does help plants grow, this is only true up to a point. Many plants actually grow less efficiently at very high CO2 levels, and the nutritional quality of crops decreases. More importantly, the warming effect of CO2 far outweighs any benefits to plant growth. **On the 280 to 420 ppm increase**: This 50% increase may seem small, but it's actually massive in geological terms. The Earth's climate is very sensitive to CO2 changes. During the last ice age, CO2 levels were only about 180 ppm - a difference of just 100 ppm caused global temperatures to be 5-6°C colder than today. **On historical CO2 levels**: We know pre-industrial CO2 levels from ice cores, which preserve ancient air bubbles. These measurements are extremely accurate and show that current CO2 levels are higher than they've been in at least 800,000 years. The rate of increase is also unprecedented - it's happening 100 times faster than natural changes. **On the "grain of sand" analogy**: This is misleading. A better analogy would be: if you're driving a car and gradually turn the steering wheel, even a small turn can eventually cause a major change in direction. CO2 is like that steering wheel for Earth's climate. The physics of the greenhouse effect is well-understood and has been known since the 1800s. CO2 traps heat in the atmosphere - this is basic physics, not speculation. Sources: IPCC reports, ice core data from Vostok and EPICA, peer-reviewed studies on CO2 effects on plants.