One of the most common instances of fallacious logic1 used by deniers is stating that, as carbon dioxide comprises only four molecules out of every ten thousand (400 parts per million [ppm]) in the atmosphere, it must only have a minor effect on the Earth. This is an instance of using a misleading statistic1; the fact that carbon dioxide is a minor part of the atmosphere does not prevent it from being vital for the growth of all green plants. If there was no carbon dioxide in the atmosphere, there would be no green plants. It only takes those four molecules out of every ten thousand to green much of this planet. Indeed, if the concentration of carbon dioxide in the atmosphere were to fall below 150 ppm, most photosynthesis would shut down, which would be catastrophic for most complex life on this planet.
The greenhouse effect makes possible life as we know it, and carbon dioxide plays a significant role in that effect. Without the greenhouse effect, Earth’s average surface temperature would be about -18 degrees C, whereas it is about 14 degrees C. It is estimated based on various studies that roughly 50% of the greenhouse effect is due to water vapour, while the rest is almost all due to carbon dioxide. In the IPCC Fifth Assessment Report, the increase in carbon dioxide was estimated to be responsible for 1.82 watts per square metre (W/sq.m.) of the 2.63 W/sq.m. increase in net energy received3.
So, to say that because of its relative rarity in the atmosphere, carbon dioxide has little effect on the climate, is to ignore reality; a common problem among climate change deniers.