1. Here it’s important to differentiate legacy carbon—CO2 that’s been released through human activity, including farming and industry—from ambient carbon, naturally occurring CO2 that’s vital for biological processes, notably plant growth. These sources of CO2 coexist (actually, commingle), so it’s a distinction we need to maintain in our minds. Just as there’s an upper limit to the CO2 in the atmosphere that earth’s systems can tolerate, there’s a threshold below which plants and animals cannot survive.
2. But there is only one atom of carbon in each of the molecules. The twenty-five-times-more-potent figure is calculated by comparing the two gases on a kilogram-for-kilogram basis, not a molecule-for-molecule basis. The differing molecular weights mean that there are nearly three times as many molecules of methane—and, therefore, atoms of carbon—in a kilogram as there are molecules of carbon dioxide. The more accurate global warming potential of methane is around eight or nine times that of carbon dioxide, and even this figure depends on some further assumptions.
3. Yeomans, P. A., The City Forest: The Keyline Plan for the Human Environment Revolution, Keyline Publishing Pty., Sydney (1971). Thanks to Abe Collins for alerting me to this.