Andrew Sheldon's June 28 letter, "Warming up to third-grade math," was an interesting but not enlightening read. As a physicist, I am skeptical of initial claims but am always willing to accept them after getting sufficient proof. The proof of global warming lies in our knowledge of the amount of greenhouse gases emitted into the atmosphere, our understanding of the effects these gases have, and our observations of global temperatures over hundreds of years.
Sheldon is right that the climate is always changing, but the difference this time is that it is being changed rapidly by human intervention. On a geological time scale, such a rapid change in our atmosphere has never been seen. What the final effects of these changes will be are uncertain, but most climate scientists think widespread disruption of weather systems will ensue — resulting in disruption to agriculture on a global scale.
Sheldon's "proof" (that there is no crisis) seems to neglect all the other things that go along with his representative human with 7 hectares about him and a hundred miles of air above him. What about his home, car, the power plants that supply him with electricity, the factories that provide him with goods, etc.? If Sheldon factored in all related factors into his reasoning, I'm sure his proof would not only be more engaging but his conclusions might be more in line with current climate science. Sheldon's analysis is, as he said, third grade — interesting and inventive but, on further examination, flawed.