David Dilley, in his letter of April 28 (“Global warming cycles part of normal sequence of events”), claims that the idea that global warming is man-made is only “politicizing.” He is among the 2 to 3 percent of scientists who still deny that human activity is the main contributing factor in global warming. His denial of anthropogenic global warming has to be called into question.

Denying that human activity is a key factor in global warming is as rational as denying that smoking can cause lung cancer because there are other causes of lung cancer besides cigarettes.

Citing the past two winters as evidence of a normal sequence of events seems, for a climatologist, to be a bit insufficient. According to NASA’s website, “scientists define climate as the average weather for a particular region and time period, usually taken over 30 years.” That leaves Mr. Dilley 28 years short on observations, at least in this letter.

Evidence of increased carbon dioxide in the atmosphere has been tracked through scientific studies, and it can be identified as a product of human activity because the carbon isotopes (there are three types) can be measured and distinguished from each other.

Measurements from tree rings, ice cores and the surface ocean waters are also used to gauge the rise in global CO2 levels. Time lines can be established because we can look at the historical records of human activity.

We know when the industrial revolution began. No one piece of information is standing alone, and all of it points to human activity as the prime source of global warming.

Linda Pankewicz