To the editor:

I have been urged to reply to the op-ed of Lee Gerhard, a petroleum geologist. Most of the arguments he presented are partial truths. Space limits me to responding to his first two points:

1. “The most effective greenhouse gas is water vapor, comprising approximately 95 percent of the total greenhouse effect.” The real number is closer to 56 percent, but we won’t quibble. It is well known that an increase in carbon dioxide increases the average temperature some, and this causes a bit more water to evaporate, which, in turn, increases global warming, etc. So, water vapor is a feedback that amplifies the effect of CO2. CO2, however, is a “forcing,” since it tends to stay in the atmosphere once placed there, for example by burning fossil fuels. Conclusion: His statement was almost true, but misleading.

2. CO2 “…continues to rise, but … concentrations at present are near the lowest in geologic history.” This is true, if one looks at the record on a coarse scale with emphasis on pre-human time, which was on average much hotter with life unlike most of what we have now. Humans evolved in a cool environment of low CO2 over the last million years or so. However, CO2 is higher now than it has been at any time for nearly a million years. The risk associated with such a change is of great concern. Conclusion: His statement was true on one level, but incomplete and misleading.

This is true for most of his remaining points, except that some of them are matters of opinion or interpretation.